Writing an ALSA Driver¶
- Author:
Takashi Iwai <tiwai@suse.de>
Preface¶
This document describes how to write an ALSA (Advanced Linux Sound Architecture) driver. The document focuses mainly on PCI soundcards. In the case of other device types, the API might be different, too. However, at least the ALSA kernel API is consistent, and therefore it would be still a bit help for writing them.
This document targets people who already have enough C language skills and have basic linux kernel programming knowledge. This document doesn’t explain the general topic of linux kernel coding and doesn’t cover low-level driver implementation details. It only describes the standard way to write a PCI sound driver on ALSA.
File Tree Structure¶
General¶
The file tree structure of ALSA driver is depicted below:
sound
/core
/oss
/seq
/oss
/include
/drivers
/mpu401
/opl3
/i2c
/synth
/emux
/pci
/(cards)
/isa
/(cards)
/arm
/ppc
/sparc
/usb
/pcmcia /(cards)
/soc
/oss
core directory¶
This directory contains the middle layer which is the heart of ALSA drivers. In this directory, the native ALSA modules are stored. The sub-directories contain different modules and are dependent upon the kernel config.
core/oss¶
The code for OSS PCM and mixer emulation modules is stored in this
directory. The OSS rawmidi emulation is included in the ALSA rawmidi
code since it’s quite small. The sequencer code is stored in
core/seq/oss directory (see below).
core/seq¶
This directory and its sub-directories are for the ALSA sequencer. This
directory contains the sequencer core and primary sequencer modules such
as snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when
CONFIG_SND_SEQUENCER is set in the kernel config.
core/seq/oss¶
This contains the OSS sequencer emulation code.
include directory¶
This is the place for the public header files of ALSA drivers, which are to be exported to user-space, or included by several files in different directories. Basically, the private header files should not be placed in this directory, but you may still find files there, due to historical reasons :)
drivers directory¶
This directory contains code shared among different drivers on different architectures. They are hence supposed not to be architecture-specific. For example, the dummy PCM driver and the serial MIDI driver are found in this directory. In the sub-directories, there is code for components which are independent from bus and cpu architectures.
drivers/mpu401¶
The MPU401 and MPU401-UART modules are stored here.
drivers/opl3 and opl4¶
The OPL3 and OPL4 FM-synth stuff is found here.
i2c directory¶
This contains the ALSA i2c components.
Although there is a standard i2c layer on Linux, ALSA has its own i2c code for some cards, because the soundcard needs only a simple operation and the standard i2c API is too complicated for such a purpose.
synth directory¶
This contains the synth middle-level modules.
So far, there is only Emu8000/Emu10k1 synth driver under the
synth/emux sub-directory.
pci directory¶
This directory and its sub-directories hold the top-level card modules for PCI soundcards and the code specific to the PCI BUS.
The drivers compiled from a single file are stored directly in the pci directory, while the drivers with several source files are stored on their own sub-directory (e.g. emu10k1, ice1712).
isa directory¶
This directory and its sub-directories hold the top-level card modules for ISA soundcards.
arm, ppc, and sparc directories¶
They are used for top-level card modules which are specific to one of these architectures.
usb directory¶
This directory contains the USB-audio driver. The USB MIDI driver is integrated in the usb-audio driver.
pcmcia directory¶
The PCMCIA, especially PCCard drivers will go here. CardBus drivers will be in the pci directory, because their API is identical to that of standard PCI cards.
soc directory¶
This directory contains the codes for ASoC (ALSA System on Chip) layer including ASoC core, codec and machine drivers.
oss directory¶
This contains OSS/Lite code. At the time of writing, all code has been removed except for dmasound on m68k.
Basic Flow for PCI Drivers¶
Outline¶
The minimum flow for PCI soundcards is as follows:
define the PCI ID table (see the section PCI Entries).
create
probecallback.create
removecallback.create a
struct pci_driverstructure containing the three pointers above.create an
initfunction just calling thepci_register_driver()to register the pci_driver table defined above.create an
exitfunction to call thepci_unregister_driver()function.
Full Code Example¶
The code example is shown below. Some parts are kept unimplemented at
this moment but will be filled in the next sections. The numbers in the
comment lines of the snd_mychip_probe() function refer
to details explained in the following section.
#include <linux/init.h>
#include <linux/pci.h>
#include <linux/slab.h>
#include <sound/core.h>
#include <sound/initval.h>
/* module parameters (see "Module Parameters") */
/* SNDRV_CARDS: maximum number of cards supported by this module */
static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
/* definition of the chip-specific record */
struct mychip {
struct snd_card *card;
/* the rest of the implementation will be in section
* "PCI Resource Management"
*/
};
/* chip-specific destructor
* (see "PCI Resource Management")
*/
static int snd_mychip_free(struct mychip *chip)
{
.... /* will be implemented later... */
}
/* component-destructor
* (see "Management of Cards and Components")
*/
static int snd_mychip_dev_free(struct snd_device *device)
{
return snd_mychip_free(device->device_data);
}
/* chip-specific constructor
* (see "Management of Cards and Components")
*/
static int snd_mychip_create(struct snd_card *card,
struct pci_dev *pci,
struct mychip **rchip)
{
struct mychip *chip;
int err;
static const struct snd_device_ops ops = {
.dev_free = snd_mychip_dev_free,
};
*rchip = NULL;
/* check PCI availability here
* (see "PCI Resource Management")
*/
....
/* allocate a chip-specific data with zero filled */
chip = kzalloc(sizeof(*chip), GFP_KERNEL);
if (chip == NULL)
return -ENOMEM;
chip->card = card;
/* rest of initialization here; will be implemented
* later, see "PCI Resource Management"
*/
....
err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
if (err < 0) {
snd_mychip_free(chip);
return err;
}
*rchip = chip;
return 0;
}
/* constructor -- see "Driver Constructor" sub-section */
static int snd_mychip_probe(struct pci_dev *pci,
const struct pci_device_id *pci_id)
{
static int dev;
struct snd_card *card;
struct mychip *chip;
int err;
/* (1) */
if (dev >= SNDRV_CARDS)
return -ENODEV;
if (!enable[dev]) {
dev++;
return -ENOENT;
}
/* (2) */
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
0, &card);
if (err < 0)
return err;
/* (3) */
err = snd_mychip_create(card, pci, &chip);
if (err < 0)
goto error;
/* (4) */
strcpy(card->driver, "My Chip");
strcpy(card->shortname, "My Own Chip 123");
sprintf(card->longname, "%s at 0x%lx irq %i",
card->shortname, chip->port, chip->irq);
/* (5) */
.... /* implemented later */
/* (6) */
err = snd_card_register(card);
if (err < 0)
goto error;
/* (7) */
pci_set_drvdata(pci, card);
dev++;
return 0;
error:
snd_card_free(card);
return err;
}
/* destructor -- see the "Destructor" sub-section */
static void snd_mychip_remove(struct pci_dev *pci)
{
snd_card_free(pci_get_drvdata(pci));
}
Driver Constructor¶
The real constructor of PCI drivers is the probe callback. The
probe callback and other component-constructors which are called
from the probe callback cannot be used with the __init prefix
because any PCI device could be a hotplug device.
In the probe callback, the following scheme is often used.
1) Check and increment the device index.¶
static int dev;
....
if (dev >= SNDRV_CARDS)
return -ENODEV;
if (!enable[dev]) {
dev++;
return -ENOENT;
}
where enable[dev] is the module option.
Each time the probe callback is called, check the availability of
the device. If not available, simply increment the device index and
return. dev will be incremented also later (step 7).
2) Create a card instance¶
struct snd_card *card;
int err;
....
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
0, &card);
The details will be explained in the section Management of Cards and Components.
3) Create a main component¶
In this part, the PCI resources are allocated:
struct mychip *chip;
....
err = snd_mychip_create(card, pci, &chip);
if (err < 0)
goto error;
The details will be explained in the section PCI Resource Management.
When something goes wrong, the probe function needs to deal with the error. In this example, we have a single error handling path placed at the end of the function:
error:
snd_card_free(card);
return err;
Since each component can be properly freed, the single
snd_card_free() call should suffice in most cases.
4) Set the driver ID and name strings.¶
strcpy(card->driver, "My Chip");
strcpy(card->shortname, "My Own Chip 123");
sprintf(card->longname, "%s at 0x%lx irq %i",
card->shortname, chip->port, chip->irq);
The driver field holds the minimal ID string of the chip. This is used by alsa-lib’s configurator, so keep it simple but unique. Even the same driver can have different driver IDs to distinguish the functionality of each chip type.
The shortname field is a string shown as more verbose name. The longname
field contains the information shown in /proc/asound/cards.
5) Create other components, such as mixer, MIDI, etc.¶
Here you define the basic components such as PCM, mixer (e.g. AC97), MIDI (e.g. MPU-401), and other interfaces. Also, if you want a proc file, define it here, too.
6) Register the card instance.¶
err = snd_card_register(card);
if (err < 0)
goto error;
Will be explained in the section Management of Cards and Components, too.
7) Set the PCI driver data and return zero.¶
pci_set_drvdata(pci, card);
dev++;
return 0;
In the above, the card record is stored. This pointer is used in the remove callback and power-management callbacks, too.
Destructor¶
The destructor, the remove callback, simply releases the card instance. Then the ALSA middle layer will release all the attached components automatically.
It would be typically just calling snd_card_free():
static void snd_mychip_remove(struct pci_dev *pci)
{
snd_card_free(pci_get_drvdata(pci));
}
The above code assumes that the card pointer is set to the PCI driver data.
Header Files¶
For the above example, at least the following include files are necessary:
#include <linux/init.h>
#include <linux/pci.h>
#include <linux/slab.h>
#include <sound/core.h>
#include <sound/initval.h>
where the last one is necessary only when module options are defined in the source file. If the code is split into several files, the files without module options don’t need them.
In addition to these headers, you’ll need <linux/interrupt.h> for
interrupt handling, and <linux/io.h> for I/O access. If you use the
mdelay() or udelay() functions, you’ll need
to include <linux/delay.h> too.
The ALSA interfaces like the PCM and control APIs are defined in other
<sound/xxx.h> header files. They have to be included after
<sound/core.h>.
Management of Cards and Components¶
Card Instance¶
For each soundcard, a “card” record must be allocated.
A card record is the headquarters of the soundcard. It manages the whole list of devices (components) on the soundcard, such as PCM, mixers, MIDI, synthesizer, and so on. Also, the card record holds the ID and the name strings of the card, manages the root of proc files, and controls the power-management states and hotplug disconnections. The component list on the card record is used to manage the correct release of resources at destruction.
As mentioned above, to create a card instance, call
snd_card_new():
struct snd_card *card;
int err;
err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
The function takes six arguments: the parent device pointer, the
card-index number, the id string, the module pointer (usually
THIS_MODULE), the size of extra-data space, and the pointer to
return the card instance. The extra_size argument is used to allocate
card->private_data for the chip-specific data. Note that these data are
allocated by snd_card_new().
The first argument, the pointer of struct device, specifies the parent
device. For PCI devices, typically &pci-> is passed there.
Components¶
After the card is created, you can attach the components (devices) to the card instance. In an ALSA driver, a component is represented as a struct snd_device object. A component can be a PCM instance, a control interface, a raw MIDI interface, etc. Each such instance has one component entry.
A component can be created via the snd_device_new()
function:
snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
This takes the card pointer, the device-level (SNDRV_DEV_XXX), the
data pointer, and the callback pointers (&ops). The device-level
defines the type of components and the order of registration and
de-registration. For most components, the device-level is already
defined. For a user-defined component, you can use
SNDRV_DEV_LOWLEVEL.
This function itself doesn’t allocate the data space. The data must be
allocated manually beforehand, and its pointer is passed as the
argument. This pointer (chip in the above example) is used as the
identifier for the instance.
Each pre-defined ALSA component such as AC97 and PCM calls
snd_device_new() inside its constructor. The destructor
for each component is defined in the callback pointers. Hence, you don’t
need to take care of calling a destructor for such a component.
If you wish to create your own component, you need to set the destructor
function to the dev_free callback in the ops, so that it can be
released automatically via snd_card_free(). The next
example will show an implementation of chip-specific data.
Chip-Specific Data¶
Chip-specific information, e.g. the I/O port address, its resource pointer, or the irq number, is stored in the chip-specific record:
struct mychip {
....
};
In general, there are two ways of allocating the chip record.
1. Allocating via snd_card_new().¶
As mentioned above, you can pass the extra-data-length to the 5th
argument of snd_card_new(), e.g.:
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
sizeof(struct mychip), &card);
struct mychip is the type of the chip record.
In return, the allocated record can be accessed as
struct mychip *chip = card->private_data;
With this method, you don’t have to allocate twice. The record is released together with the card instance.
2. Allocating an extra device.¶
After allocating a card instance via snd_card_new()
(with 0 on the 4th arg), call kzalloc():
struct snd_card *card;
struct mychip *chip;
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
0, &card);
.....
chip = kzalloc(sizeof(*chip), GFP_KERNEL);
The chip record should have the field to hold the card pointer at least,
struct mychip {
struct snd_card *card;
....
};
Then, set the card pointer in the returned chip instance:
chip->card = card;
Next, initialize the fields, and register this chip record as a
low-level device with a specified ops:
static const struct snd_device_ops ops = {
.dev_free = snd_mychip_dev_free,
};
....
snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
snd_mychip_dev_free() is the device-destructor
function, which will call the real destructor:
static int snd_mychip_dev_free(struct snd_device *device)
{
return snd_mychip_free(device->device_data);
}
where snd_mychip_free() is the real destructor.
