[Intel-gfx] [PATCH] drm: Return current vblank value for drmWaitVBlank queries

[Chris Wilson]

When userspace queries the current vblank for the CRTC, we can reply
with the cached value (using atomic reads to serialise with the vblank
interrupt as necessary) without having to touch registers. In the
instant disable case, this saves us from enabling/disabling the vblank
around every query, greatly reducing the number of registers read and
written.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Imre Deak <imre.deak at intel.com>
Cc: Daniel Vetter <daniel.vetter at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Cc: Dave Airlie <airlied at redhat.com>
---
 drivers/gpu/drm/drm_irq.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index c8a34476570a..6c4570082b65 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1585,7 +1585,18 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 	if (crtc >= dev->num_crtcs)
 		return -EINVAL;
 
-	vblank = &dev->vblank[crtc];
+	/* Fast-path the query for the current value (without an event)
+	 * to avoid having to enable/disable the vblank interrupts.
+	 */
+	if ((vblwait->request.type & (_DRM_VBLANK_TYPES_MASK | _DRM_VBLANK_FLAGS_MASK)) == _DRM_VBLANK_RELATIVE &&
+	    vblwait->request.sequence == 0) {
+		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) {
@@ -1619,6 +1630,8 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 
 	DRM_DEBUG("waiting on vblank count %d, crtc %d\n",
 		  vblwait->request.sequence, crtc);
+
+	vblank = &dev->vblank[crtc];
 	vblank->last_wait = vblwait->request.sequence;
 	DRM_WAIT_ON(ret, vblank->queue, 3 * HZ,
 		    (((drm_vblank_count(dev, crtc) -
-- 
2.1.4