drm/i915/gt: Only retire on the last breadcrumb if the last request

We use the completion of the last active breadcrumb to retire the
requests along a timeline. This is purely opportunistic as nothing
guarantees that any particular timeline is terminated by a breadcrumb;
except for parking the engine where we explicitly add a breadcrumb so
that we park quickly and do an explicit retire upon signaling to reduce
the latency dramatically (avoiding a retire worker roundtrip).

With scheduling, we anticipate retiring completed timelines as a matter
of course. Performing the same action from inside the breadcrumbs is
intended to provide similar functionality for legacy ringbuffer
submission.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108204026.20682-5-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_breadcrumbs.c

@@ -257,17 +257,17 @@ static void signal_irq_work(struct irq_work *work)
 			list_del_rcu(&rq->signal_link);
 			release = remove_signaling_context(b, ce);
 			spin_unlock(&ce->signal_lock);
+			if (release) {
+				if (intel_timeline_is_last(ce->timeline, rq))
+					add_retire(b, ce->timeline);
+				intel_context_put(ce);
+			}
 
 			if (__dma_fence_signal(&rq->fence))
 				/* We own signal_node now, xfer to local list */
 				signal = slist_add(&rq->signal_node, signal);
 			else
 				i915_request_put(rq);
-
-			if (release) {
-				add_retire(b, ce->timeline);
-				intel_context_put(ce);
-			}
 		}
 	}
 	atomic_dec(&b->signaler_active);

drivers/gpu/drm/i915/gt/intel_execlists_submission.c

@@ -640,7 +640,7 @@ static inline void __execlists_schedule_out(struct i915_request *rq)
 	 * If we have just completed this context, the engine may now be
 	 * idle and we want to re-enter powersaving.
 	 */
-	if (list_is_last_rcu(&rq->link, &ce->timeline->requests) &&
+	if (intel_timeline_is_last(ce->timeline, rq) &&
 	    __i915_request_is_complete(rq))
 		intel_engine_add_retire(engine, ce->timeline);
 

drivers/gpu/drm/i915/gt/intel_timeline.h

@@ -110,4 +110,11 @@ void intel_gt_show_timelines(struct intel_gt *gt,
 						  const char *prefix,
 						  int indent));
 
+static inline bool
+intel_timeline_is_last(const struct intel_timeline *tl,
+		       const struct i915_request *rq)
+{
+	return list_is_last_rcu(&rq->link, &tl->requests);
+}
+
 #endif