stm class: Document the MIPI SyS-T protocol usage

.. SPDX-License-Identifier: GPL-2.0

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MIPI SyS-T over STP

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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