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stm class: Update documentation to match the new identification rules 

The rules and order of identification of trace sources against the
"stp-policy" have changed; update the documentation to reflect that.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Alexander Shishkin 2018-10-05 15:43:02 +03:00 committed by Greg Kroah-Hartman
parent 4cb3653df0
commit 3ef230370e
1 changed files with 27 additions and 9 deletions
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36
Documentation/trace/stm.rst
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@ -53,12 +53,30 @@ 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.
trace data into its file descriptor.
In order to find an appropriate policy node for a given trace source,
several mechanisms can be used. First, a trace source can explicitly
identify itself by calling an STP_POLICY_ID_SET ioctl on the character
device's file descriptor, providing their id string, before they write
any data there. Secondly, if they chose not to perform the explicit
identification (because you may not want to patch existing software
to do this), they can just start writing the data, at which point the
stm core will try to find a policy node with the name matching the
task's name (e.g., "syslogd") and if one exists, it will be used.
Thirdly, if the task name can't be found among the policy nodes, the
catch-all entry "default" will be used, if it exists. This entry also
needs to be created and configured by the system administrator or
whatever tools are taking care of the policy configuration. Finally,
if all the above steps failed, the write() to an stm file descriptor
will return a error (EINVAL).
Previously, if no policy nodes were found for a trace source, the stm
class would silently fall back to allocating the first available
contiguous range of master/channels from the beginning of the device's
master/channel range. The new requirement for a policy node to exist
will help programmers and sysadmins identify gaps in configuration
and have better control over the un-identified sources.
Some STM devices may allow direct mapping of the channel mmio regions
to userspace for zero-copy writing. One mappable page (in terms of
@ -92,9 +110,9 @@ 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.
node, the stm core will use the catch-all entry "default", if one
exists. If neither policy nodes exist, the write() to stm_source_link
will return an error.
stm_console
===========