The demerit of this method is the obviously larger amount of code. The merit is, however, that you can trigger your own callback at registering and disconnecting the card via a setting in snd_device_ops. About registering and disconnecting the card, see the subsections below.
Registration and Release¶
After all components are assigned, register the card instance by calling
snd_card_register(). Access to the device files is
enabled at this point. That is, before
snd_card_register() is called, the components are safely
inaccessible from external side. If this call fails, exit the probe
function after releasing the card via snd_card_free().
For releasing the card instance, you can call simply
snd_card_free(). As mentioned earlier, all components
are released automatically by this call.
For a device which allows hotplugging, you can use
snd_card_free_when_closed(). This one will postpone
the destruction until all devices are closed.
PCI Resource Management¶
Full Code Example¶
In this section, we’ll complete the chip-specific constructor, destructor and PCI entries. Example code is shown first, below:
struct mychip {
struct snd_card *card;
struct pci_dev *pci;
unsigned long port;
int irq;
};
static int snd_mychip_free(struct mychip *chip)
{
/* disable hardware here if any */
.... /* (not implemented in this document) */
/* release the irq */
if (chip->irq >= 0)
free_irq(chip->irq, chip);
/* release the I/O ports & memory */
pci_release_regions(chip->pci);
/* disable the PCI entry */
pci_disable_device(chip->pci);
/* release the data */
kfree(chip);
return 0;
}
/* chip-specific constructor */
static int snd_mychip_create(struct snd_card *card,
struct pci_dev *pci,
struct mychip **rchip)
{
struct mychip *chip;
int err;
static const struct snd_device_ops ops = {
.dev_free = snd_mychip_dev_free,
};
*rchip = NULL;
/* initialize the PCI entry */
err = pci_enable_device(pci);
if (err < 0)
return err;
/* check PCI availability (28bit DMA) */
if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
printk(KERN_ERR "error to set 28bit mask DMA\n");
pci_disable_device(pci);
return -ENXIO;
}
chip = kzalloc(sizeof(*chip), GFP_KERNEL);
if (chip == NULL) {
pci_disable_device(pci);
return -ENOMEM;
}
/* initialize the stuff */
chip->card = card;
chip->pci = pci;
chip->irq = -1;
/* (1) PCI resource allocation */
err = pci_request_regions(pci, "My Chip");
if (err < 0) {
kfree(chip);
pci_disable_device(pci);
return err;
}
chip->port = pci_resource_start(pci, 0);
if (request_irq(pci->irq, snd_mychip_interrupt,
IRQF_SHARED, KBUILD_MODNAME, chip)) {
printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
snd_mychip_free(chip);
return -EBUSY;
}
chip->irq = pci->irq;
card->sync_irq = chip->irq;
/* (2) initialization of the chip hardware */
.... /* (not implemented in this document) */
err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
if (err < 0) {
snd_mychip_free(chip);
return err;
}
*rchip = chip;
return 0;
}
/* PCI IDs */
static struct pci_device_id snd_mychip_ids[] = {
{ PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
....
{ 0, }
};
MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
/* pci_driver definition */
static struct pci_driver driver = {
.name = KBUILD_MODNAME,
.id_table = snd_mychip_ids,
.probe = snd_mychip_probe,
.remove = snd_mychip_remove,
};
/* module initialization */
static int __init alsa_card_mychip_init(void)
{
return pci_register_driver(&driver);
}
/* module clean up */
static void __exit alsa_card_mychip_exit(void)
{
pci_unregister_driver(&driver);
}
module_init(alsa_card_mychip_init)
module_exit(alsa_card_mychip_exit)
EXPORT_NO_SYMBOLS; /* for old kernels only */
Some Hafta’s¶
The allocation of PCI resources is done in the probe function, and
usually an extra xxx_create() function is written for this
purpose.
In the case of PCI devices, you first have to call the
pci_enable_device() function before allocating
resources. Also, you need to set the proper PCI DMA mask to limit the
accessed I/O range. In some cases, you might need to call
pci_set_master() function, too.
Suppose a 28bit mask, the code to be added would look like:
err = pci_enable_device(pci);
if (err < 0)
return err;
if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
printk(KERN_ERR "error to set 28bit mask DMA\n");
pci_disable_device(pci);
return -ENXIO;
}
Resource Allocation¶
The allocation of I/O ports and irqs is done via standard kernel functions. These resources must be released in the destructor function (see below).
Now assume that the PCI device has an I/O port with 8 bytes and an interrupt. Then struct mychip will have the following fields:
struct mychip {
struct snd_card *card;
unsigned long port;
int irq;
};
For an I/O port (and also a memory region), you need to have the
resource pointer for the standard resource management. For an irq, you
have to keep only the irq number (integer). But you need to initialize
this number to -1 before actual allocation, since irq 0 is valid. The
port address and its resource pointer can be initialized as null by
kzalloc() automatically, so you don’t have to take care of
resetting them.
The allocation of an I/O port is done like this:
err = pci_request_regions(pci, "My Chip");
if (err < 0) {
kfree(chip);
pci_disable_device(pci);
return err;
}
chip->port = pci_resource_start(pci, 0);
It will reserve the I/O port region of 8 bytes of the given PCI device.
The returned value, chip->res_port, is allocated via
kmalloc() by request_region(). The pointer
must be released via kfree(), but there is a problem with
this. This issue will be explained later.
The allocation of an interrupt source is done like this:
if (request_irq(pci->irq, snd_mychip_interrupt,
IRQF_SHARED, KBUILD_MODNAME, chip)) {
printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
snd_mychip_free(chip);
return -EBUSY;
}
chip->irq = pci->irq;
where snd_mychip_interrupt() is the interrupt handler
defined later. Note that
chip->irq should be defined only when request_irq()
succeeded.
On the PCI bus, interrupts can be shared. Thus, IRQF_SHARED is used
as the interrupt flag of request_irq().
The last argument of request_irq() is the data pointer
passed to the interrupt handler. Usually, the chip-specific record is
used for that, but you can use what you like, too.
I won’t give details about the interrupt handler at this point, but at least its appearance can be explained now. The interrupt handler looks usually as follows:
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
{
struct mychip *chip = dev_id;
....
return IRQ_HANDLED;
}
After requesting the IRQ, you can passed it to card->sync_irq
field:
card->irq = chip->irq;
This allows the PCM core to automatically call
synchronize_irq() at the right time, like before hw_free.
See the later section sync_stop callback for details.
Now let’s write the corresponding destructor for the resources above. The role of destructor is simple: disable the hardware (if already activated) and release the resources. So far, we have no hardware part, so the disabling code is not written here.
To release the resources, the “check-and-release” method is a safer way. For the interrupt, do like this:
if (chip->irq >= 0)
free_irq(chip->irq, chip);
Since the irq number can start from 0, you should initialize
chip->irq with a negative value (e.g. -1), so that you can check
the validity of the irq number as above.
When you requested I/O ports or memory regions via
pci_request_region() or
pci_request_regions() like in this example, release the
resource(s) using the corresponding function,
pci_release_region() or
pci_release_regions():
pci_release_regions(chip->pci);
When you requested manually via request_region() or
request_mem_region(), you can release it via
release_resource(). Suppose that you keep the resource
pointer returned from request_region() in
chip->res_port, the release procedure looks like:
release_and_free_resource(chip->res_port);
Don’t forget to call pci_disable_device() before the
end.
And finally, release the chip-specific record:
kfree(chip);
We didn’t implement the hardware disabling part above. If you need to do this, please note that the destructor may be called even before the initialization of the chip is completed. It would be better to have a flag to skip hardware disabling if the hardware was not initialized yet.
When the chip-data is assigned to the card using
snd_device_new() with SNDRV_DEV_LOWLELVEL, its
destructor is called last. That is, it is assured that all other
components like PCMs and controls have already been released. You don’t
have to stop PCMs, etc. explicitly, but just call low-level hardware
stopping.
The management of a memory-mapped region is almost as same as the management of an I/O port. You’ll need two fields as follows:
struct mychip {
....
unsigned long iobase_phys;
void __iomem *iobase_virt;
};
and the allocation would look like below:
err = pci_request_regions(pci, "My Chip");
if (err < 0) {
kfree(chip);
return err;
}
chip->iobase_phys = pci_resource_start(pci, 0);
chip->iobase_virt = ioremap(chip->iobase_phys,
pci_resource_len(pci, 0));
and the corresponding destructor would be:
static int snd_mychip_free(struct mychip *chip)
{
....
if (chip->iobase_virt)
iounmap(chip->iobase_virt);
....
pci_release_regions(chip->pci);
....
}
Of course, a modern way with pci_iomap() will make things a
bit easier, too:
err = pci_request_regions(pci, "My Chip");
if (err < 0) {
kfree(chip);
return err;
}
chip->iobase_virt = pci_iomap(pci, 0, 0);
which is paired with pci_iounmap() at destructor.
PCI Entries¶
So far, so good. Let’s finish the missing PCI stuff. At first, we need a
struct pci_device_id table for
this chipset. It’s a table of PCI vendor/device ID number, and some
masks.
For example:
static struct pci_device_id snd_mychip_ids[] = {
{ PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
....
{ 0, }
};
MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
The first and second fields of the struct pci_device_id are the vendor
and device IDs. If you have no reason to filter the matching devices, you can
leave the remaining fields as above. The last field of the
struct pci_device_id contains private data for this entry. You can specify
any value here, for example, to define specific operations for supported
device IDs. Such an example is found in the intel8x0 driver.
The last entry of this list is the terminator. You must specify this all-zero entry.
Then, prepare the struct pci_driver
record:
static struct pci_driver driver = {
.name = KBUILD_MODNAME,
.id_table = snd_mychip_ids,
.probe = snd_mychip_probe,
.remove = snd_mychip_remove,
};
The probe and remove functions have already been defined in
the previous sections. The name field is the name string of this
device. Note that you must not use slashes (“/”) in this string.
And at last, the module entries:
static int __init alsa_card_mychip_init(void)
{
return pci_register_driver(&driver);
}
static void __exit alsa_card_mychip_exit(void)
{
pci_unregister_driver(&driver);
}
module_init(alsa_card_mychip_init)
module_exit(alsa_card_mychip_exit)
Note that these module entries are tagged with __init and __exit
prefixes.
That’s all!
PCM Interface¶
General¶
The PCM middle layer of ALSA is quite powerful and it is only necessary for each driver to implement the low-level functions to access its hardware.
To access the PCM layer, you need to include <sound/pcm.h>
first. In addition, <sound/pcm_params.h> might be needed if you
access some functions related with hw_param.
Each card device can have up to four PCM instances. A PCM instance corresponds to a PCM device file. The limitation of number of instances comes only from the available bit size of Linux’ device numbers. Once 64bit device numbers are used, we’ll have more PCM instances available.
A PCM instance consists of PCM playback and capture streams, and each
PCM stream consists of one or more PCM substreams. Some soundcards
support multiple playback functions. For example, emu10k1 has a PCM
playback of 32 stereo substreams. In this case, at each open, a free
substream is (usually) automatically chosen and opened. Meanwhile, when
only one substream exists and it was already opened, a subsequent open
will either block or error with EAGAIN according to the file open
mode. But you don’t have to care about such details in your driver. The
PCM middle layer will take care of such work.
Full Code Example¶
The example code below does not include any hardware access routines but shows only the skeleton, how to build up the PCM interfaces:
#include <sound/pcm.h>
....
/* hardware definition */
static struct snd_pcm_hardware snd_mychip_playback_hw = {
.info = (SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP_VALID),
.formats = SNDRV_PCM_FMTBIT_S16_LE,
.rates = SNDRV_PCM_RATE_8000_48000,
.rate_min = 8000,
.rate_max = 48000,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 32768,
.period_bytes_min = 4096,
.period_bytes_max = 32768,
.periods_min = 1,
.periods_max = 1024,
};
/* hardware definition */
static struct snd_pcm_hardware snd_mychip_capture_hw = {
.info = (SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP_VALID),
.formats = SNDRV_PCM_FMTBIT_S16_LE,
.rates = SNDRV_PCM_RATE_8000_48000,
.rate_min = 8000,
.rate_max = 48000,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 32768,
.period_bytes_min = 4096,
.period_bytes_max = 32768,
.periods_min = 1,
.periods_max = 1024,
};
/* open callback */
static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
runtime->hw = snd_mychip_playback_hw;
/* more hardware-initialization will be done here */
....
return 0;
}
/* close callback */
static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
/* the hardware-specific codes will be here */
....
return 0;
}
/* open callback */
static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
runtime->hw = snd_mychip_capture_hw;
/* more hardware-initialization will be done here */
....
return 0;
}
/* close callback */
static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
/* the hardware-specific codes will be here */
....
return 0;
}
/* hw_params callback */
static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
struct snd_pcm_hw_params *hw_params)
{
/* the hardware-specific codes will be here */
....
return 0;
}
/* hw_free callback */
static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
{
/* the hardware-specific codes will be here */
....
return 0;
}
/* prepare callback */
static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
/* set up the hardware with the current configuration
* for example...
