.. SPDX-License-Identifier: GPL-2.0+

============================================ 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

.. kernel-doc:: drivers/hte/hte.c :functions: devm_hte_register_chip hte_push_ts_ns

The HTE framework APIs for the consumers

.. kernel-doc:: drivers/hte/hte.c :functions: hte_init_line_attr hte_ts_get hte_ts_put devm_hte_request_ts_ns hte_request_ts_ns hte_enable_ts hte_disable_ts of_hte_req_count hte_get_clk_src_info

The HTE framework public structures

.. kernel-doc:: include/linux/hte.h

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>/.