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

[Dipen Patel]

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>
---
 Documentation/hte/hte.rst | 198 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 198 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
+------------
+
+The certain devices have the built in hardware timestamping engine which can
+monitor sets of system signals, lines, buses etc... in realtime for the state
+change; upon detecting the change it 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.
+
+The HTE framework APIs for the providers
+----------------------------------------
+Each driver must #include <linux/hte.h>. The ``linux/hte.h`` declares the
+following functions for the provider:
+
+.. c:function:: int hte_register_chip( struct hte_chip *chip )
+		int hte_unregister_chip( struct hte_chip *chip )
+
+	The provider uses these APIs to un/register itself with HTE framework.
+
+.. c:function:: int hte_push_ts_ns_atomic( const struct hte_chip *chip, u32 xlated_id, struct hte_ts_data *data, size_t n )
+
+	The provider pushes timestamp data in nano seconds unit using this API.
+
+The detail about parameters and API usage are described in each functions
+definitions in ``drivers/hte/hte.c`` file.
+
+The HTE framework APIs for the consumers
+----------------------------------------
+The consumers use following APIs to control the line for the timestamp:
+
+.. c:function:: int hte_release_ts( struct hte_ts_desc *desc )
+		int devm_hte_release_ts( struct device *dev, struct hte_ts_desc *desc )
+
+	The consumer uses API to release specified desc from timestamping.
+	The API frees resources associated with the desc and disables the
+	timestamping on it. The later is managed version of the same API.
+
+.. c:function:: struct hte_ts_desc *of_hte_request_ts( struct device *dev, const char *label, void (*cb)(enum hte_notify n) )
+		struct hte_ts_desc *devm_of_hte_request_ts( struct device *dev, const char *label, void (*cb)(enum hte_notify n) )
+
+	The consumers can use above request APIs to request real timestamp
+	capability on specified entity. The later is resource managed version
+	of the of_hte_request_ts API. Both the APIs expect consumer to follow
+	device tree bindings for the HTE consumer. The details about binding
+	is in ``Documentation/devicetree/bindings/hte/hte-consumer.yaml``.
+
+.. c:function:: struct hte_ts_desc *hte_req_ts_by_dt_node( struct device_node *of_node, unsigned int id, void (*cb)(enum hte_notify n) )
+
+	The consumer can request timestamping directly specifying provider
+	device tree node.
+
+.. c:function:: int hte_enable_ts( struct hte_ts_desc *desc )
+.. c:function:: int hte_disable_ts( struct hte_ts_desc *desc )
+
+	The consumer can enable/disable timestamping on given desc.
+
+.. c:function:: int hte_retrieve_ts_ns( const struct hte_ts_desc *desc, struct hte_ts_data *el, size_t n )
+		int hte_retrieve_ts_ns_wait( const struct hte_ts_desc *desc, struct hte_ts_data *el, size_t n )
+
+	The consumer uses above two API versions to get/retrieve timestamp data
+	for the given desc. The later is blocking version.
+
+.. c:function:: hte_get_clk_src_info(const struct hte_line_desc *desc, struct hte_clk_info *ci)
+
+	The consumer retrieves clock source information that provider uses to
+	timestamp entity in the structure hte_clk_info. This information
+	specifies clock rate in HZ and clock.
+
+The details on struct hte_clk_info
+-----------------------------------
+This structure presents detail of the hardware clock that provider uses for
+realtime timestamping purposes. The consumer can use hte_get_clk_src_info API
+to get the information in hte_clk_info structure. It has hz and type parameters
+where hz represents clock rate in HZ and type is clock type of clockid_t and
+of CLOCK_* family (for example, CLOCK_MONOTONIC).
+
+The consumers calling of_hte_request_ts or hte_req_ts_by_dt_node APIs with
+cb parameter set, usually will call hte_retrieve_ts (non blocking
+version) after being notified by the callbacks from HTE subsystem. The
+consumers calling those requests APIs with cb parameter NULL, usually will call
+hte_retrieve_ts_wait API.
+
+The HTE subsystem provides software buffer per requested id/entity to store
+timestamp data (struct hte_ts_data type). The consumers can manage the buffer.
+It also provides buffer watermark which can notify (if cb parameter is provided
+during request API call) consumer or unblock consumers calling
+hte_retrieve_ts_wait API. The following APIs are used to manipulate the
+software buffer:
+
+.. c:function:: int hte_set_buf_len( const struct hte_ts_desc *desc,unsigned int len )
+		int hte_get_buf_len( const struct hte_ts_desc *desc )
+
+	The consumer uses above APIs to set/get software buffer depth.
+
+.. c:function:: int hte_set_buf_watermark( const struct hte_ts_desc *desc, unsigned int val )
+		int hte_get_buf_watermark( const struct hte_ts_desc *desc )
+
+	The consumer uses above APIs to set/get software threshold, threshold
+	can be used to notity or unblock waiting consumer when data becomes
+	available equal or above to threshold value.
+
+.. c:function:: size_t hte_available_ts( const struct hte_ts_desc *desc )
+
+	The consumer uses above API to get available timestamp data stored
+	in the software buffer for the desc.
+
+The detail about parameters and API usage are described in each functions
+definitions in ``drivers/hte/hte.c`` file.
+
+The HTE timestamp element detail
+--------------------------------
+The struct hte_ts_data, declared at ``include/linux/hte.h``, 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. The timestamp
+element structure stores below information along with timestamp data::
+
+ struct hte_ts_data {
+	/*
+	 * Timestamp value
+	 */
+	u64 tsc;
+	/*
+	 * The sequence counter, keep track of the number of timestamps.
+	 * It can be used to check if data is dropped in between.
+	 */
+	u64 seq;
+	/* Direction of the event, i.e. falling or rising */
+	int dir;
+ };
+
+The typical hte_ts_data data life cycle::
+In this example the provider provides timestamp in nano seconds and for the
+GPIO line::
+
+ - 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_timestamp_el object to HTE subsystem.
+ - HTE subsystem increments seq counter and stores it in software buffer
+ dedicated to requested GPIO line.
+ - Waiting consumer gets notified.
+ - The consumer calls the retrieve timestamp API.
+
+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
+
+`ts_buffer_depth`
+		The software buffer lenth to store timestamp data.
+                The attribute will be availble at
+		``/sys/kernel/debug/hte/<provider>/<label or id>/``.
+
+		Read only value
+
+`ts_buffer_watermark`
+		The software buffer watermark or threshold.
+                The attribute will be availble at
+		``/sys/kernel/debug/hte/<provider>/<label or line id>/``.
+
+		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