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>

Documentation/trace/stm.rst

@@ -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
 ===========