The Linux Hardware Timestamping Engine (HTE)

Author:Dipen Patel

Introduction

Certain devices have built in hardware timestamping engines which can monitor sets of system signals, lines, buses etc… in realtime for state change; upon detecting the change they can automatically store the timestamp at the moment of occurrence. Such functionality may help achieve better accuracy in obtaining timestamps than using software counterparts i.e. ktime and friends.

This document describes the API that can be used by hardware timestamping engine provider and consumer drivers that want to use the hardware timestamping engine (HTE) framework. Both consumers and providers must include #include <linux/hte.h>.

The HTE framework APIs for the providers

int hte_push_ts_ns(const struct hte_chip *chip, u32 xlated_id, struct hte_ts_data *data)

Push timestamp data in nanoseconds.

Parameters

const struct hte_chip *chip
The HTE chip, used during the registration.
u32 xlated_id
entity id understood by both subsystem and provider, this is obtained from xlate callback during request API.
struct hte_ts_data *data
timestamp data.

Description

It is used by the provider to push timestamp data.

Return

0 on success or a negative error code on failure.

int devm_hte_register_chip(struct hte_chip *chip)

Resource managed API to register HTE chip.

Parameters

struct hte_chip *chip
the HTE chip to add to subsystem.

Description

It is used by the provider to register itself with the HTE subsystem. The unregistration is done automatically when the provider exits.

Return

0 on success or a negative error code on failure.

The HTE framework APIs for the consumers

int hte_ts_put(struct hte_ts_desc *desc)

Release and disable timestamp for the given desc.

Parameters

struct hte_ts_desc *desc
timestamp descriptor.

Context

debugfs_remove_recursive() function call may use sleeping locks, not suitable from atomic context.

Return

0 on success or a negative error code on failure.

int hte_disable_ts(struct hte_ts_desc *desc)

Disable timestamp on given descriptor.

Parameters

struct hte_ts_desc *desc
ts descriptor, this is the same as returned by the request API.

Description

The API does not release any resources associated with desc.

Context

Holds mutex lock, not suitable from atomic context.

Return

0 on success or a negative error code on failure.

int hte_enable_ts(struct hte_ts_desc *desc)

Enable timestamp on given descriptor.

Parameters

struct hte_ts_desc *desc
ts descriptor, this is the same as returned by the request API.

Context

Holds mutex lock, not suitable from atomic context.

Return

0 on success or a negative error code on failure.

int of_hte_req_count(struct device *dev)

Return the number of entities to timestamp.

Parameters

struct device *dev
The HTE consumer.

Description

The function returns the total count of the requested entities to timestamp by parsing device tree.

Return

Positive number on success, -ENOENT if no entries, -EINVAL for other errors.

int hte_ts_get(struct device *dev, struct hte_ts_desc *desc, int index)

The function to initialize and obtain HTE desc.

Parameters

struct device *dev
HTE consumer/client device, used in case of parsing device tree node.
struct hte_ts_desc *desc
Pre-allocated timestamp descriptor.
int index
The index will be used as an index to parse line_id from the device tree node if node is present.

Description

The function initializes the consumer provided HTE descriptor. If consumer has device tree node, index is used to parse the line id and other details. The function needs to be called before using any request APIs.

Context

Holds mutex lock.

Return

Returns 0 on success or negative error code on failure.

int hte_request_ts_ns(struct hte_ts_desc *desc, hte_ts_cb_t cb, hte_ts_sec_cb_t tcb, void *data)

The API to request and enable hardware timestamp in nanoseconds.

Parameters

struct hte_ts_desc *desc
Pre-allocated and initialized timestamp descriptor.
hte_ts_cb_t cb
Callback to push the timestamp data to consumer.
hte_ts_sec_cb_t tcb
Optional callback. If its provided, subsystem initializes workqueue. It is called when cb returns HTE_RUN_SECOND_CB.
void *data
Client data, used during cb and tcb callbacks.

Description

The entity is provider specific for example, GPIO lines, signals, buses etc…The API allocates necessary resources and enables the timestamp.

Context

Holds mutex lock.

