[Intel-gfx] [PATCH 3/3] drm: Peek at the current counter/timestamp for 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.

This is dependent upon the both the hardware and driver to provide the
proper barriers to facilitate reading our bookkeeping outside of the
vblank interrupt and outside of the explicit vblank locks.

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 866cf58a36c5..00298b39e7fc 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1659,6 +1659,17 @@ 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.
  *
@@ -1708,6 +1719,21 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 
 	vblank = &dev->vblank[pipe];
 
+	/* 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, pipe, &now);
+		vblwait->reply.tval_sec = now.tv_sec;
+		vblwait->reply.tval_usec = now.tv_usec;
+		return 0;
+	}
+
 	ret = drm_vblank_get(dev, pipe);
 	if (ret) {
 		DRM_DEBUG("crtc %d failed to acquire vblank counter, %d\n", pipe, ret);
-- 
2.7.0.rc3