*/
mychip_set_sample_format(chip, runtime->format);
mychip_set_sample_rate(chip, runtime->rate);
mychip_set_channels(chip, runtime->channels);
mychip_set_dma_setup(chip, runtime->dma_addr,
chip->buffer_size,
chip->period_size);
return 0;
}
/* trigger callback */
static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
int cmd)
{
switch (cmd) {
case SNDRV_PCM_TRIGGER_START:
/* do something to start the PCM engine */
....
break;
case SNDRV_PCM_TRIGGER_STOP:
/* do something to stop the PCM engine */
....
break;
default:
return -EINVAL;
}
}
/* pointer callback */
static snd_pcm_uframes_t
snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
unsigned int current_ptr;
/* get the current hardware pointer */
current_ptr = mychip_get_hw_pointer(chip);
return current_ptr;
}
/* operators */
static struct snd_pcm_ops snd_mychip_playback_ops = {
.open = snd_mychip_playback_open,
.close = snd_mychip_playback_close,
.hw_params = snd_mychip_pcm_hw_params,
.hw_free = snd_mychip_pcm_hw_free,
.prepare = snd_mychip_pcm_prepare,
.trigger = snd_mychip_pcm_trigger,
.pointer = snd_mychip_pcm_pointer,
};
/* operators */
static struct snd_pcm_ops snd_mychip_capture_ops = {
.open = snd_mychip_capture_open,
.close = snd_mychip_capture_close,
.hw_params = snd_mychip_pcm_hw_params,
.hw_free = snd_mychip_pcm_hw_free,
.prepare = snd_mychip_pcm_prepare,
.trigger = snd_mychip_pcm_trigger,
.pointer = snd_mychip_pcm_pointer,
};
/*
* definitions of capture are omitted here...
*/
/* create a pcm device */
static int snd_mychip_new_pcm(struct mychip *chip)
{
struct snd_pcm *pcm;
int err;
err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
if (err < 0)
return err;
pcm->private_data = chip;
strcpy(pcm->name, "My Chip");
chip->pcm = pcm;
/* set operators */
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
&snd_mychip_playback_ops);
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
&snd_mychip_capture_ops);
/* pre-allocation of buffers */
/* NOTE: this may fail */
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
&chip->pci->dev,
64*1024, 64*1024);
return 0;
}
PCM Constructor¶
A PCM instance is allocated by the snd_pcm_new()
function. It would be better to create a constructor for the PCM, namely:
static int snd_mychip_new_pcm(struct mychip *chip)
{
struct snd_pcm *pcm;
int err;
err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
if (err < 0)
return err;
pcm->private_data = chip;
strcpy(pcm->name, "My Chip");
chip->pcm = pcm;
...
return 0;
}
The snd_pcm_new() function takes six arguments. The
first argument is the card pointer to which this PCM is assigned, and
the second is the ID string.
The third argument (index, 0 in the above) is the index of this new
PCM. It begins from zero. If you create more than one PCM instances,
specify the different numbers in this argument. For example, index =
1 for the second PCM device.
The fourth and fifth arguments are the number of substreams for playback and capture, respectively. Here 1 is used for both arguments. When no playback or capture substreams are available, pass 0 to the corresponding argument.
If a chip supports multiple playbacks or captures, you can specify more numbers, but they must be handled properly in open/close, etc. callbacks. When you need to know which substream you are referring to, then it can be obtained from struct snd_pcm_substream data passed to each callback as follows:
struct snd_pcm_substream *substream;
int index = substream->number;
After the PCM is created, you need to set operators for each PCM stream:
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
&snd_mychip_playback_ops);
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
&snd_mychip_capture_ops);
The operators are defined typically like this:
static struct snd_pcm_ops snd_mychip_playback_ops = {
.open = snd_mychip_pcm_open,
.close = snd_mychip_pcm_close,
.hw_params = snd_mychip_pcm_hw_params,
.hw_free = snd_mychip_pcm_hw_free,
.prepare = snd_mychip_pcm_prepare,
.trigger = snd_mychip_pcm_trigger,
.pointer = snd_mychip_pcm_pointer,
};
All the callbacks are described in the Operators subsection.
After setting the operators, you probably will want to pre-allocate the buffer and set up the managed allocation mode. For that, simply call the following:
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
&chip->pci->dev,
64*1024, 64*1024);
It will allocate a buffer up to 64kB by default. Buffer management details will be described in the later section Buffer and Memory Management.
Additionally, you can set some extra information for this PCM in
pcm->info_flags. The available values are defined as
SNDRV_PCM_INFO_XXX in <sound/asound.h>, which is used for the
hardware definition (described later). When your soundchip supports only
half-duplex, specify it like this:
pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
... And the Destructor?¶
The destructor for a PCM instance is not always necessary. Since the PCM device will be released by the middle layer code automatically, you don’t have to call the destructor explicitly.
The destructor would be necessary if you created special records
internally and needed to release them. In such a case, set the
destructor function to pcm->private_free:
static void mychip_pcm_free(struct snd_pcm *pcm)
{
struct mychip *chip = snd_pcm_chip(pcm);
/* free your own data */
kfree(chip->my_private_pcm_data);
/* do what you like else */
....
}
static int snd_mychip_new_pcm(struct mychip *chip)
{
struct snd_pcm *pcm;
....
/* allocate your own data */
chip->my_private_pcm_data = kmalloc(...);
/* set the destructor */
pcm->private_data = chip;
pcm->private_free = mychip_pcm_free;
....
}
Runtime Pointer - The Chest of PCM Information¶
When the PCM substream is opened, a PCM runtime instance is allocated
and assigned to the substream. This pointer is accessible via
substream->runtime. This runtime pointer holds most information you
need to control the PCM: a copy of hw_params and sw_params
configurations, the buffer pointers, mmap records, spinlocks, etc.
The definition of runtime instance is found in <sound/pcm.h>. Here
is the relevant part of this file:
struct _snd_pcm_runtime {
/* -- Status -- */
struct snd_pcm_substream *trigger_master;
snd_timestamp_t trigger_tstamp; /* trigger timestamp */
int overrange;
snd_pcm_uframes_t avail_max;
snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */
snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
/* -- HW params -- */
snd_pcm_access_t access; /* access mode */
snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */
snd_pcm_subformat_t subformat; /* subformat */
unsigned int rate; /* rate in Hz */
unsigned int channels; /* channels */
snd_pcm_uframes_t period_size; /* period size */
unsigned int periods; /* periods */
snd_pcm_uframes_t buffer_size; /* buffer size */
unsigned int tick_time; /* tick time */
snd_pcm_uframes_t min_align; /* Min alignment for the format */
size_t byte_align;
unsigned int frame_bits;
unsigned int sample_bits;
unsigned int info;
unsigned int rate_num;
unsigned int rate_den;
/* -- SW params -- */
struct timespec tstamp_mode; /* mmap timestamp is updated */
unsigned int period_step;
unsigned int sleep_min; /* min ticks to sleep */
snd_pcm_uframes_t start_threshold;
/*
* The following two thresholds alleviate playback buffer underruns; when
* hw_avail drops below the threshold, the respective action is triggered:
*/
snd_pcm_uframes_t stop_threshold; /* - stop playback */
snd_pcm_uframes_t silence_threshold; /* - pre-fill buffer with silence */
snd_pcm_uframes_t silence_size; /* max size of silence pre-fill; when >= boundary,
* fill played area with silence immediately */
snd_pcm_uframes_t boundary; /* pointers wrap point */
/* internal data of auto-silencer */
snd_pcm_uframes_t silence_start; /* starting pointer to silence area */
snd_pcm_uframes_t silence_filled; /* size filled with silence */
snd_pcm_sync_id_t sync; /* hardware synchronization ID */
/* -- mmap -- */
volatile struct snd_pcm_mmap_status *status;
volatile struct snd_pcm_mmap_control *control;
atomic_t mmap_count;
/* -- locking / scheduling -- */
spinlock_t lock;
wait_queue_head_t sleep;
struct timer_list tick_timer;
struct fasync_struct *fasync;
/* -- private section -- */
void *private_data;
void (*private_free)(struct snd_pcm_runtime *runtime);
/* -- hardware description -- */
struct snd_pcm_hardware hw;
struct snd_pcm_hw_constraints hw_constraints;
/* -- timer -- */
unsigned int timer_resolution; /* timer resolution */
/* -- DMA -- */
unsigned char *dma_area; /* DMA area */
dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */
size_t dma_bytes; /* size of DMA area */
struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */
#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
/* -- OSS things -- */
struct snd_pcm_oss_runtime oss;
#endif
};
For the operators (callbacks) of each sound driver, most of these records are supposed to be read-only. Only the PCM middle-layer changes / updates them. The exceptions are the hardware description (hw) DMA buffer information and the private data. Besides, if you use the standard managed buffer allocation mode, you don’t need to set the DMA buffer information by yourself.
In the sections below, important records are explained.
Hardware Description¶
The hardware descriptor (struct snd_pcm_hardware) contains the definitions of
the fundamental hardware configuration. Above all, you’ll need to define this
in the PCM open callback. Note that the runtime instance holds a copy of
the descriptor, not a pointer to the existing descriptor. That is,
in the open callback, you can modify the copied descriptor
(runtime->hw) as you need. For example, if the maximum number of
channels is 1 only on some chip models, you can still use the same
hardware descriptor and change the channels_max later:
struct snd_pcm_runtime *runtime = substream->runtime;
...
runtime->hw = snd_mychip_playback_hw; /* common definition */
if (chip->model == VERY_OLD_ONE)
runtime->hw.channels_max = 1;
Typically, you’ll have a hardware descriptor as below:
static struct snd_pcm_hardware snd_mychip_playback_hw = {
.info = (SNDRV_PCM_INFO_MMAP |
SNDRV_PCM_INFO_INTERLEAVED |
SNDRV_PCM_INFO_BLOCK_TRANSFER |
SNDRV_PCM_INFO_MMAP_VALID),
.formats = SNDRV_PCM_FMTBIT_S16_LE,
.rates = SNDRV_PCM_RATE_8000_48000,
.rate_min = 8000,
.rate_max = 48000,
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = 32768,
.period_bytes_min = 4096,
.period_bytes_max = 32768,
.periods_min = 1,
.periods_max = 1024,
};
The
infofield contains the type and capabilities of this PCM. The bit flags are defined in<sound/asound.h>asSNDRV_PCM_INFO_XXX. Here, at least, you have to specify whether mmap is supported and which interleaving formats are supported. When the hardware supports mmap, add theSNDRV_PCM_INFO_MMAPflag here. When the hardware supports the interleaved or the non-interleaved formats, theSNDRV_PCM_INFO_INTERLEAVEDorSNDRV_PCM_INFO_NONINTERLEAVEDflag must be set, respectively. If both are supported, you can set both, too.In the above example,
MMAP_VALIDandBLOCK_TRANSFERare specified for the OSS mmap mode. Usually both are set. Of course,MMAP_VALIDis set only if mmap is really supported.The other possible flags are
SNDRV_PCM_INFO_PAUSEandSNDRV_PCM_INFO_RESUME. ThePAUSEbit means that the PCM supports the “pause” operation, while theRESUMEbit means that the PCM supports the full “suspend/resume” operation. If thePAUSEflag is set, thetriggercallback below must handle the corresponding (pause push/release) commands. The suspend/resume trigger commands can be defined even without theRESUMEflag. See the Power Management section for details.When the PCM substreams can be synchronized (typically, synchronized start/stop of a playback and a capture stream), you can give
SNDRV_PCM_INFO_SYNC_START, too. In this case, you’ll need to check the linked-list of PCM substreams in the trigger callback. This will be described in a later section.The
formatsfield contains the bit-flags of supported formats (SNDRV_PCM_FMTBIT_XXX). If the hardware supports more than one format, give all or’ed bits. In the example above, the signed 16bit little-endian format is specified.The
ratesfield contains the bit-flags of supported rates (SNDRV_PCM_RATE_XXX). When the chip supports continuous rates, pass theCONTINUOUSbit additionally. The pre-defined rate bits are provided only for typical rates. If your chip supports unconventional rates, you need to add theKNOTbit and set up the hardware constraint manually (explained later).rate_minandrate_maxdefine the minimum and maximum sample rate. This should correspond somehow toratesbits.channels_minandchannels_maxdefine, as you might have already expected, the minimum and maximum number of channels.buffer_bytes_maxdefines the maximum buffer size in bytes. There is nobuffer_bytes_minfield, since it can be calculated from the minimum period size and the minimum number of periods. Meanwhile,period_bytes_minandperiod_bytes_maxdefine the minimum and maximum size of the period in bytes.periods_maxandperiods_mindefine the maximum and minimum number of periods in the buffer.The “period” is a term that corresponds to a fragment in the OSS world. The period defines the point at which a PCM interrupt is generated. This point strongly depends on the hardware. Generally, a smaller period size will give you more interrupts, which results in being able to fill/drain the buffer more timely. In the case of capture, this size defines the input latency. On the other hand, the whole buffer size defines the output latency for the playback direction.
There is also a field
fifo_size. This specifies the size of the hardware FIFO, but currently it is neither used by the drivers nor in the alsa-lib. So, you can ignore this field.
PCM Configurations¶
Ok, let’s go back again to the PCM runtime records. The most
frequently referred records in the runtime instance are the PCM
configurations. The PCM configurations are stored in the runtime
instance after the application sends hw_params data via
alsa-lib. There are many fields copied from hw_params and sw_params
structs. For example, format holds the format type chosen by the
application. This field contains the enum value
SNDRV_PCM_FORMAT_XXX.
One thing to be noted is that the configured buffer and period sizes
are stored in “frames” in the runtime. In the ALSA world, 1 frame =
channels \* samples-size. For conversion between frames and bytes,
you can use the frames_to_bytes() and
bytes_to_frames() helper functions:
period_bytes = frames_to_bytes(runtime, runtime->period_size);
Also, many software parameters (sw_params) are stored in frames, too.
