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1System Trace Module
2===================
3
4System Trace Module (STM) is a device described in MIPI STP specs as
5STP trace stream generator. STP (System Trace Protocol) is a trace
6protocol multiplexing data from multiple trace sources, each one of
7which is assigned a unique pair of master and channel. While some of
8these masters and channels are statically allocated to certain
9hardware trace sources, others are available to software. Software
10trace sources are usually free to pick for themselves any
11master/channel combination from this pool.
12
13On the receiving end of this STP stream (the decoder side), trace
14sources can only be identified by master/channel combination, so in
15order for the decoder to be able to make sense of the trace that
16involves multiple trace sources, it needs to be able to map those
17master/channel pairs to the trace sources that it understands.
18
19For instance, it is helpful to know that syslog messages come on
20master 7 channel 15, while arbitrary user applications can use masters
2148 to 63 and channels 0 to 127.
22
23To solve this mapping problem, stm class provides a policy management
24mechanism via configfs, that allows defining rules that map string
25identifiers to ranges of masters and channels. If these rules (policy)
26are consistent with what decoder expects, it will be able to properly
27process the trace data.
28
29This policy is a tree structure containing rules (policy_node) that
30have a name (string identifier) and a range of masters and channels
31associated with it, located in "stp-policy" subsystem directory in
32configfs. The topmost directory's name (the policy) is formatted as
33the STM device name to which this policy applies and and arbitrary
34string identifier separated by a stop. From the examle above, a rule
35may look like this:
36
37$ ls /config/stp-policy/dummy_stm.my-policy/user
38channels masters
39$ cat /config/stp-policy/dummy_stm.my-policy/user/masters
4048 63
41$ cat /config/stp-policy/dummy_stm.my-policy/user/channels
420 127
43
44which means that the master allocation pool for this rule consists of
45masters 48 through 63 and channel allocation pool has channels 0
46through 127 in it. Now, any producer (trace source) identifying itself
47with "user" identification string will be allocated a master and
48channel from within these ranges.
49
50These rules can be nested, for example, one can define a rule "dummy"
51under "user" directory from the example above and this new rule will
52be used for trace sources with the id string of "user/dummy".
53
54Trace sources have to open the stm class device's node and write their
55trace data into its file descriptor. In order to identify themselves
56to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file
57descriptor providing their id string. Otherwise, they will be
58automatically allocated a master/channel pair upon first write to this
59file descriptor according to the "default" rule of the policy, if such
60exists.
61
62Some STM devices may allow direct mapping of the channel mmio regions
63to userspace for zero-copy writing. One mappable page (in terms of
64mmu) will usually contain multiple channels' mmios, so the user will
65need to allocate that many channels to themselves (via the
66aforementioned ioctl() call) to be able to do this. That is, if your
67stm device's channel mmio region is 64 bytes and hardware page size is
684096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with
69width==64, you should be able to mmap() one page on this file
70descriptor and obtain direct access to an mmio region for 64 channels.
71
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72Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM
73[2].
74
75stm_source
76==========
77
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78For kernel-based trace sources, there is "stm_source" device
79class. Devices of this class can be connected and disconnected to/from
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80stm devices at runtime via a sysfs attribute called "stm_source_link"
81by writing the name of the desired stm device there, for example:
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83$ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link
84
85For examples on how to use stm_source interface in the kernel, refer
39fccd2f 86to stm_console, stm_heartbeat or stm_ftrace drivers.
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88Each stm_source device will need to assume a master and a range of
89channels, depending on how many channels it requires. These are
90allocated for the device according to the policy configuration. If
91there's a node in the root of the policy directory that matches the
92stm_source device's name (for example, "console"), this node will be
93used to allocate master and channel numbers. If there's no such policy
94node, the stm core will pick the first contiguous chunk of channels
95within the first available master. Note that the node must exist
96before the stm_source device is connected to its stm device.
97
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98stm_console
99===========
100
101One implementation of this interface also used in the example above is
102the "stm_console" driver, which basically provides a one-way console
103for kernel messages over an stm device.
104
105To configure the master/channel pair that will be assigned to this
106console in the STP stream, create a "console" policy entry (see the
107beginning of this text on how to do that). When initialized, it will
108consume one channel.
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110stm_ftrace
111==========
112
113This is another "stm_source" device, once the stm_ftrace has been
114linked with an stm device, and if "function" tracer is enabled,
115function address and parent function address which Ftrace subsystem
116would store into ring buffer will be exported via the stm device at
117the same time.
118
119Currently only Ftrace "function" tracer is supported.
120
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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