1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 =================== 3 ===================
4 System Trace Module 4 System Trace Module
5 =================== 5 ===================
6 6
7 System Trace Module (STM) is a device describe 7 System Trace Module (STM) is a device described in MIPI STP specs as
8 STP trace stream generator. STP (System Trace 8 STP trace stream generator. STP (System Trace Protocol) is a trace
9 protocol multiplexing data from multiple trace 9 protocol multiplexing data from multiple trace sources, each one of
10 which is assigned a unique pair of master and 10 which is assigned a unique pair of master and channel. While some of
11 these masters and channels are statically allo 11 these masters and channels are statically allocated to certain
12 hardware trace sources, others are available t 12 hardware trace sources, others are available to software. Software
13 trace sources are usually free to pick for the 13 trace sources are usually free to pick for themselves any
14 master/channel combination from this pool. 14 master/channel combination from this pool.
15 15
16 On the receiving end of this STP stream (the d 16 On the receiving end of this STP stream (the decoder side), trace
17 sources can only be identified by master/chann 17 sources can only be identified by master/channel combination, so in
18 order for the decoder to be able to make sense 18 order for the decoder to be able to make sense of the trace that
19 involves multiple trace sources, it needs to b 19 involves multiple trace sources, it needs to be able to map those
20 master/channel pairs to the trace sources that 20 master/channel pairs to the trace sources that it understands.
21 21
22 For instance, it is helpful to know that syslo 22 For instance, it is helpful to know that syslog messages come on
23 master 7 channel 15, while arbitrary user appl 23 master 7 channel 15, while arbitrary user applications can use masters
24 48 to 63 and channels 0 to 127. 24 48 to 63 and channels 0 to 127.
25 25
26 To solve this mapping problem, stm class provi 26 To solve this mapping problem, stm class provides a policy management
27 mechanism via configfs, that allows defining r 27 mechanism via configfs, that allows defining rules that map string
28 identifiers to ranges of masters and channels. 28 identifiers to ranges of masters and channels. If these rules (policy)
29 are consistent with what decoder expects, it w 29 are consistent with what decoder expects, it will be able to properly
30 process the trace data. 30 process the trace data.
31 31
32 This policy is a tree structure containing rul 32 This policy is a tree structure containing rules (policy_node) that
33 have a name (string identifier) and a range of 33 have a name (string identifier) and a range of masters and channels
34 associated with it, located in "stp-policy" su 34 associated with it, located in "stp-policy" subsystem directory in
35 configfs. The topmost directory's name (the po 35 configfs. The topmost directory's name (the policy) is formatted as
36 the STM device name to which this policy appli 36 the STM device name to which this policy applies and an arbitrary
37 string identifier separated by a stop. From th 37 string identifier separated by a stop. From the example above, a rule
38 may look like this:: 38 may look like this::
39 39
40 $ ls /config/stp-policy/dummy_stm.my-p 40 $ ls /config/stp-policy/dummy_stm.my-policy/user
41 channels masters 41 channels masters
42 $ cat /config/stp-policy/dummy_stm.my- 42 $ cat /config/stp-policy/dummy_stm.my-policy/user/masters
43 48 63 43 48 63
44 $ cat /config/stp-policy/dummy_stm.my- 44 $ cat /config/stp-policy/dummy_stm.my-policy/user/channels
45 0 127 45 0 127
46 46
47 which means that the master allocation pool fo 47 which means that the master allocation pool for this rule consists of
48 masters 48 through 63 and channel allocation p 48 masters 48 through 63 and channel allocation pool has channels 0
49 through 127 in it. Now, any producer (trace so 49 through 127 in it. Now, any producer (trace source) identifying itself
50 with "user" identification string will be allo 50 with "user" identification string will be allocated a master and
51 channel from within these ranges. 51 channel from within these ranges.
52 52
53 These rules can be nested, for example, one ca 53 These rules can be nested, for example, one can define a rule "dummy"
54 under "user" directory from the example above 54 under "user" directory from the example above and this new rule will
55 be used for trace sources with the id string o 55 be used for trace sources with the id string of "user/dummy".
56 56
57 Trace sources have to open the stm class devic 57 Trace sources have to open the stm class device's node and write their
58 trace data into its file descriptor. 58 trace data into its file descriptor.
59 59
60 In order to find an appropriate policy node fo 60 In order to find an appropriate policy node for a given trace source,
61 several mechanisms can be used. First, a trace 61 several mechanisms can be used. First, a trace source can explicitly
62 identify itself by calling an STP_POLICY_ID_SE 62 identify itself by calling an STP_POLICY_ID_SET ioctl on the character
63 device's file descriptor, providing their id s 63 device's file descriptor, providing their id string, before they write
64 any data there. Secondly, if they chose not to 64 any data there. Secondly, if they chose not to perform the explicit
65 identification (because you may not want to pa 65 identification (because you may not want to patch existing software
66 to do this), they can just start writing the d 66 to do this), they can just start writing the data, at which point the
67 stm core will try to find a policy node with t 67 stm core will try to find a policy node with the name matching the
68 task's name (e.g., "syslogd") and if one exist 68 task's name (e.g., "syslogd") and if one exists, it will be used.
