[Intel-gfx] [PATCH 1/2] drm/vblank: Estimate sample time

[Chris Wilson]

Since we have a precise start/end time for the sample, the actual time
the HW was read back is within that interval, and more likely closer to
the mean of the interval. Use the mean sample time when estimating the
vblank time.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/drm_vblank.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index da7b0b0c1090..79a5461d3773 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -710,15 +710,18 @@ drm_crtc_vblank_helper_get_vblank_timestamp_internal(
 	delta_ns = div_s64(1000000LL * (vpos * mode->crtc_htotal + hpos),
 			   mode->crtc_clock);
 
+	/* Estimate when the sample was taken */
+	stime += (etime - stime) >> 2;
+
 	/* Subtract time delta from raw timestamp to get final
 	 * vblank_time timestamp for end of vblank.
 	 */
-	*vblank_time = ktime_sub_ns(etime, delta_ns);
+	*vblank_time = ktime_sub_ns(stime, delta_ns);
 
 	if (!drm_debug_enabled(DRM_UT_VBL))
 		return true;
 
-	ts_etime = ktime_to_timespec64(etime);
+	ts_etime = ktime_to_timespec64(stime);
 	ts_vblank_time = ktime_to_timespec64(*vblank_time);
 
 	DRM_DEBUG_VBL("crtc %u : v p(%d,%d)@ %lld.%06ld -> %lld.%06ld [e %d us, %d rep]\n",
-- 
2.27.0