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author | Linus Torvalds <torvalds@linux-foundation.org> | 2018-04-03 13:35:51 -0700 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2018-04-03 13:35:51 -0700 |
commit | bb2407a7219760926760f0448fddf00d625e5aec (patch) | |
tree | 68d2b7a17f0bb837d04d658a56baf16dd261210f /Documentation/trace/stm.rst | |
parent | e40dc66220b7ff1b816311b135b9298f8ba14ce6 (diff) | |
parent | 86afad7d87f535ebb1a0e978bc32a8c58ac99268 (diff) | |
download | linux-bb2407a7219760926760f0448fddf00d625e5aec.tar.bz2 |
Merge tag 'docs-4.17' of git://git.lwn.net/linux
Pull documentation updates from Jonathan Corbet:
"There's been a fair amount of activity in Documentation/ this time
around:
- Lots of work aligning Documentation/ABI with reality, done by
Aishwarya Pant.
- The trace documentation has been converted to RST by Changbin Du
- I thrashed up kernel-doc to deal with a parsing issue and to try to
make the code more readable. It's still a 20+-year-old Perl hack,
though.
- Lots of other updates, typo fixes, and more"
* tag 'docs-4.17' of git://git.lwn.net/linux: (82 commits)
Documentation/process: update FUSE project website
docs: kernel-doc: fix parsing of arrays
dmaengine: Fix spelling for parenthesis in dmatest documentation
dmaengine: Make dmatest.rst indeed reST compatible
dmaengine: Add note to dmatest documentation about supported channels
Documentation: magic-numbers: Fix typo
Documentation: admin-guide: add kvmconfig, xenconfig and tinyconfig commands
Input: alps - Update documentation for trackstick v3 format
Documentation: Mention why %p prints ptrval
COPYING: use the new text with points to the license files
COPYING: create a new file with points to the Kernel license files
Input: trackpoint: document sysfs interface
xfs: Change URL for the project in xfs.txt
char/bsr: add sysfs interface documentation
acpi: nfit: document sysfs interface
block: rbd: update sysfs interface
Documentation/sparse: fix typo
Documentation/CodingStyle: Add an example for braces
docs/vm: update 00-INDEX
kernel-doc: Remove __sched markings
...
Diffstat (limited to 'Documentation/trace/stm.rst')
-rw-r--r-- | Documentation/trace/stm.rst | 123 |
1 files changed, 123 insertions, 0 deletions
diff --git a/Documentation/trace/stm.rst b/Documentation/trace/stm.rst new file mode 100644 index 000000000000..2c22ddb7fd3e --- /dev/null +++ b/Documentation/trace/stm.rst @@ -0,0 +1,123 @@ +=================== +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 |