drm/i915: Note the addition of timeslicing to the pretend scheduler

Since writing the comment that the scheduler is entirely passive, we've
added minimal timeslicing which adds the most primitive of active
elements (a timeout and reschedule).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010071434.31195-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_scheduler_types.h

@@ -49,6 +49,15 @@ struct i915_sched_attr {
  * DAG of each request, we are able to insert it into a sorted queue when it
  * is ready, and are able to reorder its portion of the graph to accommodate
  * dynamic priority changes.
+ *
+ * Ok, there is now one active element to the "scheduler" in the backends.
+ * We let a new context run for a small amount of time before re-evaluating
+ * the run order. As we re-evaluate, we maintain the strict ordering of
+ * dependencies, but attempt to rotate the active contexts (the current context
+ * is put to the back of its priority queue, then reshuffling its dependents).
+ * This provides minimal timeslicing and prevents a userspace hog (e.g.
+ * something waiting on a user semaphore [VkEvent]) from denying service to
+ * others.
  */
 struct i915_sched_node {
 	struct list_head signalers_list; /* those before us, we depend upon */