Please check the type of the field. snd_pcm_uframes_t is for
frames as unsigned integer while snd_pcm_sframes_t is for
frames as signed integer.
DMA Buffer Information¶
The DMA buffer is defined by the following four fields: dma_area,
dma_addr, dma_bytes and dma_private. dma_area
holds the buffer pointer (the logical address). You can call
memcpy() from/to this pointer. Meanwhile, dma_addr holds
the physical address of the buffer. This field is specified only when
the buffer is a linear buffer. dma_bytes holds the size of the
buffer in bytes. dma_private is used for the ALSA DMA allocator.
If you use either the managed buffer allocation mode or the standard
API function snd_pcm_lib_malloc_pages() for allocating the buffer,
these fields are set by the ALSA middle layer, and you should not
change them by yourself. You can read them but not write them. On the
other hand, if you want to allocate the buffer by yourself, you’ll
need to manage it in the hw_params callback. At least, dma_bytes is
mandatory. dma_area is necessary when the buffer is mmapped. If
your driver doesn’t support mmap, this field is not
necessary. dma_addr is also optional. You can use dma_private as
you like, too.
Running Status¶
The running status can be referred via runtime->status. This is
a pointer to a struct snd_pcm_mmap_status record.
For example, you can get the current
DMA hardware pointer via runtime->status->hw_ptr.
The DMA application pointer can be referred via runtime->control,
which points to a struct snd_pcm_mmap_control record.
However, accessing this value directly is not recommended.
Private Data¶
You can allocate a record for the substream and store it in
runtime->private_data. Usually, this is done in the PCM open
callback. Don’t mix this with pcm->private_data. The
pcm->private_data usually points to the chip instance assigned
statically at creation time of the PCM device, while
runtime->private_data
points to a dynamic data structure created in the PCM open
callback:
static int snd_xxx_open(struct snd_pcm_substream *substream)
{
struct my_pcm_data *data;
....
data = kmalloc(sizeof(*data), GFP_KERNEL);
substream->runtime->private_data = data;
....
}
The allocated object must be released in the close callback.
Operators¶
OK, now let me give details about each PCM callback (ops). In
general, every callback must return 0 if successful, or a negative
error number such as -EINVAL. To choose an appropriate error
number, it is advised to check what value other parts of the kernel
return when the same kind of request fails.
Each callback function takes at least one argument containing a struct snd_pcm_substream pointer. To retrieve the chip record from the given substream instance, you can use the following macro:
int xxx(...) {
struct mychip *chip = snd_pcm_substream_chip(substream);
....
}
The macro reads substream->private_data, which is a copy of
pcm->private_data. You can override the former if you need to
assign different data records per PCM substream. For example, the
cmi8330 driver assigns different private_data for playback and
capture directions, because it uses two different codecs (SB- and
AD-compatible) for different directions.
PCM open callback¶
static int snd_xxx_open(struct snd_pcm_substream *substream);
This is called when a PCM substream is opened.
At least, here you have to initialize the runtime->hw
record. Typically, this is done like this:
static int snd_xxx_open(struct snd_pcm_substream *substream)
{
struct mychip *chip = snd_pcm_substream_chip(substream);
struct snd_pcm_runtime *runtime = substream->runtime;
runtime->hw = snd_mychip_playback_hw;
return 0;
}
where snd_mychip_playback_hw is the pre-defined hardware
description.
You can allocate private data in this callback, as described in the Private Data section.
If the hardware configuration needs more constraints, set the hardware constraints here, too. See Constraints for more details.
close callback¶
static int snd_xxx_close(struct snd_pcm_substream *substream);
Obviously, this is called when a PCM substream is closed.
Any private instance for a PCM substream allocated in the open
callback will be released here:
static int snd_xxx_close(struct snd_pcm_substream *substream)
{
....
kfree(substream->runtime->private_data);
....
}
ioctl callback¶
This is used for any special call to PCM ioctls. But usually you can
leave it NULL, then the PCM core calls the generic ioctl callback
function snd_pcm_lib_ioctl(). If you need to deal with a
unique setup of channel info or reset procedure, you can pass your own
callback function here.
hw_params callback¶
static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
struct snd_pcm_hw_params *hw_params);
This is called when the hardware parameters (hw_params) are set up
by the application, that is, once when the buffer size, the period
size, the format, etc. are defined for the PCM substream.
Many hardware setups should be done in this callback, including the allocation of buffers.
Parameters to be initialized are retrieved by the
params_xxx() macros.
When you choose managed buffer allocation mode for the substream, a buffer is already allocated before this callback gets called. Alternatively, you can call a helper function below for allocating the buffer:
snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
snd_pcm_lib_malloc_pages() is available only when the
DMA buffers have been pre-allocated. See the section Buffer Types
for more details.
Note that this one and the prepare callback may be called multiple
times per initialization. For example, the OSS emulation may call these
callbacks at each change via its ioctl.
Thus, you need to be careful not to allocate the same buffers many times, which will lead to memory leaks! Calling the helper function above many times is OK. It will release the previous buffer automatically when it was already allocated.
Another note is that this callback is non-atomic (schedulable) by
default, i.e. when no nonatomic flag set. This is important,
because the trigger callback is atomic (non-schedulable). That is,
mutexes or any schedule-related functions are not available in the
trigger callback. Please see the subsection Atomicity for
details.
hw_free callback¶
static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
This is called to release the resources allocated via
hw_params.
This function is always called before the close callback is called. Also, the callback may be called multiple times, too. Keep track whether each resource was already released.
When you have chosen managed buffer allocation mode for the PCM
substream, the allocated PCM buffer will be automatically released
after this callback gets called. Otherwise you’ll have to release the
buffer manually. Typically, when the buffer was allocated from the
pre-allocated pool, you can use the standard API function
snd_pcm_lib_malloc_pages() like:
snd_pcm_lib_free_pages(substream);
prepare callback¶
static int snd_xxx_prepare(struct snd_pcm_substream *substream);
This callback is called when the PCM is “prepared”. You can set the
format type, sample rate, etc. here. The difference from hw_params
is that the prepare callback will be called each time
snd_pcm_prepare() is called, i.e. when recovering after
underruns, etc.
Note that this callback is non-atomic. You can use schedule-related functions safely in this callback.
In this and the following callbacks, you can refer to the values via
the runtime record, substream->runtime. For example, to get the
current rate, format or channels, access to runtime->rate,
runtime->format or runtime->channels, respectively. The
physical address of the allocated buffer is set to
runtime->dma_area. The buffer and period sizes are in
runtime->buffer_size and runtime->period_size, respectively.
Be careful that this callback will be called many times at each setup, too.
trigger callback¶
static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
This is called when the PCM is started, stopped or paused.
The action is specified in the second argument, SNDRV_PCM_TRIGGER_XXX
defined in <sound/pcm.h>. At least, the START
and STOP commands must be defined in this callback:
switch (cmd) {
case SNDRV_PCM_TRIGGER_START:
/* do something to start the PCM engine */
break;
case SNDRV_PCM_TRIGGER_STOP:
/* do something to stop the PCM engine */
break;
default:
return -EINVAL;
}
When the PCM supports the pause operation (given in the info field of
the hardware table), the PAUSE_PUSH and PAUSE_RELEASE commands
must be handled here, too. The former is the command to pause the PCM,
and the latter to restart the PCM again.
When the PCM supports the suspend/resume operation, regardless of full
or partial suspend/resume support, the SUSPEND and RESUME
commands must be handled, too. These commands are issued when the
power-management status is changed. Obviously, the SUSPEND and
RESUME commands suspend and resume the PCM substream, and usually,
they are identical to the STOP and START commands, respectively.
See the Power Management section for details.
As mentioned, this callback is atomic by default unless the nonatomic
flag set, and you cannot call functions which may sleep. The
trigger callback should be as minimal as possible, just really
triggering the DMA. The other stuff should be initialized in
hw_params and prepare callbacks properly beforehand.
sync_stop callback¶
static int snd_xxx_sync_stop(struct snd_pcm_substream *substream);
This callback is optional, and NULL can be passed. It’s called after
the PCM core stops the stream, before it changes the stream state via
prepare, hw_params or hw_free.
Since the IRQ handler might be still pending, we need to wait until
the pending task finishes before moving to the next step; otherwise it
might lead to a crash due to resource conflicts or access to freed
resources. A typical behavior is to call a synchronization function
like synchronize_irq() here.
For the majority of drivers that need only a call of
synchronize_irq(), there is a simpler setup, too.
While keeping the sync_stop PCM callback NULL, the driver can set
the card->sync_irq field to the returned interrupt number after
requesting an IRQ, instead. Then PCM core will call
synchronize_irq() with the given IRQ appropriately.
If the IRQ handler is released by the card destructor, you don’t need
to clear card->sync_irq, as the card itself is being released.
So, usually you’ll need to add just a single line for assigning
card->sync_irq in the driver code unless the driver re-acquires
the IRQ. When the driver frees and re-acquires the IRQ dynamically
(e.g. for suspend/resume), it needs to clear and re-set
card->sync_irq again appropriately.
pointer callback¶
static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
This callback is called when the PCM middle layer inquires the current
hardware position in the buffer. The position must be returned in
frames, ranging from 0 to buffer_size - 1.
This is usually called from the buffer-update routine in the PCM
middle layer, which is invoked when snd_pcm_period_elapsed()
is called by the interrupt routine. Then the PCM middle layer updates
the position and calculates the available space, and wakes up the
sleeping poll threads, etc.
This callback is also atomic by default.
copy and fill_silence ops¶
These callbacks are not mandatory, and can be omitted in most cases. These callbacks are used when the hardware buffer cannot be in the normal memory space. Some chips have their own buffer in the hardware which is not mappable. In such a case, you have to transfer the data manually from the memory buffer to the hardware buffer. Or, if the buffer is non-contiguous on both physical and virtual memory spaces, these callbacks must be defined, too.
If these two callbacks are defined, copy and set-silence operations are done by them. The details will be described in the later section Buffer and Memory Management.
ack callback¶
This callback is also not mandatory. This callback is called when the
appl_ptr is updated in read or write operations. Some drivers like
emu10k1-fx and cs46xx need to track the current appl_ptr for the
internal buffer, and this callback is useful only for such a purpose.
The callback function may return 0 or a negative error. When the
return value is -EPIPE, PCM core treats that as a buffer XRUN,
and changes the state to SNDRV_PCM_STATE_XRUN automatically.
This callback is atomic by default.
page callback¶
This callback is optional too. The mmap calls this callback to get the page fault address.
You need no special callback for the standard SG-buffer or vmalloc- buffer. Hence this callback should be rarely used.
mmap callback¶
This is another optional callback for controlling mmap behavior. When defined, the PCM core calls this callback when a page is memory-mapped, instead of using the standard helper. If you need special handling (due to some architecture or device-specific issues), implement everything here as you like.
PCM Interrupt Handler¶
The remainder of the PCM stuff is the PCM interrupt handler. The role
of the PCM
interrupt handler in the sound driver is to update the buffer position
and to tell the PCM middle layer when the buffer position goes across
the specified period boundary. To inform about this, call the
snd_pcm_period_elapsed() function.
There are several ways sound chips can generate interrupts.
Interrupts at the period (fragment) boundary¶
This is the most frequently found type: the hardware generates an
interrupt at each period boundary. In this case, you can call
snd_pcm_period_elapsed() at each interrupt.
snd_pcm_period_elapsed() takes the substream pointer as
its argument. Thus, you need to keep the substream pointer accessible
from the chip instance. For example, define substream field in the
chip record to hold the current running substream pointer, and set the
pointer value at open callback (and reset at close callback).
If you acquire a spinlock in the interrupt handler, and the lock is used
in other PCM callbacks, too, then you have to release the lock before
calling snd_pcm_period_elapsed(), because
snd_pcm_period_elapsed() calls other PCM callbacks
inside.
Typical code would look like:
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
{
struct mychip *chip = dev_id;
spin_lock(&chip->lock);
....
if (pcm_irq_invoked(chip)) {
/* call updater, unlock before it */
spin_unlock(&chip->lock);
snd_pcm_period_elapsed(chip->substream);
spin_lock(&chip->lock);
/* acknowledge the interrupt if necessary */
}
....
spin_unlock(&chip->lock);
return IRQ_HANDLED;
}
Also, when the device can detect a buffer underrun/overrun, the driver
can notify the XRUN status to the PCM core by calling
snd_pcm_stop_xrun(). This function stops the stream and sets
the PCM state to SNDRV_PCM_STATE_XRUN. Note that it must be called
outside the PCM stream lock, hence it can’t be called from the atomic
callback.
High frequency timer interrupts¶
This happens when the hardware doesn’t generate interrupts at the period
boundary but issues timer interrupts at a fixed timer rate (e.g. es1968
or ymfpci drivers). In this case, you need to check the current hardware
position and accumulate the processed sample length at each interrupt.
When the accumulated size exceeds the period size, call
snd_pcm_period_elapsed() and reset the accumulator.
