docs: trace: Add HiSilicon PTT device driver documentation

Document the introduction and usage of HiSilicon PTT device driver as well
as the sysfs attributes description provided by the driver.
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
[Fixed month and kernel version]
Link: https://lore.kernel.org/r/20220816114414.4092-5-yangyicong@huawei.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

Documentation/ABI/testing/sysfs-devices-hisi_ptt

+What:		/sys/devices/hisi_ptt<sicl_id>_<core_id>/tune
+Date:		October 2022
+KernelVersion:	6.1
+Contact:	Yicong Yang <yangyicong@hisilicon.com>
+Description:	This directory contains files for tuning the PCIe link
+		parameters(events). Each file is named after the event
+		of the PCIe link.
+
+		See Documentation/trace/hisi-ptt.rst for more information.
+
+What:		/sys/devices/hisi_ptt<sicl_id>_<core_id>/tune/qos_tx_cpl
+Date:		October 2022
+KernelVersion:	6.1
+Contact:	Yicong Yang <yangyicong@hisilicon.com>
+Description:	(RW) Controls the weight of Tx completion TLPs, which influence
+		the proportion of outbound completion TLPs on the PCIe link.
+		The available tune data is [0, 1, 2]. Writing a negative value
+		will return an error, and out of range values will be converted
+		to 2. The value indicates a probable level of the event.
+
+What:		/sys/devices/hisi_ptt<sicl_id>_<core_id>/tune/qos_tx_np
+Date:		October 2022
+KernelVersion:	6.1
+Contact:	Yicong Yang <yangyicong@hisilicon.com>
+Description:	(RW) Controls the weight of Tx non-posted TLPs, which influence
+		the proportion of outbound non-posted TLPs on the PCIe link.
+		The available tune data is [0, 1, 2]. Writing a negative value
+		will return an error, and out of range values will be converted
+		to 2. The value indicates a probable level of the event.
+
+What:		/sys/devices/hisi_ptt<sicl_id>_<core_id>/tune/qos_tx_p
+Date:		October 2022
+KernelVersion:	6.1
+Contact:	Yicong Yang <yangyicong@hisilicon.com>
+Description:	(RW) Controls the weight of Tx posted TLPs, which influence the
+		proportion of outbound posted TLPs on the PCIe link.
+		The available tune data is [0, 1, 2]. Writing a negative value
+		will return an error, and out of range values will be converted
+		to 2. The value indicates a probable level of the event.
+
+What:		/sys/devices/hisi_ptt<sicl_id>_<core_id>/tune/rx_alloc_buf_level
+Date:		October 2022
+KernelVersion:	6.1
+Contact:	Yicong Yang <yangyicong@hisilicon.com>
+Description:	(RW) Control the allocated buffer watermark for inbound packets.
+		The packets will be stored in the buffer first and then transmitted
+		either when the watermark reached or when timed out.
+		The available tune data is [0, 1, 2]. Writing a negative value
+		will return an error, and out of range values will be converted
+		to 2. The value indicates a probable level of the event.
+
+What:		/sys/devices/hisi_ptt<sicl_id>_<core_id>/tune/tx_alloc_buf_level
+Date:		October 2022
+KernelVersion:	6.1
+Contact:	Yicong Yang <yangyicong@hisilicon.com>
+Description:	(RW) Control the allocated buffer watermark of outbound packets.
+		The packets will be stored in the buffer first and then transmitted
+		either when the watermark reached or when timed out.
+		The available tune data is [0, 1, 2]. Writing a negative value
+		will return an error, and out of range values will be converted
+		to 2. The value indicates a probable level of the event.

Documentation/trace/index.rst

@@ -33,3 +33,4 @@ Linux Tracing Technologies
    coresight/index
    user_events
    rv/index
+   hisi-ptt