trace doc: convert trace/stm.txt to rst format

This converts the plain text documentation to reStructuredText format and
add it into Sphinx TOC tree. No essential content change.

Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 123 insertions, 0 deletions

diff --git a/Documentation/trace/stm.rst b/Documentation/trace/stm.rst
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+===================
+System Trace Module
+===================
+
+System Trace Module (STM) is a device described in MIPI STP specs as
+STP trace stream generator. STP (System Trace Protocol) is a trace
+protocol multiplexing data from multiple trace sources, each one of
+which is assigned a unique pair of master and channel. While some of
+these masters and channels are statically allocated to certain
+hardware trace sources, others are available to software. Software
+trace sources are usually free to pick for themselves any
+master/channel combination from this pool.
+
+On the receiving end of this STP stream (the decoder side), trace
+sources can only be identified by master/channel combination, so in
+order for the decoder to be able to make sense of the trace that
+involves multiple trace sources, it needs to be able to map those
+master/channel pairs to the trace sources that it understands.
+
+For instance, it is helpful to know that syslog messages come on
+master 7 channel 15, while arbitrary user applications can use masters
+48 to 63 and channels 0 to 127.
+
+To solve this mapping problem, stm class provides a policy management
+mechanism via configfs, that allows defining rules that map string
+identifiers to ranges of masters and channels. If these rules (policy)
+are consistent with what decoder expects, it will be able to properly
+process the trace data.
+
+This policy is a tree structure containing rules (policy_node) that
+have a name (string identifier) and a range of masters and channels
+associated with it, located in "stp-policy" subsystem directory in
+configfs. The topmost directory's name (the policy) is formatted as
+the STM device name to which this policy applies and and arbitrary
+string identifier separated by a stop. From the examle above, a rule
+may look like this::
+
+	$ ls /config/stp-policy/dummy_stm.my-policy/user
+	channels masters
+	$ cat /config/stp-policy/dummy_stm.my-policy/user/masters
+	48 63
+	$ cat /config/stp-policy/dummy_stm.my-policy/user/channels
+	0 127
+
+which means that the master allocation pool for this rule consists of
+masters 48 through 63 and channel allocation pool has channels 0
+through 127 in it. Now, any producer (trace source) identifying itself
+with "user" identification string will be allocated a master and
+channel from within these ranges.
+
+These rules can be nested, for example, one can define a rule "dummy"
+under "user" directory from the example above and this new rule will
+be used for trace sources with the id string of "user/dummy".
+
+Trace sources have to open the stm class device's node and write their
+trace data into its file descriptor. In order to identify themselves
+to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file
+descriptor providing their id string. Otherwise, they will be
+automatically allocated a master/channel pair upon first write to this
+file descriptor according to the "default" rule of the policy, if such
+exists.
+
+Some STM devices may allow direct mapping of the channel mmio regions
+to userspace for zero-copy writing. One mappable page (in terms of
+mmu) will usually contain multiple channels' mmios, so the user will
+need to allocate that many channels to themselves (via the
+aforementioned ioctl() call) to be able to do this. That is, if your
+stm device's channel mmio region is 64 bytes and hardware page size is
+4096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with
+width==64, you should be able to mmap() one page on this file
+descriptor and obtain direct access to an mmio region for 64 channels.
+
+Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM
+[2].
+
+stm_source
+==========
+
+For kernel-based trace sources, there is "stm_source" device
+class. Devices of this class can be connected and disconnected to/from
+stm devices at runtime via a sysfs attribute called "stm_source_link"
+by writing the name of the desired stm device there, for example::
+
+	$ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link
+
+For examples on how to use stm_source interface in the kernel, refer
+to stm_console, stm_heartbeat or stm_ftrace drivers.
+
+Each stm_source device will need to assume a master and a range of
+channels, depending on how many channels it requires. These are
+allocated for the device according to the policy configuration. If
+there's a node in the root of the policy directory that matches the
+stm_source device's name (for example, "console"), this node will be
+used to allocate master and channel numbers. If there's no such policy
+node, the stm core will pick the first contiguous chunk of channels
+within the first available master. Note that the node must exist
+before the stm_source device is connected to its stm device.
+
+stm_console
+===========
+
+One implementation of this interface also used in the example above is
+the "stm_console" driver, which basically provides a one-way console
+for kernel messages over an stm device.
+
+To configure the master/channel pair that will be assigned to this
+console in the STP stream, create a "console" policy entry (see the
+beginning of this text on how to do that). When initialized, it will
+consume one channel.
+
+stm_ftrace
+==========
+
+This is another "stm_source" device, once the stm_ftrace has been
+linked with an stm device, and if "function" tracer is enabled,
+function address and parent function address which Ftrace subsystem
+would store into ring buffer will be exported via the stm device at
+the same time.
+
+Currently only Ftrace "function" tracer is supported.
+
+* [1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
+* [2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html