Return

Returns 0 on success or negative error code on failure.

int devm_hte_request_ts_ns(struct device *dev, struct hte_ts_desc *desc, hte_ts_cb_t cb, hte_ts_sec_cb_t tcb, void *data)

Resource managed API to request and enable hardware timestamp in nanoseconds.

Parameters

struct device *dev
HTE consumer/client device.
struct hte_ts_desc *desc
Pre-allocated and initialized timestamp descriptor.
hte_ts_cb_t cb
Callback to push the timestamp data to consumer.
hte_ts_sec_cb_t tcb
Optional callback. If its provided, subsystem initializes workqueue. It is called when cb returns HTE_RUN_SECOND_CB.
void *data
Client data, used during cb and tcb callbacks.

Description

The entity is provider specific for example, GPIO lines, signals, buses etc…The API allocates necessary resources and enables the timestamp. It deallocates and disables automatically when the consumer exits.

Context

Holds mutex lock.

Return

Returns 0 on success or negative error code on failure.

int hte_init_line_attr(struct hte_ts_desc *desc, u32 line_id, unsigned long edge_flags, const char *name, void *data)

Initialize line attributes.

Parameters

struct hte_ts_desc *desc
Pre-allocated timestamp descriptor.
u32 line_id
line id.
unsigned long edge_flags
edge flags related to line_id.
const char *name
name of the line.
void *data
line data related to line_id.

Description

Zeroes out line attributes and initializes with provided arguments. The function needs to be called before calling any consumer facing functions.

Context

Any.

Return

0 on success or negative error code for the failure.

int hte_get_clk_src_info(const struct hte_ts_desc *desc, struct hte_clk_info *ci)

Get the clock source information for a ts descriptor.

Parameters

const struct hte_ts_desc *desc
ts descriptor, same as returned from request API.
struct hte_clk_info *ci
The API fills this structure with the clock information data.

Context

Any context.

Return

0 on success else negative error code on failure.

The HTE framework public structures

enum hte_edge

HTE line edge flags.

Constants

HTE_EDGE_NO_SETUP
No edge setup. In this case consumer will setup edges, for example during request irq call.
HTE_RISING_EDGE_TS
Rising edge.
HTE_FALLING_EDGE_TS
Falling edge.
enum hte_return

HTE subsystem return values used during callback.

Constants

HTE_CB_HANDLED
The consumer handled the data.
HTE_RUN_SECOND_CB
The consumer needs further processing, in that case HTE subsystem calls secondary callback provided by the consumer where it is allowed to sleep.
struct hte_ts_data

HTE timestamp data.

Definition:

struct hte_ts_data {
    u64 tsc;
    u64 seq;
    int raw_level;
};

Members

tsc
Timestamp value.
seq
Sequence counter of the timestamps.
raw_level
Level of the line at the timestamp if provider supports it, -1 otherwise.
struct hte_clk_info

Clock source info that HTE provider uses to timestamp.

Definition:

struct hte_clk_info {
    u64 hz;
    clockid_t type;
};

Members

hz
Supported clock rate in HZ, for example 1KHz clock = 1000.
type
Supported clock type.
hte_ts_cb_t

Typedef: HTE timestamp data processing primary callback.

Syntax

enum hte_return hte_ts_cb_t (struct hte_ts_data *ts, void *data)

Parameters

struct hte_ts_data *ts
HW timestamp data.
void *data
Client supplied data.

Description

The callback is used to push timestamp data to the client and it is not allowed to sleep.

hte_ts_sec_cb_t

Typedef: HTE timestamp data processing secondary callback.

Syntax

enum hte_return hte_ts_sec_cb_t (void *data)

Parameters

void *data
Client supplied data.

Description

This is used when the client needs further processing where it is allowed to sleep.

struct hte_line_attr

Line attributes.

Definition:

struct hte_line_attr {
    u32 line_id;
    void *line_data;
    unsigned long edge_flags;
    const char *name;
};

Members

line_id
The logical ID understood by the consumers and providers.
line_data
Line data related to line_id.
edge_flags
Edge setup flags.
name
Descriptive name of the entity that is being monitored for the hardware timestamping. If null, HTE core will construct the name.
struct hte_ts_desc

HTE timestamp descriptor.

