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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	System Trace Module
2	===================
4	System Trace Module (STM) is a device described in MIPI STP specs as
5	STP trace stream generator. STP (System Trace Protocol) is a trace
6	protocol multiplexing data from multiple trace sources, each one of
7	which is assigned a unique pair of master and channel. While some of
8	these masters and channels are statically allocated to certain
9	hardware trace sources, others are available to software. Software
10	trace sources are usually free to pick for themselves any
11	master/channel combination from this pool.
13	On the receiving end of this STP stream (the decoder side), trace
14	sources can only be identified by master/channel combination, so in
15	order for the decoder to be able to make sense of the trace that
16	involves multiple trace sources, it needs to be able to map those
17	master/channel pairs to the trace sources that it understands.
19	For instance, it is helpful to know that syslog messages come on
20	master 7 channel 15, while arbitrary user applications can use masters
21	48 to 63 and channels 0 to 127.
23	To solve this mapping problem, stm class provides a policy management
24	mechanism via configfs, that allows defining rules that map string
25	identifiers to ranges of masters and channels. If these rules (policy)
26	are consistent with what decoder expects, it will be able to properly
27	process the trace data.
29	This policy is a tree structure containing rules (policy_node) that
30	have a name (string identifier) and a range of masters and channels
31	associated with it, located in "stp-policy" subsystem directory in
32	configfs. The topmost directory's name (the policy) is formatted as
33	the STM device name to which this policy applies and and arbitrary
34	string identifier separated by a stop. From the examle above, a rule
35	may look like this:
37	$ ls /config/stp-policy/dummy_stm.my-policy/user
38	channels masters
39	$ cat /config/stp-policy/dummy_stm.my-policy/user/masters
40	48 63
41	$ cat /config/stp-policy/dummy_stm.my-policy/user/channels
42	0 127
44	which means that the master allocation pool for this rule consists of
45	masters 48 through 63 and channel allocation pool has channels 0
46	through 127 in it. Now, any producer (trace source) identifying itself
47	with "user" identification string will be allocated a master and
48	channel from within these ranges.
50	These rules can be nested, for example, one can define a rule "dummy"
51	under "user" directory from the example above and this new rule will
52	be used for trace sources with the id string of "user/dummy".
54	Trace sources have to open the stm class device's node and write their
55	trace data into its file descriptor. In order to identify themselves
56	to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file
57	descriptor providing their id string. Otherwise, they will be
58	automatically allocated a master/channel pair upon first write to this
59	file descriptor according to the "default" rule of the policy, if such
60	exists.
62	Some STM devices may allow direct mapping of the channel mmio regions
63	to userspace for zero-copy writing. One mappable page (in terms of
64	mmu) will usually contain multiple channels' mmios, so the user will
65	need to allocate that many channels to themselves (via the
66	aforementioned ioctl() call) to be able to do this. That is, if your
67	stm device's channel mmio region is 64 bytes and hardware page size is
68	4096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with
69	width==64, you should be able to mmap() one page on this file
70	descriptor and obtain direct access to an mmio region for 64 channels.
72	Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM
73	[2].
75	stm_source
76	==========
78	For kernel-based trace sources, there is "stm_source" device
79	class. Devices of this class can be connected and disconnected to/from
80	stm devices at runtime via a sysfs attribute called "stm_source_link"
81	by writing the name of the desired stm device there, for example:
83	$ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link
85	For examples on how to use stm_source interface in the kernel, refer
86	to stm_console, stm_heartbeat or stm_ftrace drivers.
88	Each stm_source device will need to assume a master and a range of
89	channels, depending on how many channels it requires. These are
90	allocated for the device according to the policy configuration. If
91	there's a node in the root of the policy directory that matches the
92	stm_source device's name (for example, "console"), this node will be
93	used to allocate master and channel numbers. If there's no such policy
94	node, the stm core will pick the first contiguous chunk of channels
95	within the first available master. Note that the node must exist
96	before the stm_source device is connected to its stm device.
98	stm_console
99	===========
101	One implementation of this interface also used in the example above is
102	the "stm_console" driver, which basically provides a one-way console
103	for kernel messages over an stm device.
105	To configure the master/channel pair that will be assigned to this
106	console in the STP stream, create a "console" policy entry (see the
107	beginning of this text on how to do that). When initialized, it will
108	consume one channel.
110	stm_ftrace
111	==========
113	This is another "stm_source" device, once the stm_ftrace has been
114	linked with an stm device, and if "function" tracer is enabled,
115	function address and parent function address which Ftrace subsystem
116	would store into ring buffer will be exported via the stm device at
117	the same time.
119	Currently only Ftrace "function" tracer is supported.
121	[1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
122	[2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html
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