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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>
authored by
Alexander Shishkin
and committed by
Greg Kroah-Hartman
4cb3653d
95323943
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Documentation/trace/sys-t.rst
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Documentation/trace/sys-t.rst
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.. SPDX-License-Identifier: GPL-2.0
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===================
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MIPI SyS-T over STP
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===================
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The MIPI SyS-T protocol driver can be used with STM class devices to
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generate standardized trace stream. Aside from being a standard, it
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provides better trace source identification and timestamp correlation.
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In order to use the MIPI SyS-T protocol driver with your STM device,
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first, you'll need CONFIG_STM_PROTO_SYS_T.
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Now, you can select which protocol driver you want to use when you create
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a policy for your STM device, by specifying it in the policy name:
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# mkdir /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/
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In other words, the policy name format is extended like this:
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<device_name>:<protocol_name>.<policy_name>
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With Intel TH, therefore it can look like "0-sth:p_sys-t.my-policy".
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If the protocol name is omitted, the STM class will chose whichever
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protocol driver was loaded first.
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You can also double check that everything is working as expected by
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# cat /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/protocol
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p_sys-t
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Now, with the MIPI SyS-T protocol driver, each policy node in the
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configfs gets a few additional attributes, which determine per-source
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parameters specific to the protocol:
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# mkdir /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/default
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# ls /config/stp-policy/dummy_stm.0:p_sys-t.my-policy/default
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channels
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clocksync_interval
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do_len
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masters
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ts_interval
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uuid
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The most important one here is the "uuid", which determines the UUID
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that will be used to tag all data coming from this source. It is
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automatically generated when a new node is created, but it is likely
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that you would want to change it.
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do_len switches on/off the additional "payload length" field in the
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MIPI SyS-T message header. It is off by default as the STP already
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marks message boundaries.
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ts_interval and clocksync_interval determine how much time in milliseconds
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can pass before we need to include a protocol (not transport, aka STP)
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timestamp in a message header or send a CLOCKSYNC packet, respectively.
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See Documentation/ABI/testing/configfs-stp-policy-p_sys-t for more
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details.
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* [1] https://www.mipi.org/specifications/sys-t