Typical code would look as follows:
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
{
struct mychip *chip = dev_id;
spin_lock(&chip->lock);
....
if (pcm_irq_invoked(chip)) {
unsigned int last_ptr, size;
/* get the current hardware pointer (in frames) */
last_ptr = get_hw_ptr(chip);
/* calculate the processed frames since the
* last update
*/
if (last_ptr < chip->last_ptr)
size = runtime->buffer_size + last_ptr
- chip->last_ptr;
else
size = last_ptr - chip->last_ptr;
/* remember the last updated point */
chip->last_ptr = last_ptr;
/* accumulate the size */
chip->size += size;
/* over the period boundary? */
if (chip->size >= runtime->period_size) {
/* reset the accumulator */
chip->size %= runtime->period_size;
/* call updater */
spin_unlock(&chip->lock);
snd_pcm_period_elapsed(substream);
spin_lock(&chip->lock);
}
/* acknowledge the interrupt if necessary */
}
....
spin_unlock(&chip->lock);
return IRQ_HANDLED;
}
On calling snd_pcm_period_elapsed()¶
In both cases, even if more than one period has elapsed, you don’t have
to call snd_pcm_period_elapsed() many times. Call only
once. And the PCM layer will check the current hardware pointer and
update to the latest status.
Atomicity¶
One of the most important (and thus difficult to debug) problems in kernel programming are race conditions. In the Linux kernel, they are usually avoided via spin-locks, mutexes or semaphores. In general, if a race condition can happen in an interrupt handler, it has to be managed atomically, and you have to use a spinlock to protect the critical section. If the critical section is not in interrupt handler code and if taking a relatively long time to execute is acceptable, you should use mutexes or semaphores instead.
As already seen, some PCM callbacks are atomic and some are not. For
example, the hw_params callback is non-atomic, while the trigger
callback is atomic. This means, the latter is called already in a
spinlock held by the PCM middle layer, the PCM stream lock. Please
take this atomicity into account when you choose a locking scheme in
the callbacks.
In the atomic callbacks, you cannot use functions which may call
schedule() or go to sleep(). Semaphores and
mutexes can sleep, and hence they cannot be used inside the atomic
callbacks (e.g. trigger callback). To implement some delay in such a
callback, please use udelay() or mdelay().
All three atomic callbacks (trigger, pointer, and ack) are called with local interrupts disabled.
However, it is possible to request all PCM operations to be non-atomic.
This assumes that all call sites are in
non-atomic contexts. For example, the function
snd_pcm_period_elapsed() is called typically from the
interrupt handler. But, if you set up the driver to use a threaded
interrupt handler, this call can be in non-atomic context, too. In such
a case, you can set the nonatomic field of the struct snd_pcm object
after creating it. When this flag is set, mutex and rwsem are used internally
in the PCM core instead of spin and rwlocks, so that you can call all PCM
functions safely in a non-atomic
context.
Also, in some cases, you might need to call
snd_pcm_period_elapsed() in the atomic context (e.g. the
period gets elapsed during ack or other callback). There is a
variant that can be called inside the PCM stream lock
snd_pcm_period_elapsed_under_stream_lock() for that purpose,
too.
Constraints¶
Due to physical limitations, hardware is not infinitely configurable. These limitations are expressed by setting constraints.
For example, in order to restrict the sample rates to some supported
values, use snd_pcm_hw_constraint_list(). You need to
call this function in the open callback:
static unsigned int rates[] =
{4000, 10000, 22050, 44100};
static struct snd_pcm_hw_constraint_list constraints_rates = {
.count = ARRAY_SIZE(rates),
.list = rates,
.mask = 0,
};
static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
{
int err;
....
err = snd_pcm_hw_constraint_list(substream->runtime, 0,
SNDRV_PCM_HW_PARAM_RATE,
&constraints_rates);
if (err < 0)
return err;
....
}
There are many different constraints. Look at sound/pcm.h for a
complete list. You can even define your own constraint rules. For
example, let’s suppose my_chip can manage a substream of 1 channel if
and only if the format is S16_LE, otherwise it supports any format
specified in struct snd_pcm_hardware (or in any other
constraint_list). You can build a rule like this:
static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
struct snd_pcm_hw_rule *rule)
{
struct snd_interval *c = hw_param_interval(params,
SNDRV_PCM_HW_PARAM_CHANNELS);
struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
struct snd_interval ch;
snd_interval_any(&ch);
if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
ch.min = ch.max = 1;
ch.integer = 1;
return snd_interval_refine(c, &ch);
}
return 0;
}
Then you need to call this function to add your rule:
snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
hw_rule_channels_by_format, NULL,
SNDRV_PCM_HW_PARAM_FORMAT, -1);
The rule function is called when an application sets the PCM format, and it refines the number of channels accordingly. But an application may set the number of channels before setting the format. Thus you also need to define the inverse rule:
static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
struct snd_pcm_hw_rule *rule)
{
struct snd_interval *c = hw_param_interval(params,
SNDRV_PCM_HW_PARAM_CHANNELS);
struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
struct snd_mask fmt;
snd_mask_any(&fmt); /* Init the struct */
if (c->min < 2) {
fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
return snd_mask_refine(f, &fmt);
}
return 0;
}
... and in the open callback:
snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
hw_rule_format_by_channels, NULL,
SNDRV_PCM_HW_PARAM_CHANNELS, -1);
One typical usage of the hw constraints is to align the buffer size with the period size. By default, ALSA PCM core doesn’t enforce the buffer size to be aligned with the period size. For example, it’d be possible to have a combination like 256 period bytes with 999 buffer bytes.
Many device chips, however, require the buffer to be a multiple of
periods. In such a case, call
snd_pcm_hw_constraint_integer() for
SNDRV_PCM_HW_PARAM_PERIODS:
snd_pcm_hw_constraint_integer(substream->runtime,
SNDRV_PCM_HW_PARAM_PERIODS);
This assures that the number of periods is integer, hence the buffer size is aligned with the period size.
The hw constraint is a very powerful mechanism to define the preferred PCM configuration, and there are relevant helpers. I won’t give more details here, rather I would like to say, “Luke, use the source.”
Control Interface¶
General¶
The control interface is used widely for many switches, sliders, etc. which are accessed from user-space. Its most important use is the mixer interface. In other words, since ALSA 0.9.x, all the mixer stuff is implemented on the control kernel API.
ALSA has a well-defined AC97 control module. If your chip supports only the AC97 and nothing else, you can skip this section.
The control API is defined in <sound/control.h>. Include this file
if you want to add your own controls.
Definition of Controls¶
To create a new control, you need to define the following three
callbacks: info, get and put. Then, define a
struct snd_kcontrol_new record, such as:
static struct snd_kcontrol_new my_control = {
.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
.name = "PCM Playback Switch",
.index = 0,
.access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
.private_value = 0xffff,
.info = my_control_info,
.get = my_control_get,
.put = my_control_put
};
The iface field specifies the control type,
SNDRV_CTL_ELEM_IFACE_XXX, which is usually MIXER. Use CARD
for global controls that are not logically part of the mixer. If the
control is closely associated with some specific device on the sound
card, use HWDEP, PCM, RAWMIDI, TIMER, or SEQUENCER,
and specify the device number with the device and subdevice
fields.
The name is the name identifier string. Since ALSA 0.9.x, the
control name is very important, because its role is classified from
its name. There are pre-defined standard control names. The details
are described in the Control Names subsection.
The index field holds the index number of this control. If there
are several different controls with the same name, they can be
distinguished by the index number. This is the case when several
codecs exist on the card. If the index is zero, you can omit the
definition above.
The access field contains the access type of this control. Give
the combination of bit masks, SNDRV_CTL_ELEM_ACCESS_XXX,
there. The details will be explained in the Access Flags
subsection.
The private_value field contains an arbitrary long integer value
for this record. When using the generic info, get and put
callbacks, you can pass a value through this field. If several small
numbers are necessary, you can combine them in bitwise. Or, it’s
possible to store a pointer (casted to unsigned long) of some record in
this field, too.
The tlv field can be used to provide metadata about the control;
see the Metadata subsection.
The other three are Control Callbacks.
Control Names¶
There are some standards to define the control names. A control is usually defined from the three parts as “SOURCE DIRECTION FUNCTION”.
The first, SOURCE, specifies the source of the control, and is a
string such as “Master”, “PCM”, “CD” and “Line”. There are many
pre-defined sources.
The second, DIRECTION, is one of the following strings according to
the direction of the control: “Playback”, “Capture”, “Bypass Playback”
and “Bypass Capture”. Or, it can be omitted, meaning both playback and
capture directions.
The third, FUNCTION, is one of the following strings according to
the function of the control: “Switch”, “Volume” and “Route”.
The example of control names are, thus, “Master Capture Switch” or “PCM Playback Volume”.
There are some exceptions:
Global capture and playback¶
“Capture Source”, “Capture Switch” and “Capture Volume” are used for the global capture (input) source, switch and volume. Similarly, “Playback Switch” and “Playback Volume” are used for the global output gain switch and volume.
Tone-controls¶
tone-control switch and volumes are specified like “Tone Control - XXX”, e.g. “Tone Control - Switch”, “Tone Control - Bass”, “Tone Control - Center”.
3D controls¶
3D-control switches and volumes are specified like “3D Control - XXX”, e.g. “3D Control - Switch”, “3D Control - Center”, “3D Control - Space”.
Mic boost¶
Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”.
More precise information can be found in
Documentation/sound/designs/control-names.rst.
Access Flags¶
The access flag is the bitmask which specifies the access type of the
given control. The default access type is
SNDRV_CTL_ELEM_ACCESS_READWRITE, which means both read and write are
allowed to this control. When the access flag is omitted (i.e. = 0), it
is considered as READWRITE access by default.
When the control is read-only, pass SNDRV_CTL_ELEM_ACCESS_READ
instead. In this case, you don’t have to define the put callback.
Similarly, when the control is write-only (although it’s a rare case),
you can use the WRITE flag instead, and you don’t need the get
callback.
If the control value changes frequently (e.g. the VU meter),
VOLATILE flag should be given. This means that the control may be
changed without Change notification. Applications should poll such
a control constantly.
When the control may be updated, but currently has no effect on anything,
setting the INACTIVE flag may be appropriate. For example, PCM
controls should be inactive while no PCM device is open.
There are LOCK and OWNER flags to change the write permissions.
Control Callbacks¶
info callback¶
The info callback is used to get detailed information on this
control. This must store the values of the given
struct snd_ctl_elem_info object. For example,
for a boolean control with a single element:
static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_info *uinfo)
{
uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
uinfo->count = 1;
uinfo->value.integer.min = 0;
uinfo->value.integer.max = 1;
return 0;
}
The type field specifies the type of the control. There are
BOOLEAN, INTEGER, ENUMERATED, BYTES, IEC958 and
INTEGER64. The count field specifies the number of elements in
this control. For example, a stereo volume would have count = 2. The
value field is a union, and the values stored depend on the
type. The boolean and integer types are identical.
The enumerated type is a bit different from the others. You’ll need to set the string for the selectec item index:
static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_info *uinfo)
{
static char *texts[4] = {
"First", "Second", "Third", "Fourth"
};
uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
uinfo->count = 1;
uinfo->value.enumerated.items = 4;
if (uinfo->value.enumerated.item > 3)
uinfo->value.enumerated.item = 3;
strcpy(uinfo->value.enumerated.name,
texts[uinfo->value.enumerated.item]);
return 0;
}
The above callback can be simplified with a helper function,
snd_ctl_enum_info(). The final code looks like below.
(You can pass ARRAY_SIZE(texts) instead of 4 in the third argument;
it’s a matter of taste.)
static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_info *uinfo)
{
static char *texts[4] = {
"First", "Second", "Third", "Fourth"
};
return snd_ctl_enum_info(uinfo, 1, 4, texts);
}
Some common info callbacks are available for your convenience:
snd_ctl_boolean_mono_info() and
snd_ctl_boolean_stereo_info(). Obviously, the former
is an info callback for a mono channel boolean item, just like
snd_myctl_mono_info() above, and the latter is for a
stereo channel boolean item.
get callback¶
This callback is used to read the current value of the control, so it can be returned to user-space.
For example:
static int snd_myctl_get(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *ucontrol)
{
struct mychip *chip = snd_kcontrol_chip(kcontrol);
ucontrol->value.integer.value[0] = get_some_value(chip);
return 0;
}
The value field depends on the type of control as well as on the
info callback. For example, the sb driver uses this field to store the
register offset, the bit-shift and the bit-mask. The private_value
field is set as follows:
.private_value = reg | (shift << 16) | (mask << 24)
and is retrieved in callbacks like:
static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *ucontrol)
{
int reg = kcontrol->private_value & 0xff;
int shift = (kcontrol->private_value >> 16) & 0xff;
int mask = (kcontrol->private_value >> 24) & 0xff;
....
}
In the get callback, you have to fill all the elements if the
control has more than one element, i.e. count > 1. In the example
above, we filled only one element (value.integer.value[0]) since
count = 1 is assumed.
put callback¶
This callback is used to write a value coming from user-space.
For example:
static int snd_myctl_put(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *ucontrol)
{
struct mychip *chip = snd_kcontrol_chip(kcontrol);
int changed = 0;
if (chip->current_value !=
ucontrol->value.integer.value[0]) {
change_current_value(chip,
ucontrol->value.integer.value[0]);
changed = 1;
}
return changed;
}
As seen above, you have to return 1 if the value is changed. If the value is not changed, return 0 instead. If any fatal error happens, return a negative error code as usual.
As in the get callback, when the control has more than one
element, all elements must be evaluated in this callback, too.
Callbacks are not atomic¶
All these three callbacks are not-atomic.
Control Constructor¶
When everything is ready, finally we can create a new control. To create
a control, there are two functions to be called,
snd_ctl_new1() and snd_ctl_add().