69 Thirdly, if the task name can't be found among 69 Thirdly, if the task name can't be found among the policy nodes, the
70 catch-all entry "default" will be used, if it 70 catch-all entry "default" will be used, if it exists. This entry also
71 needs to be created and configured by the syst 71 needs to be created and configured by the system administrator or
72 whatever tools are taking care of the policy c 72 whatever tools are taking care of the policy configuration. Finally,
73 if all the above steps failed, the write() to 73 if all the above steps failed, the write() to an stm file descriptor
74 will return a error (EINVAL). 74 will return a error (EINVAL).
75 75
76 Previously, if no policy nodes were found for 76 Previously, if no policy nodes were found for a trace source, the stm
77 class would silently fall back to allocating t 77 class would silently fall back to allocating the first available
78 contiguous range of master/channels from the b 78 contiguous range of master/channels from the beginning of the device's
79 master/channel range. The new requirement for 79 master/channel range. The new requirement for a policy node to exist
80 will help programmers and sysadmins identify g 80 will help programmers and sysadmins identify gaps in configuration
81 and have better control over the un-identified 81 and have better control over the un-identified sources.
82 82
83 Some STM devices may allow direct mapping of t 83 Some STM devices may allow direct mapping of the channel mmio regions
84 to userspace for zero-copy writing. One mappab 84 to userspace for zero-copy writing. One mappable page (in terms of
85 mmu) will usually contain multiple channels' m 85 mmu) will usually contain multiple channels' mmios, so the user will
86 need to allocate that many channels to themsel 86 need to allocate that many channels to themselves (via the
87 aforementioned ioctl() call) to be able to do 87 aforementioned ioctl() call) to be able to do this. That is, if your
88 stm device's channel mmio region is 64 bytes a 88 stm device's channel mmio region is 64 bytes and hardware page size is
89 4096 bytes, after a successful STP_POLICY_ID_S 89 4096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with
90 width==64, you should be able to mmap() one pa 90 width==64, you should be able to mmap() one page on this file
91 descriptor and obtain direct access to an mmio 91 descriptor and obtain direct access to an mmio region for 64 channels.
92 92
93 Examples of STM devices are Intel(R) Trace Hub 93 Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM
94 [2]. 94 [2].
95 95
96 stm_source 96 stm_source
97 ========== 97 ==========
98 98
99 For kernel-based trace sources, there is "stm_ 99 For kernel-based trace sources, there is "stm_source" device
100 class. Devices of this class can be connected 100 class. Devices of this class can be connected and disconnected to/from
101 stm devices at runtime via a sysfs attribute c 101 stm devices at runtime via a sysfs attribute called "stm_source_link"
102 by writing the name of the desired stm device 102 by writing the name of the desired stm device there, for example::
103 103
104 $ echo dummy_stm.0 > /sys/class/stm_so 104 $ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link
105 105
106 For examples on how to use stm_source interfac 106 For examples on how to use stm_source interface in the kernel, refer
107 to stm_console, stm_heartbeat or stm_ftrace dr 107 to stm_console, stm_heartbeat or stm_ftrace drivers.
108 108
109 Each stm_source device will need to assume a m 109 Each stm_source device will need to assume a master and a range of
110 channels, depending on how many channels it re 110 channels, depending on how many channels it requires. These are
111 allocated for the device according to the poli 111 allocated for the device according to the policy configuration. If
112 there's a node in the root of the policy direc 112 there's a node in the root of the policy directory that matches the
113 stm_source device's name (for example, "consol 113 stm_source device's name (for example, "console"), this node will be
114 used to allocate master and channel numbers. I 114 used to allocate master and channel numbers. If there's no such policy
115 node, the stm core will use the catch-all entr 115 node, the stm core will use the catch-all entry "default", if one
116 exists. If neither policy nodes exist, the wri 116 exists. If neither policy nodes exist, the write() to stm_source_link
117 will return an error. 117 will return an error.
118 118
119 stm_console 119 stm_console
120 =========== 120 ===========
121 121
122 One implementation of this interface also used 122 One implementation of this interface also used in the example above is
123 the "stm_console" driver, which basically prov 123 the "stm_console" driver, which basically provides a one-way console
124 for kernel messages over an stm device. 124 for kernel messages over an stm device.
125 125
126 To configure the master/channel pair that will 126 To configure the master/channel pair that will be assigned to this
127 console in the STP stream, create a "console" 127 console in the STP stream, create a "console" policy entry (see the
128 beginning of this text on how to do that). Whe 128 beginning of this text on how to do that). When initialized, it will
129 consume one channel. 129 consume one channel.
130 130
131 stm_ftrace 131 stm_ftrace
132 ========== 132 ==========
133 133
134 This is another "stm_source" device, once the 134 This is another "stm_source" device, once the stm_ftrace has been
135 linked with an stm device, and if "function" t 135 linked with an stm device, and if "function" tracer is enabled,
136 function address and parent function address w 136 function address and parent function address which Ftrace subsystem
137 would store into ring buffer will be exported 137 would store into ring buffer will be exported via the stm device at
138 the same time. 138 the same time.
139 139
140 Currently only Ftrace "function" tracer is sup 140 Currently only Ftrace "function" tracer is supported.
141 141
142 * [1] https://software.intel.com/sites/default 142 * [1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
143 * [2] http://infocenter.arm.com/help/index.jsp 143 * [2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html
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