[Intel-gfx] [PATCH v2] drm/vblank: Estimate sample time

[Ville Syrjälä]

On Thu, Jun 11, 2020 at 01:34:47PM +0100, Chris Wilson wrote:
> 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>

Seems reasonable.

Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

> ---
>  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..a7043d268cca 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) >> 1;
> +
>  	/* 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
> 
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-- 
Ville Syrjälä
Intel