In the simplest way, you can do it like this:
err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
if (err < 0)
return err;
where my_control is the struct snd_kcontrol_new object defined above,
and chip is the object pointer to be passed to kcontrol->private_data which
can be referred to in callbacks.
snd_ctl_new1() allocates a new struct snd_kcontrol instance, and
snd_ctl_add() assigns the given control component to the
card.
Change Notification¶
If you need to change and update a control in the interrupt routine, you
can call snd_ctl_notify(). For example:
snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
This function takes the card pointer, the event-mask, and the control id
pointer for the notification. The event-mask specifies the types of
notification, for example, in the above example, the change of control
values is notified. The id pointer is the pointer of struct snd_ctl_elem_id
to be notified. You can find some examples in es1938.c or es1968.c
for hardware volume interrupts.
Metadata¶
To provide information about the dB values of a mixer control, use one of
the DECLARE_TLV_xxx macros from <sound/tlv.h> to define a
variable containing this information, set the tlv.p field to point to
this variable, and include the SNDRV_CTL_ELEM_ACCESS_TLV_READ flag
in the access field; like this:
static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
static struct snd_kcontrol_new my_control = {
...
.access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
SNDRV_CTL_ELEM_ACCESS_TLV_READ,
...
.tlv.p = db_scale_my_control,
};
The DECLARE_TLV_DB_SCALE() macro defines information
about a mixer control where each step in the control’s value changes the
dB value by a constant dB amount. The first parameter is the name of the
variable to be defined. The second parameter is the minimum value, in
units of 0.01 dB. The third parameter is the step size, in units of 0.01
dB. Set the fourth parameter to 1 if the minimum value actually mutes
the control.
The DECLARE_TLV_DB_LINEAR() macro defines information
about a mixer control where the control’s value affects the output
linearly. The first parameter is the name of the variable to be defined.
The second parameter is the minimum value, in units of 0.01 dB. The
third parameter is the maximum value, in units of 0.01 dB. If the
minimum value mutes the control, set the second parameter to
TLV_DB_GAIN_MUTE.
API for AC97 Codec¶
General¶
The ALSA AC97 codec layer is a well-defined one, and you don’t have to
write much code to control it. Only low-level control routines are
necessary. The AC97 codec API is defined in <sound/ac97_codec.h>.
Full Code Example¶
struct mychip {
....
struct snd_ac97 *ac97;
....
};
static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
unsigned short reg)
{
struct mychip *chip = ac97->private_data;
....
/* read a register value here from the codec */
return the_register_value;
}
static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
unsigned short reg, unsigned short val)
{
struct mychip *chip = ac97->private_data;
....
/* write the given register value to the codec */
}
static int snd_mychip_ac97(struct mychip *chip)
{
struct snd_ac97_bus *bus;
struct snd_ac97_template ac97;
int err;
static struct snd_ac97_bus_ops ops = {
.write = snd_mychip_ac97_write,
.read = snd_mychip_ac97_read,
};
err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
if (err < 0)
return err;
memset(&ac97, 0, sizeof(ac97));
ac97.private_data = chip;
return snd_ac97_mixer(bus, &ac97, &chip->ac97);
}
AC97 Constructor¶
To create an ac97 instance, first call snd_ac97_bus()
with an ac97_bus_ops_t record with callback functions:
struct snd_ac97_bus *bus;
static struct snd_ac97_bus_ops ops = {
.write = snd_mychip_ac97_write,
.read = snd_mychip_ac97_read,
};
snd_ac97_bus(card, 0, &ops, NULL, &pbus);
The bus record is shared among all belonging ac97 instances.
And then call snd_ac97_mixer() with a struct snd_ac97_template
record together with the bus pointer created above:
struct snd_ac97_template ac97;
int err;
memset(&ac97, 0, sizeof(ac97));
ac97.private_data = chip;
snd_ac97_mixer(bus, &ac97, &chip->ac97);
where chip->ac97 is a pointer to a newly created ac97_t
instance. In this case, the chip pointer is set as the private data,
so that the read/write callback functions can refer to this chip
instance. This instance is not necessarily stored in the chip
record. If you need to change the register values from the driver, or
need the suspend/resume of ac97 codecs, keep this pointer to pass to
the corresponding functions.
AC97 Callbacks¶
The standard callbacks are read and write. Obviously they
correspond to the functions for read and write accesses to the
hardware low-level codes.
The read callback returns the register value specified in the
argument:
static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
unsigned short reg)
{
struct mychip *chip = ac97->private_data;
....
return the_register_value;
}
Here, the chip can be cast from ac97->private_data.
Meanwhile, the write callback is used to set the register
value:
static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
unsigned short reg, unsigned short val)
These callbacks are non-atomic like the control API callbacks.
There are also other callbacks: reset, wait and init.
The reset callback is used to reset the codec. If the chip
requires a special kind of reset, you can define this callback.
The wait callback is used to add some waiting time in the standard
initialization of the codec. If the chip requires the extra waiting
time, define this callback.
The init callback is used for additional initialization of the
codec.
Updating Registers in The Driver¶
If you need to access to the codec from the driver, you can call the
following functions: snd_ac97_write(),
snd_ac97_read(), snd_ac97_update() and
snd_ac97_update_bits().
Both snd_ac97_write() and
snd_ac97_update() functions are used to set a value to
the given register (AC97_XXX). The difference between them is that
snd_ac97_update() doesn’t write a value if the given
value has been already set, while snd_ac97_write()
always rewrites the value:
snd_ac97_write(ac97, AC97_MASTER, 0x8080);
snd_ac97_update(ac97, AC97_MASTER, 0x8080);
snd_ac97_read() is used to read the value of the given
register. For example:
value = snd_ac97_read(ac97, AC97_MASTER);
snd_ac97_update_bits() is used to update some bits in
the given register:
snd_ac97_update_bits(ac97, reg, mask, value);
Also, there is a function to change the sample rate (of a given register
such as AC97_PCM_FRONT_DAC_RATE) when VRA or DRA is supported by the
codec: snd_ac97_set_rate():
snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
The following registers are available to set the rate:
AC97_PCM_MIC_ADC_RATE, AC97_PCM_FRONT_DAC_RATE,
AC97_PCM_LR_ADC_RATE, AC97_SPDIF. When AC97_SPDIF is
specified, the register is not really changed but the corresponding
IEC958 status bits will be updated.
Clock Adjustment¶
In some chips, the clock of the codec isn’t 48000 but using a PCI clock
(to save a quartz!). In this case, change the field bus->clock to
the corresponding value. For example, intel8x0 and es1968 drivers have
their own function to read from the clock.
Proc Files¶
The ALSA AC97 interface will create a proc file such as
/proc/asound/card0/codec97#0/ac97#0-0 and ac97#0-0+regs. You
can refer to these files to see the current status and registers of
the codec.
Multiple Codecs¶
When there are several codecs on the same card, you need to call
snd_ac97_mixer() multiple times with ac97.num=1 or
greater. The num field specifies the codec number.
If you set up multiple codecs, you either need to write different
callbacks for each codec or check ac97->num in the callback
routines.
MIDI (MPU401-UART) Interface¶
General¶
Many soundcards have built-in MIDI (MPU401-UART) interfaces. When the
soundcard supports the standard MPU401-UART interface, most likely you
can use the ALSA MPU401-UART API. The MPU401-UART API is defined in
<sound/mpu401.h>.
Some soundchips have a similar but slightly different implementation of mpu401 stuff. For example, emu10k1 has its own mpu401 routines.
MIDI Constructor¶
To create a rawmidi object, call snd_mpu401_uart_new():
struct snd_rawmidi *rmidi;
snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
irq, &rmidi);
The first argument is the card pointer, and the second is the index of this component. You can create up to 8 rawmidi devices.
The third argument is the type of the hardware, MPU401_HW_XXX. If
it’s not a special one, you can use MPU401_HW_MPU401.
The 4th argument is the I/O port address. Many backward-compatible MPU401 have an I/O port such as 0x330. Or, it might be a part of its own PCI I/O region. It depends on the chip design.
The 5th argument is a bitflag for additional information. When the I/O
port address above is part of the PCI I/O region, the MPU401 I/O port
might have been already allocated (reserved) by the driver itself. In
such a case, pass a bit flag MPU401_INFO_INTEGRATED, and the
mpu401-uart layer will allocate the I/O ports by itself.
When the controller supports only the input or output MIDI stream, pass
the MPU401_INFO_INPUT or MPU401_INFO_OUTPUT bitflag,
respectively. Then the rawmidi instance is created as a single stream.
MPU401_INFO_MMIO bitflag is used to change the access method to MMIO
(via readb and writeb) instead of iob and outb. In this case, you have
to pass the iomapped address to snd_mpu401_uart_new().
When MPU401_INFO_TX_IRQ is set, the output stream isn’t checked in
the default interrupt handler. The driver needs to call
snd_mpu401_uart_interrupt_tx() by itself to start
processing the output stream in the irq handler.
If the MPU-401 interface shares its interrupt with the other logical
devices on the card, set MPU401_INFO_IRQ_HOOK (see
below).
Usually, the port address corresponds to the command port and port + 1
corresponds to the data port. If not, you may change the cport
field of struct snd_mpu401 manually afterward.
However, struct snd_mpu401 pointer is
not returned explicitly by snd_mpu401_uart_new(). You
need to cast rmidi->private_data to struct snd_mpu401 explicitly:
struct snd_mpu401 *mpu;
mpu = rmidi->private_data;
and reset the cport as you like:
mpu->cport = my_own_control_port;
The 6th argument specifies the ISA irq number that will be allocated. If no interrupt is to be allocated (because your code is already allocating a shared interrupt, or because the device does not use interrupts), pass -1 instead. For a MPU-401 device without an interrupt, a polling timer will be used instead.
MIDI Interrupt Handler¶
When the interrupt is allocated in
snd_mpu401_uart_new(), an exclusive ISA interrupt
handler is automatically used, hence you don’t have anything else to do
than creating the mpu401 stuff. Otherwise, you have to set
MPU401_INFO_IRQ_HOOK, and call
snd_mpu401_uart_interrupt() explicitly from your own
interrupt handler when it has determined that a UART interrupt has
occurred.
In this case, you need to pass the private_data of the returned rawmidi
object from snd_mpu401_uart_new() as the second
argument of snd_mpu401_uart_interrupt():
snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
RawMIDI Interface¶
Overview¶
The raw MIDI interface is used for hardware MIDI ports that can be accessed as a byte stream. It is not used for synthesizer chips that do not directly understand MIDI.
ALSA handles file and buffer management. All you have to do is to write some code to move data between the buffer and the hardware.
The rawmidi API is defined in <sound/rawmidi.h>.
RawMIDI Constructor¶
To create a rawmidi device, call the snd_rawmidi_new()
function:
struct snd_rawmidi *rmidi;
err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
if (err < 0)
return err;
rmidi->private_data = chip;
strcpy(rmidi->name, "My MIDI");
rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
SNDRV_RAWMIDI_INFO_INPUT |
SNDRV_RAWMIDI_INFO_DUPLEX;
The first argument is the card pointer, the second argument is the ID string.
The third argument is the index of this component. You can create up to 8 rawmidi devices.
The fourth and fifth arguments are the number of output and input substreams, respectively, of this device (a substream is the equivalent of a MIDI port).
Set the info_flags field to specify the capabilities of the
device. Set SNDRV_RAWMIDI_INFO_OUTPUT if there is at least one
output port, SNDRV_RAWMIDI_INFO_INPUT if there is at least one
input port, and SNDRV_RAWMIDI_INFO_DUPLEX if the device can handle
output and input at the same time.
After the rawmidi device is created, you need to set the operators (callbacks) for each substream. There are helper functions to set the operators for all the substreams of a device:
snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
The operators are usually defined like this:
static struct snd_rawmidi_ops snd_mymidi_output_ops = {
.open = snd_mymidi_output_open,
.close = snd_mymidi_output_close,
.trigger = snd_mymidi_output_trigger,
};
These callbacks are explained in the RawMIDI Callbacks section.
If there are more than one substream, you should give a unique name to each of them:
struct snd_rawmidi_substream *substream;
list_for_each_entry(substream,
&rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
list {
sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
}
/* same for SNDRV_RAWMIDI_STREAM_INPUT */
RawMIDI Callbacks¶
In all the callbacks, the private data that you’ve set for the rawmidi
device can be accessed as substream->rmidi->private_data.
If there is more than one port, your callbacks can determine the port index from the struct snd_rawmidi_substream data passed to each callback:
struct snd_rawmidi_substream *substream;
int index = substream->number;
RawMIDI open callback¶
static int snd_xxx_open(struct snd_rawmidi_substream *substream);
This is called when a substream is opened. You can initialize the hardware here, but you shouldn’t start transmitting/receiving data yet.
RawMIDI close callback¶
static int snd_xxx_close(struct snd_rawmidi_substream *substream);
Guess what.
The open and close callbacks of a rawmidi device are
serialized with a mutex, and can sleep.
Rawmidi trigger callback for output substreams¶
static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
This is called with a nonzero up parameter when there is some data
in the substream buffer that must be transmitted.
To read data from the buffer, call
snd_rawmidi_transmit_peek(). It will return the number
of bytes that have been read; this will be less than the number of bytes
requested when there are no more data in the buffer. After the data have
been transmitted successfully, call
snd_rawmidi_transmit_ack() to remove the data from the
substream buffer:
unsigned char data;
while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
if (snd_mychip_try_to_transmit(data))
snd_rawmidi_transmit_ack(substream, 1);
else
break; /* hardware FIFO full */
}
If you know beforehand that the hardware will accept data, you can use
the snd_rawmidi_transmit() function which reads some
data and removes them from the buffer at once:
while (snd_mychip_transmit_possible()) {
unsigned char data;
if (snd_rawmidi_transmit(substream, &data, 1) != 1)
break; /* no more data */
snd_mychip_transmit(data);
}
If you know beforehand how many bytes you can accept, you can use a
buffer size greater than one with the snd_rawmidi_transmit*() functions.
