Documentation: coresight: Update coresight configuration docs

Update the CoreSight System Configuration document to cover the
use of loadable modules to add configurations and features
to the system.

Signed-off-by: Mike Leach <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mathieu Poirier <[e-mail]>

Documentation/trace/coresight/coresight-config.rst

···
     autofdo
     $ cd autofdo/
     $ ls
-    description   preset1  preset3  preset5  preset7  preset9
-    feature_refs  preset2  preset4  preset6  preset8
+    description  feature_refs  preset1  preset3  preset5  preset7  preset9
+    enable       preset        preset2  preset4  preset6  preset8
     $ cat description
     Setup ETMs with strobing for autofdo
     $ cat feature_refs
     strobing
 
-Each preset declared has a preset<n> subdirectory declared. The values for
+Each preset declared has a 'preset<n>' subdirectory declared. The values for
 the preset can be examined::
 
     $ cat preset1/values
     strobing.window = 0x1388 strobing.period = 0x2
     $ cat preset2/values
     strobing.window = 0x1388 strobing.period = 0x4
+
+The 'enable' and 'preset' files allow the control of a configuration when
+using CoreSight with sysfs.
 
 The features referenced by the configuration can be examined in the features
 directory::
···
 
 When configurations are selected in this way, then the trace sink used is
 automatically selected.
+
+Using Configurations in sysfs
+=============================
+
+Coresight can be controlled using sysfs. When this is in use then a configuration
+can be made active for the devices that are used in the sysfs session.
+
+In a configuration there are 'enable' and 'preset' files.
+
+To enable a configuration for use with sysfs::
+
+    $ cd configurations/autofdo
+    $ echo 1 > enable
+
+This will then use any default parameter values in the features - which can be
+adjusted as described above.
+
+To use a preset<n> set of parameter values::
+
+    $ echo 3 > preset
+
+This will select preset3 for the configuration.
+The valid values for preset are 0 - to deselect presets, and any value of
+<n> where a preset<n> sub-directory is present.
+
+Note that the active sysfs configuration is a global parameter, therefore
+only a single configuration can be active for sysfs at any one time.
+Attempting to enable a second configuration will result in an error.
+Additionally, attempting to disable the configuration while in use will
+also result in an error.
+
+The use of the active configuration by sysfs is independent of the configuration
+used in perf.
+
+
+Creating and Loading Custom Configurations
+==========================================
+
+Custom configurations and / or features can be dynamically loaded into the
+system by using a loadable module.
+
+An example of a custom configuration is found in ./samples/coresight.
+
+This creates a new configuration that uses the existing built in
+strobing feature, but provides a different set of presets.
+
+When the module is loaded, then the configuration appears in the configfs
+file system and is selectable in the same way as the built in configuration
+described above.
+
+Configurations can use previously loaded features. The system will ensure
+that it is not possible to unload a feature that is currently in use, by
+enforcing the unload order as the strict reverse of the load order.