1.. SPDX-License-Identifier: GPL-2.0
2
3===================
4MIPI SyS-T over STP
5===================
6
7The MIPI SyS-T protocol driver can be used with STM class devices to
8generate standardized trace stream. Aside from being a standard, it
9provides better trace source identification and timestamp correlation.
10
11In order to use the MIPI SyS-T protocol driver with your STM device,
12first, you'll need CONFIG_STM_PROTO_SYS_T.
13
14Now, you can select which protocol driver you want to use when you create
15a policy for your STM device, by specifying it in the policy name:
16
17# mkdir /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/
18
19In other words, the policy name format is extended like this:
20
21  <device_name>:<protocol_name>.<policy_name>
22
23With Intel TH, therefore it can look like "0-sth:p_sys-t.my-policy".
24
25If the protocol name is omitted, the STM class will chose whichever
26protocol driver was loaded first.
27
28You can also double check that everything is working as expected by
29
30# cat /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/protocol
31p_sys-t
32
33Now, with the MIPI SyS-T protocol driver, each policy node in the
34configfs gets a few additional attributes, which determine per-source
35parameters specific to the protocol:
36
37# mkdir /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/default
38# ls /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/default
39channels
40clocksync_interval
41do_len
42masters
43ts_interval
44uuid
45
46The most important one here is the "uuid", which determines the UUID
47that will be used to tag all data coming from this source. It is
48automatically generated when a new node is created, but it is likely
49that you would want to change it.
50
51do_len switches on/off the additional "payload length" field in the
52MIPI SyS-T message header. It is off by default as the STP already
53marks message boundaries.
54
55ts_interval and clocksync_interval determine how much time in milliseconds
56can pass before we need to include a protocol (not transport, aka STP)
57timestamp in a message header or send a CLOCKSYNC packet, respectively.
58
59See Documentation/ABI/testing/configfs-stp-policy-p_sys-t for more
60details.
61
62* [1] https://www.mipi.org/specifications/sys-t
63