The trigger callback must not sleep. If the hardware FIFO is full
before the substream buffer has been emptied, you have to continue
transmitting data later, either in an interrupt handler, or with a
timer if the hardware doesn’t have a MIDI transmit interrupt.
The trigger callback is called with a zero up parameter when
the transmission of data should be aborted.
RawMIDI trigger callback for input substreams¶
static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
This is called with a nonzero up parameter to enable receiving data,
or with a zero up parameter do disable receiving data.
The trigger callback must not sleep; the actual reading of data
from the device is usually done in an interrupt handler.
When data reception is enabled, your interrupt handler should call
snd_rawmidi_receive() for all received data:
void snd_mychip_midi_interrupt(...)
{
while (mychip_midi_available()) {
unsigned char data;
data = mychip_midi_read();
snd_rawmidi_receive(substream, &data, 1);
}
}
drain callback¶
static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
This is only used with output substreams. This function should wait until all data read from the substream buffer have been transmitted. This ensures that the device can be closed and the driver unloaded without losing data.
This callback is optional. If you do not set drain in the struct
snd_rawmidi_ops structure, ALSA will simply wait for 50 milliseconds
instead.
Miscellaneous Devices¶
FM OPL3¶
The FM OPL3 is still used in many chips (mainly for backward
compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API
is defined in <sound/opl3.h>.
FM registers can be directly accessed through the direct-FM API, defined
in <sound/asound_fm.h>. In ALSA native mode, FM registers are
accessed through the Hardware-Dependent Device direct-FM extension API,
whereas in OSS compatible mode, FM registers can be accessed with the
OSS direct-FM compatible API in /dev/dmfmX device.
To create the OPL3 component, you have two functions to call. The first
one is a constructor for the opl3_t instance:
struct snd_opl3 *opl3;
snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
integrated, &opl3);
The first argument is the card pointer, the second one is the left port address, and the third is the right port address. In most cases, the right port is placed at the left port + 2.
The fourth argument is the hardware type.
When the left and right ports have been already allocated by the card
driver, pass non-zero to the fifth argument (integrated). Otherwise,
the opl3 module will allocate the specified ports by itself.
When the accessing the hardware requires special method instead of the
standard I/O access, you can create opl3 instance separately with
snd_opl3_new():
struct snd_opl3 *opl3;
snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
Then set command, private_data and private_free for the
private access function, the private data and the destructor. The
l_port and r_port are not necessarily set. Only the command
must be set properly. You can retrieve the data from the
opl3->private_data field.
After creating the opl3 instance via snd_opl3_new(),
call snd_opl3_init() to initialize the chip to the
proper state. Note that snd_opl3_create() always calls
it internally.
If the opl3 instance is created successfully, then create a hwdep device for this opl3:
struct snd_hwdep *opl3hwdep;
snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
The first argument is the opl3_t instance you created, and the
second is the index number, usually 0.
The third argument is the index-offset for the sequencer client assigned to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART always takes 0).
Hardware-Dependent Devices¶
Some chips need user-space access for special controls or for loading
the micro code. In such a case, you can create a hwdep
(hardware-dependent) device. The hwdep API is defined in
<sound/hwdep.h>. You can find examples in opl3 driver or
isa/sb/sb16_csp.c.
The creation of the hwdep instance is done via
snd_hwdep_new():
struct snd_hwdep *hw;
snd_hwdep_new(card, "My HWDEP", 0, &hw);
where the third argument is the index number.
You can then pass any pointer value to the private_data. If you
assign private data, you should define a destructor, too. The
destructor function is set in the private_free field:
struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
hw->private_data = p;
hw->private_free = mydata_free;
and the implementation of the destructor would be:
static void mydata_free(struct snd_hwdep *hw)
{
struct mydata *p = hw->private_data;
kfree(p);
}
The arbitrary file operations can be defined for this instance. The file
operators are defined in the ops table. For example, assume that
this chip needs an ioctl:
hw->ops.open = mydata_open;
hw->ops.ioctl = mydata_ioctl;
hw->ops.release = mydata_release;
And implement the callback functions as you like.
IEC958 (S/PDIF)¶
Usually the controls for IEC958 devices are implemented via the control
interface. There is a macro to compose a name string for IEC958
controls, SNDRV_CTL_NAME_IEC958() defined in
<include/asound.h>.
There are some standard controls for IEC958 status bits. These controls
use the type SNDRV_CTL_ELEM_TYPE_IEC958, and the size of element is
fixed as 4 bytes array (value.iec958.status[x]). For the info
callback, you don’t specify the value field for this type (the count
field must be set, though).
“IEC958 Playback Con Mask” is used to return the bit-mask for the IEC958 status bits of consumer mode. Similarly, “IEC958 Playback Pro Mask” returns the bitmask for professional mode. They are read-only controls.
Meanwhile, “IEC958 Playback Default” control is defined for getting and setting the current default IEC958 bits.
Due to historical reasons, both variants of the Playback Mask and the
Playback Default controls can be implemented on either a
SNDRV_CTL_ELEM_IFACE_PCM or a SNDRV_CTL_ELEM_IFACE_MIXER iface.
Drivers should expose the mask and default on the same iface though.
In addition, you can define the control switches to enable/disable or to
set the raw bit mode. The implementation will depend on the chip, but
the control should be named as “IEC958 xxx”, preferably using the
SNDRV_CTL_NAME_IEC958() macro.
You can find several cases, for example, pci/emu10k1,
pci/ice1712, or pci/cmipci.c.
Buffer and Memory Management¶
Buffer Types¶
ALSA provides several different buffer allocation functions depending on
the bus and the architecture. All these have a consistent API. The
allocation of physically-contiguous pages is done via the
snd_malloc_xxx_pages() function, where xxx is the bus
type.
The allocation of pages with fallback is done via
snd_dma_alloc_pages_fallback(). This function tries
to allocate the specified number of pages, but if not enough pages are
available, it tries to reduce the request size until enough space
is found, down to one page.
To release the pages, call the snd_dma_free_pages()
function.
Usually, ALSA drivers try to allocate and reserve a large contiguous physical space at the time the module is loaded for later use. This is called “pre-allocation”. As already written, you can call the following function at PCM instance construction time (in the case of PCI bus):
snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
&pci->dev, size, max);
where size is the byte size to be pre-allocated and max is
the maximum size settable via the prealloc proc file. The
allocator will try to get an area as large as possible within the
given size.
The second argument (type) and the third argument (device pointer) are
dependent on the bus. For normal devices, pass the device pointer
(typically identical as card->dev) to the third argument with
SNDRV_DMA_TYPE_DEV type.
A continuous buffer unrelated to the
bus can be pre-allocated with SNDRV_DMA_TYPE_CONTINUOUS type.
You can pass NULL to the device pointer in that case, which is the
default mode implying to allocate with the GFP_KERNEL flag.
If you need a restricted (lower) address, set up the coherent DMA mask
bits for the device, and pass the device pointer, like the normal
device memory allocations. For this type, it’s still allowed to pass
NULL to the device pointer, too, if no address restriction is needed.
For the scatter-gather buffers, use SNDRV_DMA_TYPE_DEV_SG with the
device pointer (see the Non-Contiguous Buffers section).
Once the buffer is pre-allocated, you can use the allocator in the
hw_params callback:
snd_pcm_lib_malloc_pages(substream, size);
Note that you have to pre-allocate to use this function.
But most drivers use the “managed buffer allocation mode” instead
of manual allocation and release.
This is done by calling snd_pcm_set_managed_buffer_all()
instead of snd_pcm_lib_preallocate_pages_for_all():
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
&pci->dev, size, max);
where the passed arguments are identical for both functions.
The difference in the managed mode is that PCM core will call
snd_pcm_lib_malloc_pages() internally already before calling
the PCM hw_params callback, and call snd_pcm_lib_free_pages()
after the PCM hw_free callback automatically. So the driver
doesn’t have to call these functions explicitly in its callback any
longer. This allows many drivers to have NULL hw_params and
hw_free entries.
External Hardware Buffers¶
Some chips have their own hardware buffers and DMA transfer from the host memory is not available. In such a case, you need to either 1) copy/set the audio data directly to the external hardware buffer, or 2) make an intermediate buffer and copy/set the data from it to the external hardware buffer in interrupts (or in tasklets, preferably).
The first case works fine if the external hardware buffer is large
enough. This method doesn’t need any extra buffers and thus is more
efficient. You need to define the copy callback
for the data transfer, in addition to the fill_silence
callback for playback. However, there is a drawback: it cannot be
mmapped. The examples are GUS’s GF1 PCM or emu8000’s wavetable PCM.
The second case allows for mmap on the buffer, although you have to handle an interrupt or a tasklet to transfer the data from the intermediate buffer to the hardware buffer. You can find an example in the vxpocket driver.
Another case is when the chip uses a PCI memory-map region for the
buffer instead of the host memory. In this case, mmap is available only
on certain architectures like the Intel one. In non-mmap mode, the data
cannot be transferred as in the normal way. Thus you need to define the
copy and fill_silence callbacks as well,
as in the cases above. Examples are found in rme32.c and
rme96.c.
The implementation of the copy and
silence callbacks depends upon whether the hardware supports
interleaved or non-interleaved samples. The copy callback is
defined like below, a bit differently depending on whether the direction
is playback or capture:
static int playback_copy(struct snd_pcm_substream *substream,
int channel, unsigned long pos,
struct iov_iter *src, unsigned long count);
static int capture_copy(struct snd_pcm_substream *substream,
int channel, unsigned long pos,
struct iov_iter *dst, unsigned long count);
In the case of interleaved samples, the second argument (channel) is
not used. The third argument (pos) specifies the position in bytes.
The meaning of the fourth argument is different between playback and capture. For playback, it holds the source data pointer, and for capture, it’s the destination data pointer.
The last argument is the number of bytes to be copied.
What you have to do in this callback is again different between playback
and capture directions. In the playback case, you copy the given amount
of data (count) at the specified pointer (src) to the specified
offset (pos) in the hardware buffer. When coded like memcpy-like
way, the copy would look like:
my_memcpy_from_iter(my_buffer + pos, src, count);
For the capture direction, you copy the given amount of data (count)
at the specified offset (pos) in the hardware buffer to the
specified pointer (dst):
my_memcpy_to_iter(dst, my_buffer + pos, count);
The given src or dst a struct iov_iter pointer containing the
pointer and the size. Use the existing helpers to copy or access the
data as defined in linux/uio.h.
Careful readers might notice that these callbacks receive the arguments in bytes, not in frames like other callbacks. It’s because this makes coding easier like in the examples above, and also it makes it easier to unify both the interleaved and non-interleaved cases, as explained below.
In the case of non-interleaved samples, the implementation will be a bit more complicated. The callback is called for each channel, passed in the second argument, so in total it’s called N times per transfer.
The meaning of the other arguments are almost the same as in the
interleaved case. The callback is supposed to copy the data from/to
the given user-space buffer, but only for the given channel. For
details, please check isa/gus/gus_pcm.c or pci/rme9652/rme9652.c
as examples.
Usually for the playback, another callback fill_silence is
defined. It’s implemented in a similar way as the copy callbacks
above:
static int silence(struct snd_pcm_substream *substream, int channel,
unsigned long pos, unsigned long count);
The meanings of arguments are the same as in the copy callback,
although there is no buffer pointer
argument. In the case of interleaved samples, the channel argument has
no meaning, as for the copy callback.
The role of the fill_silence callback is to set the given amount
(count) of silence data at the specified offset (pos) in the
hardware buffer. Suppose that the data format is signed (that is, the
silent-data is 0), and the implementation using a memset-like function
would look like:
my_memset(my_buffer + pos, 0, count);
In the case of non-interleaved samples, again, the implementation
becomes a bit more complicated, as it’s called N times per transfer
for each channel. See, for example, isa/gus/gus_pcm.c.
Non-Contiguous Buffers¶
If your hardware supports a page table as in emu10k1 or buffer
descriptors as in via82xx, you can use scatter-gather (SG) DMA. ALSA
provides an interface for handling SG-buffers. The API is provided in
<sound/pcm.h>.
For creating the SG-buffer handler, call
snd_pcm_set_managed_buffer() or
snd_pcm_set_managed_buffer_all() with
SNDRV_DMA_TYPE_DEV_SG in the PCM constructor like for other PCI
pre-allocations. You need to pass &pci->dev, where pci is
the struct pci_dev pointer of the chip as well:
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV_SG,
&pci->dev, size, max);
The struct snd_sg_buf instance is created as
substream->dma_private in turn. You can cast the pointer like:
struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
Then in the snd_pcm_lib_malloc_pages() call, the common SG-buffer
handler will allocate the non-contiguous kernel pages of the given size
and map them as virtually contiguous memory. The virtual pointer
is addressed via runtime->dma_area. The physical address
(runtime->dma_addr) is set to zero, because the buffer is
physically non-contiguous. The physical address table is set up in
sgbuf->table. You can get the physical address at a certain offset
via snd_pcm_sgbuf_get_addr().
If you need to release the SG-buffer data explicitly, call the
standard API function snd_pcm_lib_free_pages() as usual.
