Documentation/trace/stm.rst

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