[PATCH 2/2] drm: Shortcircuit vblank queries

[Chris Wilson]

Bypass all the spinlocks and return the last timestamp and counter from
the last vblank if the driver delcares that it is accurate (and stable
across on/off), and the vblank is currently enabled.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Michel Dänzer <michel at daenzer.net>
Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Cc: Dave Airlie <airlied at redhat.com>,
Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
---
 drivers/gpu/drm/drm_irq.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index ba80b51b4b00..be9c210bb22e 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1538,6 +1538,17 @@ err_put:
 	return ret;
 }
 
+static bool drm_wait_vblank_is_query(union drm_wait_vblank *vblwait)
+{
+	if (vblwait->request.sequence)
+		return false;
+
+	return _DRM_VBLANK_RELATIVE ==
+		(vblwait->request.type & (_DRM_VBLANK_TYPES_MASK |
+					  _DRM_VBLANK_EVENT |
+					  _DRM_VBLANK_NEXTONMISS));
+}
+
 /*
  * Wait for VBLANK.
  *
@@ -1587,6 +1598,21 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 
 	vblank = &dev->vblank[crtc];
 
+	/* If the counter is currently enabled and accurate, short-circuit queries
+	 * to return the cached timestamp of the last vblank.
+	 */
+	if (dev->vblank_disable_immediate &&
+	    drm_wait_vblank_is_query(vblwait) &&
+	    vblank->enabled) {
+		struct timeval now;
+
+		vblwait->reply.sequence =
+			drm_vblank_count_and_time(dev, crtc, &now);
+		vblwait->reply.tval_sec = now.tv_sec;
+		vblwait->reply.tval_usec = now.tv_usec;
+		return 0;
+	}
+
 	ret = drm_vblank_get(dev, crtc);
 	if (ret) {
 		DRM_DEBUG("failed to acquire vblank counter, %d\n", ret);
-- 
2.1.4