drm/i915: Only wake the waiter from the interrupt if passed

As we now check if the seqno is complete in order to signal the fence,
we can also decide not to wake up the first_waiter until it is ready
(since it is waiting on the same seqno). The only caveat is that if we
need the engine->irq_seqno_barrier to enforce some coherency between an
interrupt and the seqno read, we have to always wake the waiter in order
to perform that heavyweight barrier.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170918162734.21294-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -1038,6 +1038,8 @@ static void notify_ring(struct intel_engine_cs *engine)
 	spin_lock(&engine->breadcrumbs.irq_lock);
 	wait = engine->breadcrumbs.irq_wait;
 	if (wait) {
+		bool wakeup = engine->irq_seqno_barrier;
+
 		/* We use a callback from the dma-fence to submit
 		 * requests after waiting on our own requests. To
 		 * ensure minimum delay in queuing the next request to
@@ -1050,12 +1052,15 @@ static void notify_ring(struct intel_engine_cs *engine)
 		 * and many waiters.
 		 */
 		if (i915_seqno_passed(intel_engine_get_seqno(engine),
-				      wait->seqno) &&
-		    !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
-			      &wait->request->fence.flags))
-			rq = i915_gem_request_get(wait->request);
+				      wait->seqno)) {
+			wakeup = true;
+			if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
+				      &wait->request->fence.flags))
+				rq = i915_gem_request_get(wait->request);
+		}
 
-		wake_up_process(wait->tsk);
+		if (wakeup)
+			wake_up_process(wait->tsk);
 	} else {
 		__intel_engine_disarm_breadcrumbs(engine);
 	}