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