Vmalloc’ed Buffers¶
It’s possible to use a buffer allocated via vmalloc(), for
example, for an intermediate buffer.
You can simply allocate it via the standard
snd_pcm_lib_malloc_pages() and co. after setting up the
buffer preallocation with SNDRV_DMA_TYPE_VMALLOC type:
snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_VMALLOC,
NULL, 0, 0);
NULL is passed as the device pointer argument, which indicates that default pages (GFP_KERNEL and GFP_HIGHMEM) will be allocated.
Also, note that zero is passed as both the size and the max size argument here. Since each vmalloc call should succeed at any time, we don’t need to pre-allocate the buffers like other continuous pages.
Proc Interface¶
ALSA provides an easy interface for procfs. The proc files are very
useful for debugging. I recommend you set up proc files if you write a
driver and want to get a running status or register dumps. The API is
found in <sound/info.h>.
To create a proc file, call snd_card_proc_new():
struct snd_info_entry *entry;
int err = snd_card_proc_new(card, "my-file", &entry);
where the second argument specifies the name of the proc file to be
created. The above example will create a file my-file under the
card directory, e.g. /proc/asound/card0/my-file.
Like other components, the proc entry created via
snd_card_proc_new() will be registered and released
automatically in the card registration and release functions.
When the creation is successful, the function stores a new instance in
the pointer given in the third argument. It is initialized as a text
proc file for read only. To use this proc file as a read-only text file
as-is, set the read callback with private data via
snd_info_set_text_ops():
snd_info_set_text_ops(entry, chip, my_proc_read);
where the second argument (chip) is the private data to be used in
the callback. The third parameter specifies the read buffer size and
the fourth (my_proc_read) is the callback function, which is
defined like:
static void my_proc_read(struct snd_info_entry *entry,
struct snd_info_buffer *buffer);
In the read callback, use snd_iprintf() for output
strings, which works just like normal printf(). For
example:
static void my_proc_read(struct snd_info_entry *entry,
struct snd_info_buffer *buffer)
{
struct my_chip *chip = entry->private_data;
snd_iprintf(buffer, "This is my chip!\n");
snd_iprintf(buffer, "Port = %ld\n", chip->port);
}
The file permissions can be changed afterwards. By default, they are read only for all users. If you want to add write permission for the user (root by default), do as follows:
entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
and set the write buffer size and the callback:
entry->c.text.write = my_proc_write;
In the write callback, you can use snd_info_get_line()
to get a text line, and snd_info_get_str() to retrieve
a string from the line. Some examples are found in
core/oss/mixer_oss.c, core/oss/and pcm_oss.c.
For a raw-data proc-file, set the attributes as follows:
static const struct snd_info_entry_ops my_file_io_ops = {
.read = my_file_io_read,
};
entry->content = SNDRV_INFO_CONTENT_DATA;
entry->private_data = chip;
entry->c.ops = &my_file_io_ops;
entry->size = 4096;
entry->mode = S_IFREG | S_IRUGO;
For raw data, size field must be set properly. This specifies
the maximum size of the proc file access.
The read/write callbacks of raw mode are more direct than the text mode.
You need to use a low-level I/O functions such as
copy_from_user() and copy_to_user() to transfer the
data:
static ssize_t my_file_io_read(struct snd_info_entry *entry,
void *file_private_data,
struct file *file,
char *buf,
size_t count,
loff_t pos)
{
if (copy_to_user(buf, local_data + pos, count))
return -EFAULT;
return count;
}
If the size of the info entry has been set up properly, count and
pos are guaranteed to fit within 0 and the given size. You don’t
have to check the range in the callbacks unless any other condition is
required.
Power Management¶
If the chip is supposed to work with suspend/resume functions, you need
to add power-management code to the driver. The additional code for
power-management should be ifdef-ed with CONFIG_PM, or annotated
with __maybe_unused attribute; otherwise the compiler will complain.
If the driver fully supports suspend/resume that is, the device can be
properly resumed to its state when suspend was called, you can set the
SNDRV_PCM_INFO_RESUME flag in the PCM info field. Usually, this is
possible when the registers of the chip can be safely saved and restored
to RAM. If this is set, the trigger callback is called with
SNDRV_PCM_TRIGGER_RESUME after the resume callback completes.
Even if the driver doesn’t support PM fully but partial suspend/resume
is still possible, it’s still worthy to implement suspend/resume
callbacks. In such a case, applications would reset the status by
calling snd_pcm_prepare() and restart the stream
appropriately. Hence, you can define suspend/resume callbacks below but
don’t set the SNDRV_PCM_INFO_RESUME info flag to the PCM.
Note that the trigger with SUSPEND can always be called when
snd_pcm_suspend_all() is called, regardless of the
SNDRV_PCM_INFO_RESUME flag. The RESUME flag affects only the
behavior of snd_pcm_resume(). (Thus, in theory,
SNDRV_PCM_TRIGGER_RESUME isn’t needed to be handled in the trigger
callback when no SNDRV_PCM_INFO_RESUME flag is set. But, it’s better
to keep it for compatibility reasons.)
The driver needs to define the suspend/resume hooks according to the bus the device is connected to. In the case of PCI drivers, the callbacks look like below:
static int __maybe_unused snd_my_suspend(struct device *dev)
{
.... /* do things for suspend */
return 0;
}
static int __maybe_unused snd_my_resume(struct device *dev)
{
.... /* do things for suspend */
return 0;
}
The scheme of the real suspend job is as follows:
Retrieve the card and the chip data.
Call
snd_power_change_state()withSNDRV_CTL_POWER_D3hotto change the power status.If AC97 codecs are used, call
snd_ac97_suspend()for each codec.Save the register values if necessary.
Stop the hardware if necessary.
Typical code would look like:
static int __maybe_unused mychip_suspend(struct device *dev)
{
/* (1) */
struct snd_card *card = dev_get_drvdata(dev);
struct mychip *chip = card->private_data;
/* (2) */
snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
/* (3) */
snd_ac97_suspend(chip->ac97);
/* (4) */
snd_mychip_save_registers(chip);
/* (5) */
snd_mychip_stop_hardware(chip);
return 0;
}
The scheme of the real resume job is as follows:
Retrieve the card and the chip data.
Re-initialize the chip.
Restore the saved registers if necessary.
Resume the mixer, e.g. by calling
snd_ac97_resume().Restart the hardware (if any).
Call
snd_power_change_state()withSNDRV_CTL_POWER_D0to notify the processes.
Typical code would look like:
static int __maybe_unused mychip_resume(struct pci_dev *pci)
{
/* (1) */
struct snd_card *card = dev_get_drvdata(dev);
struct mychip *chip = card->private_data;
/* (2) */
snd_mychip_reinit_chip(chip);
/* (3) */
snd_mychip_restore_registers(chip);
/* (4) */
snd_ac97_resume(chip->ac97);
/* (5) */
snd_mychip_restart_chip(chip);
/* (6) */
snd_power_change_state(card, SNDRV_CTL_POWER_D0);
return 0;
}
Note that, at the time this callback gets called, the PCM stream has
been already suspended via its own PM ops calling
snd_pcm_suspend_all() internally.
OK, we have all callbacks now. Let’s set them up. In the initialization
of the card, make sure that you can get the chip data from the card
instance, typically via private_data field, in case you created the
chip data individually:
static int snd_mychip_probe(struct pci_dev *pci,
const struct pci_device_id *pci_id)
{
....
struct snd_card *card;
struct mychip *chip;
int err;
....
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
0, &card);
....
chip = kzalloc(sizeof(*chip), GFP_KERNEL);
....
card->private_data = chip;
....
}
When you created the chip data with snd_card_new(), it’s
anyway accessible via private_data field:
static int snd_mychip_probe(struct pci_dev *pci,
const struct pci_device_id *pci_id)
{
....
struct snd_card *card;
struct mychip *chip;
int err;
....
err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
sizeof(struct mychip), &card);
....
chip = card->private_data;
....
}
If you need space to save the registers, allocate the buffer for it here, too, since it would be fatal if you cannot allocate a memory in the suspend phase. The allocated buffer should be released in the corresponding destructor.
And next, set suspend/resume callbacks to the pci_driver:
static DEFINE_SIMPLE_DEV_PM_OPS(snd_my_pm_ops, mychip_suspend, mychip_resume);
static struct pci_driver driver = {
.name = KBUILD_MODNAME,
.id_table = snd_my_ids,
.probe = snd_my_probe,
.remove = snd_my_remove,
.driver = {
.pm = &snd_my_pm_ops,
},
};
Module Parameters¶
There are standard module options for ALSA. At least, each module should
have the index, id and enable options.
If the module supports multiple cards (usually up to 8 = SNDRV_CARDS
cards), they should be arrays. The default initial values are defined
already as constants for easier programming:
static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
If the module supports only a single card, they could be single
variables, instead. enable option is not always necessary in this
case, but it would be better to have a dummy option for compatibility.
The module parameters must be declared with the standard
module_param(), module_param_array() and
MODULE_PARM_DESC() macros.
Typical code would look as below:
#define CARD_NAME "My Chip"
module_param_array(index, int, NULL, 0444);
MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
module_param_array(id, charp, NULL, 0444);
MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
module_param_array(enable, bool, NULL, 0444);
MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
Also, don’t forget to define the module description and the license. Especially, the recent modprobe requires to define the module license as GPL, etc., otherwise the system is shown as “tainted”:
MODULE_DESCRIPTION("Sound driver for My Chip");
MODULE_LICENSE("GPL");
Device-Managed Resources¶
In the examples above, all resources are allocated and released
manually. But human beings are lazy in nature, especially developers
are lazier. So there are some ways to automate the release part; it’s
the (device-)managed resources aka devres or devm family. For
example, an object allocated via devm_kmalloc() will be
freed automatically at unbinding the device.
ALSA core provides also the device-managed helper, namely,
snd_devm_card_new() for creating a card object.
Call this functions instead of the normal snd_card_new(),
and you can forget the explicit snd_card_free() call, as
it’s called automagically at error and removal paths.
One caveat is that the call of snd_card_free() would be put
at the beginning of the call chain only after you call
snd_card_register().
Also, the private_free callback is always called at the card free,
so be careful to put the hardware clean-up procedure in
private_free callback. It might be called even before you
actually set up at an earlier error path. For avoiding such an
invalid initialization, you can set private_free callback after
snd_card_register() call succeeds.
Another thing to be remarked is that you should use device-managed helpers for each component as much as possible once when you manage the card in that way. Mixing up with the normal and the managed resources may screw up the release order.
How To Put Your Driver Into ALSA Tree¶
General¶
So far, you’ve learned how to write the driver codes. And you might have a question now: how to put my own driver into the ALSA driver tree? Here (finally :) the standard procedure is described briefly.
Suppose that you create a new PCI driver for the card “xyz”. The card
module name would be snd-xyz. The new driver is usually put into the
alsa-driver tree, sound/pci directory in the case of PCI
cards.
In the following sections, the driver code is supposed to be put into Linux kernel tree. The two cases are covered: a driver consisting of a single source file and one consisting of several source files.
Driver with A Single Source File¶
Modify sound/pci/Makefile
Suppose you have a file xyz.c. Add the following two lines:
snd-xyz-y := xyz.o obj-$(CONFIG_SND_XYZ) += snd-xyz.o
Create the Kconfig entry
Add the new entry of Kconfig for your xyz driver:
config SND_XYZ tristate "Foobar XYZ" depends on SND select SND_PCM help Say Y here to include support for Foobar XYZ soundcard. To compile this driver as a module, choose M here: the module will be called snd-xyz.
The line select SND_PCM specifies that the driver xyz supports PCM.
In addition to SND_PCM, the following components are supported for
select command: SND_RAWMIDI, SND_TIMER, SND_HWDEP, SND_MPU401_UART,
SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB, SND_AC97_CODEC.
Add the select command for each supported component.
Note that some selections imply the lowlevel selections. For example, PCM includes TIMER, MPU401_UART includes RAWMIDI, AC97_CODEC includes PCM, and OPL3_LIB includes HWDEP. You don’t need to give the lowlevel selections again.
For the details of Kconfig script, refer to the kbuild documentation.
Drivers with Several Source Files¶
Suppose that the driver snd-xyz have several source files. They are located in the new subdirectory, sound/pci/xyz.
Add a new directory (
sound/pci/xyz) insound/pci/Makefileas below:obj-$(CONFIG_SND) += sound/pci/xyz/
Under the directory
sound/pci/xyz, create a Makefile:snd-xyz-y := xyz.o abc.o def.o obj-$(CONFIG_SND_XYZ) += snd-xyz.o
Create the Kconfig entry
This procedure is as same as in the last section.
Useful Functions¶
snd_BUG()¶
It shows the BUG? message and stack trace as well as
snd_BUG_ON() at the point. It’s useful to show that a
fatal error happens there.
When no debug flag is set, this macro is ignored.
snd_BUG_ON()¶
snd_BUG_ON() macro is similar with
WARN_ON() macro. For example, snd_BUG_ON(!pointer); or
it can be used as the condition, if (snd_BUG_ON(non_zero_is_bug))
return -EINVAL;
The macro takes an conditional expression to evaluate. When
CONFIG_SND_DEBUG, is set, if the expression is non-zero, it shows
the warning message such as BUG? (xxx) normally followed by stack
trace. In both cases it returns the evaluated value.
Acknowledgments¶
I would like to thank Phil Kerr for his help for improvement and corrections of this document.
Kevin Conder reformatted the original plain-text to the DocBook format.
Giuliano Pochini corrected typos and contributed the example codes in the hardware constraints section.