[RFC v2 01/11] Documentation: Add HTE subsystem guide

[Dipen Patel <dipenp@xxxxxxxxxx>]

Adding hte document which can help understand various APIs implemented
in HTE framework for the HTE producers and the consumers.

Signed-off-by: Dipen Patel <dipenp@xxxxxxxxxx>
---
Changes in v2:
- Removed explanation, instead added kernel-doc references.

 Documentation/hte/hte.rst | 83 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)
 create mode 100644 Documentation/hte/hte.rst

diff --git a/Documentation/hte/hte.rst b/Documentation/hte/hte.rst
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+============================================
+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 timestamp 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 <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: devm_of_hte_request_ts hte_req_ts_by_hte_name hte_release_ts hte_enable_ts hte_disable_ts 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 nano second in u64 data
+type. For now all the HTE APIs using struct hte_ts_data requires tsc to be in
+nano seconds. An example of the typical hte_ts_data data life cycle, for the
+GPIO line is as follows::
+
+ - Monitors GPIO line change.
+ - Detects the state change on GPIO line.
+ - Converts timestamps in nano seconds and stores it in tsc.
+ - Stores GPIO direction in dir 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 starts kernel thread and 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>/``.
+
+`ts_requested`
+		The total number of entities requested from the given provider,
+		where entity is the provider specific and could represent
+		lines, GPIO, chip signals, buses etc...
+                The attribute will be availble at
+		``/sys/kernel/debug/hte/<provider>/``.
+
+		Read only value
+
+`total_ts`
+		The total number of entities supported by the provider.
+                The attribute will be availble at
+		``/sys/kernel/debug/hte/<provider>/``.
+
+		Read only value
+
+`dropped_timestamps`
+		The dropped timestamps for a given line.
+                The attribute will be availble at
+		``/sys/kernel/debug/hte/<provider>/<label or line id>/``.
+
+		Read only value
-- 
2.17.1