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

[Chris Wilson]

Quoting Chris Wilson (2020-06-11 13:30:37)
> 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;

/2 != >>2
-Chris