drm/sched: Document what the timedout_job method should do

The documentation is a bit vague and doesn't really describe what the
->timedout_job() is expected to do. Let's add a few more details.

v5:
* New patch
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-2-boris.brezillon@collabora.com

include/drm/gpu_scheduler.h

@@ -239,6 +239,20 @@ struct drm_sched_backend_ops {
 	 * @timedout_job: Called when a job has taken too long to execute,
 	 * to trigger GPU recovery.
 	 *
+	 * This method is called in a workqueue context.
+	 *
+	 * Drivers typically issue a reset to recover from GPU hangs, and this
+	 * procedure usually follows the following workflow:
+	 *
+	 * 1. Stop the scheduler using drm_sched_stop(). This will park the
+	 *    scheduler thread and cancel the timeout work, guaranteeing that
+	 *    nothing is queued while we reset the hardware queue
+	 * 2. Try to gracefully stop non-faulty jobs (optional)
+	 * 3. Issue a GPU reset (driver-specific)
+	 * 4. Re-submit jobs using drm_sched_resubmit_jobs()
+	 * 5. Restart the scheduler using drm_sched_start(). At that point, new
+	 *    jobs can be queued, and the scheduler thread is unblocked
+	 *
 	 * Return DRM_GPU_SCHED_STAT_NOMINAL, when all is normal,
 	 * and the underlying driver has started or completed recovery.
 	 *