Re: [PATCH] drm/vblank: Fix delta_ns to an absolute value

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Sep 01, 2017 at 04:07:16PM +0900, Hoegeun Kwon wrote:
> If scanout started, we should reduce etime by delta_ns. But delta_ns
> is negative if scanout has not started. If delta_ns is negative,
> subtraction of delta_ns from etime increases etime. This is wrong, the
> etime should not be increased, so you have to make delta_ns an
> absolute value.
> 
> Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
> ---
> 
> Hello all,
> 
> I think that the etime should not be increased.
> In cases where delta_ns is negative, if you get time again after an
> interrupt call, there is a problem that the time obtained from the
> interrupt becomes the future time instead of the past time.
> 
> Please let me know if this patch is wrong.

It is wrong. The timestamp corresponds to the first active pixel of the
frame/field. So while between vblank start and active start we expect
to get a timestamp that is in the future.

> 
> Best regards,
> Hoegeun
> 
>  drivers/gpu/drm/drm_vblank.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 70f2b95..a3e0176 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -684,7 +684,7 @@ bool drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
>  	/* Subtract time delta from raw timestamp to get final
>  	 * vblank_time timestamp for end of vblank.
>  	 */
> -	etime = ktime_sub_ns(etime, delta_ns);
> +	etime = ktime_sub_ns(etime, abs(delta_ns));
>  	*vblank_time = ktime_to_timeval(etime);
>  
>  	DRM_DEBUG_VBL("crtc %u : v p(%d,%d)@ %ld.%ld -> %ld.%ld [e %d us, %d rep]\n",
> -- 
> 1.9.1
> 
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-- 
Ville Syrjälä
Intel OTC