stm class: Document the MIPI SyS-T protocol usage

Add a document describing MIPI SyS-T protocol driver usage.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/trace/sys-t.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+===================
+MIPI SyS-T over STP
+===================
+
+The MIPI SyS-T protocol driver can be used with STM class devices to
+generate standardized trace stream. Aside from being a standard, it
+provides better trace source identification and timestamp correlation.
+
+In order to use the MIPI SyS-T protocol driver with your STM device,
+first, you'll need CONFIG_STM_PROTO_SYS_T.
+
+Now, you can select which protocol driver you want to use when you create
+a policy for your STM device, by specifying it in the policy name:
+
+# mkdir /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/
+
+In other words, the policy name format is extended like this:
+
+  <device_name>:<protocol_name>.<policy_name>
+
+With Intel TH, therefore it can look like "0-sth:p_sys-t.my-policy".
+
+If the protocol name is omitted, the STM class will chose whichever
+protocol driver was loaded first.
+
+You can also double check that everything is working as expected by
+
+# cat /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/protocol
+p_sys-t
+
+Now, with the MIPI SyS-T protocol driver, each policy node in the
+configfs gets a few additional attributes, which determine per-source
+parameters specific to the protocol:
+
+# mkdir /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/default
+# ls /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/default
+channels
+clocksync_interval
+do_len
+masters
+ts_interval
+uuid
+
+The most important one here is the "uuid", which determines the UUID
+that will be used to tag all data coming from this source. It is
+automatically generated when a new node is created, but it is likely
+that you would want to change it.
+
+do_len switches on/off the additional "payload length" field in the
+MIPI SyS-T message header. It is off by default as the STP already
+marks message boundaries.
+
+ts_interval and clocksync_interval determine how much time in milliseconds
+can pass before we need to include a protocol (not transport, aka STP)
+timestamp in a message header or send a CLOCKSYNC packet, respectively.
+
+See Documentation/ABI/testing/configfs-stp-policy-p_sys-t for more
+details.
+
+* [1] https://www.mipi.org/specifications/sys-t