Definition:

struct hte_ts_desc {
    struct hte_line_attr attr;
    void *hte_data;
};

Members

attr
The line attributes.
hte_data
Subsystem’s private data, set by HTE subsystem.

Description

This structure is a communication token between consumers to subsystem and subsystem to providers.

struct hte_ops

HTE operations set by providers.

Definition:

struct hte_ops {
    int (*request)(struct hte_chip *chip, struct hte_ts_desc *desc, u32 xlated_id);
    int (*release)(struct hte_chip *chip, struct hte_ts_desc *desc, u32 xlated_id);
    int (*enable)(struct hte_chip *chip, u32 xlated_id);
    int (*disable)(struct hte_chip *chip, u32 xlated_id);
    int (*get_clk_src_info)(struct hte_chip *chip, struct hte_clk_info *ci);
};

Members

request
Hook for requesting a HTE timestamp. Returns 0 on success, non-zero for failures.
release
Hook for releasing a HTE timestamp. Returns 0 on success, non-zero for failures.
enable
Hook to enable the specified timestamp. Returns 0 on success, non-zero for failures.
disable
Hook to disable specified timestamp. Returns 0 on success, non-zero for failures.
get_clk_src_info
Hook to get the clock information the provider uses to timestamp. Returns 0 for success and negative error code for failure. On success HTE subsystem fills up provided struct hte_clk_info.

Description

xlated_id parameter is used to communicate between HTE subsystem and the providers and is translated by the provider.

struct hte_chip

Abstract HTE chip.

Definition:

struct hte_chip {
    const char *name;
    struct device *dev;
    const struct hte_ops *ops;
    u32 nlines;
    int (*xlate_of)(struct hte_chip *gc,const struct of_phandle_args *args, struct hte_ts_desc *desc, u32 *xlated_id);
    int (*xlate_plat)(struct hte_chip *gc, struct hte_ts_desc *desc, u32 *xlated_id);
    bool (*match_from_linedata)(const struct hte_chip *chip, const struct hte_ts_desc *hdesc);
    u8 of_hte_n_cells;
    struct hte_device *gdev;
    void *data;
};

Members

name
functional name of the HTE IP block.
dev
device providing the HTE.
ops
callbacks for this HTE.
nlines
number of lines/signals supported by this chip.
xlate_of
Callback which translates consumer supplied logical ids to physical ids, return 0 for the success and negative for the failures. It stores (between 0 to nlines) in xlated_id parameter for the success.
xlate_plat
Same as above but for the consumers with no DT node.
match_from_linedata
Match HTE device using the line_data.
of_hte_n_cells
Number of cells used to form the HTE specifier.
gdev
HTE subsystem abstract device, internal to the HTE subsystem.
data
chip specific private data.

More on the HTE timestamp data

The struct hte_ts_data is used to pass timestamp details between the consumers and the providers. It expresses timestamp data in nanoseconds in u64. An example of the typical timestamp data life cycle, for the GPIO line is as follows:

- Monitors GPIO line change.
- Detects the state change on GPIO line.
- Converts timestamps in nanoseconds.
- Stores GPIO raw level in raw_level variable if the provider has that
hardware capability.
- Pushes this hte_ts_data object to HTE subsystem.
- HTE subsystem increments seq counter and invokes consumer provided callback.
Based on callback return value, the HTE core invokes secondary callback in
the thread context.

HTE subsystem debugfs attributes

HTE subsystem creates debugfs attributes at /sys/kernel/debug/hte/. It also creates line/signal-related debugfs attributes at /sys/kernel/debug/hte/<provider>/<label or line id>/. Note that these attributes are read-only.

ts_requested
The total number of entities requested from the given provider, where entity is specified by the provider and could represent lines, GPIO, chip signals, buses etc… The attribute will be available at /sys/kernel/debug/hte/<provider>/.
total_ts
The total number of entities supported by the provider. The attribute will be available at /sys/kernel/debug/hte/<provider>/.
dropped_timestamps
The dropped timestamps for a given line. The attribute will be available at /sys/kernel/debug/hte/<provider>/<label or line id>/.