drm: Skip the waitqueue setup for vblank queries

Avoid adding to the waitqueue and reprobing the current vblank if the
caller is only querying the current vblank sequence and timestamp, where
we know that the wait would return immediately.

v2: Add CRTC identifier to debug messages
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Dave Airlie <airlied@redhat.com>,
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: NMichel Dänzer <michel@daenzer.net>
Reviewed-and-tested-by: NMario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170315204027.20160-2-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_irq.c

@@ -1610,7 +1610,7 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 
 	ret = drm_vblank_get(dev, pipe);
 	if (ret) {
-		DRM_DEBUG("failed to acquire vblank counter, %d\n", ret);
+		DRM_DEBUG("crtc %d failed to acquire vblank counter, %d\n", pipe, ret);
 		return ret;
 	}
 	seq = drm_vblank_count(dev, pipe);
@@ -1638,6 +1638,7 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 		return drm_queue_vblank_event(dev, pipe, vblwait, file_priv);
 	}
 
+	if (vblwait->request.sequence != seq) {
 		DRM_DEBUG("waiting on vblank count %u, crtc %u\n",
 			  vblwait->request.sequence, pipe);
 		DRM_WAIT_ON(ret, vblank->queue, 3 * HZ,
@@ -1645,6 +1646,7 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 			       vblwait->request.sequence) <= (1 << 23)) ||
 			     !vblank->enabled ||
 			     !dev->irq_enabled));
+	}
 
 	if (ret != -EINTR) {
 		struct timeval now;
@@ -1653,10 +1655,10 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 		vblwait->reply.tval_sec = now.tv_sec;
 		vblwait->reply.tval_usec = now.tv_usec;
 
-		DRM_DEBUG("returning %u to client\n",
-			  vblwait->reply.sequence);
+		DRM_DEBUG("crtc %d returning %u to client\n",
+			  pipe, vblwait->reply.sequence);
 	} else {
-		DRM_DEBUG("vblank wait interrupted by signal\n");
+		DRM_DEBUG("crtc %d vblank wait interrupted by signal\n", pipe);
 	}
 
 done: