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 or stm_heartbeat 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.

[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