ALSA Co-processor Acceleration API

Jaroslav Kysela <perex@perex.cz>

Overview

There is a requirement to expose the audio hardware that accelerates various tasks for user space such as sample rate converters, compressed stream decoders, etc.

This is description for the API extension for the compress ALSA API which is able to handle “tasks” that are not bound to real-time operations and allows for the serialization of operations.

Requirements

The main requirements are:

  • serialization of multiple tasks for user space to allow multiple operations without user space intervention

  • separate buffers (input + output) for each operation

  • expose buffers using mmap to user space

  • signal user space when the task is finished (standard poll mechanism)

Design

A new direction SND_COMPRESS_ACCEL is introduced to identify the passthrough API.

The API extension shares device enumeration and parameters handling from the main compressed API. All other realtime streaming ioctls are deactivated and a new set of task related ioctls are introduced. The standard read/write/mmap I/O operations are not supported in the passthrough device.

Device (“stream”) state handling is reduced to OPEN/SETUP. All other states are not available for the passthrough mode.

Data I/O mechanism is using standard dma-buf interface with all advantages like mmap, standard I/O, buffer sharing etc. One buffer is used for the input data and second (separate) buffer is used for the output data. Each task have separate I/O buffers.

For the buffering parameters, the fragments means a limit of allocated tasks for given device. The fragment_size limits the input buffer size for the given device. The output buffer size is determined by the driver (may be different from the input buffer size).

State Machine

The passthrough audio stream state machine is described below:

                                     +----------+
                                     |          |
                                     |   OPEN   |
                                     |          |
                                     +----------+
                                           |
                                           |
                                           | compr_set_params()
                                           |
                                           v
       all passthrough task ops      +----------+
+------------------------------------|          |
|                                    |   SETUP  |
|                                    |
|                                    +----------+
|                                          |
+------------------------------------------+

Passthrough operations (ioctls)

All operations are protected using stream->device->lock (mutex).

CREATE

Creates a set of input/output buffers. The input buffer size is fragment_size. Allocates unique seqno.

The hardware drivers allocate internal ‘struct dma_buf’ for both input and output buffers (using ‘dma_buf_export()’ function). The anonymous file descriptors for those buffers are passed to user space.

FREE

Free a set of input/output buffers. If a task is active, the stop operation is executed before. If seqno is zero, operation is executed for all tasks.

START

Starts (queues) a task. There are two cases of the task start - right after the task is created. In this case, origin_seqno must be zero. The second case is for reusing of already finished task. The origin_seqno must identify the task to be reused. In both cases, a new seqno value is allocated and returned to user space.

The prerequisite is that application filled input dma buffer with new source data and set input_size to pass the real data size to the driver.

The order of data processing is preserved (first started job must be finished at first).

If the multiple tasks require a state handling (e.g. resampling operation), the user space may set SND_COMPRESS_TFLG_NEW_STREAM flag to mark the start of the new stream data. It is useful to keep the allocated buffers for the new operation rather using open/close mechanism.

STOP

Stop (dequeues) a task. If seqno is zero, operation is executed for all tasks.

STATUS

Obtain the task status (active, finished). Also, the driver will set the real output data size (valid area in the output buffer).

Credits