Re: [PATCH 14/19] drm: Don't update vblank timestamp when the counter didn't change

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Aug 06, 2014 at 02:49:57PM +0300, ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> If we already have a timestamp for the current vblank counter, don't
> update it with a new timestmap. Small errors can creep in between two
> timestamp queries for the same vblank count, which could be confusing to
> userspace when it queries the timestamp for the same vblank sequence
> number twice.
> 
> This problem gets exposed when the vblank disable timer is not used
> (or is set to expire quickly) and thus we can get multiple vblank
> disable<->enable transition during the same frame which would all
> attempt to update the timestamp with the latest estimate.
> 
> Testcase: igt/kms_flip/flip-vs-expired-vblank
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_irq.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index af33df1..0523f5b 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -106,6 +106,9 @@ static void drm_update_vblank_count(struct drm_device *dev, int crtc)
>  	DRM_DEBUG("enabling vblank interrupts on crtc %d, missed %d\n",
>  		  crtc, diff);
>  
> +	if (diff == 0)
> +		return;
> +
>  	/* Reinitialize corresponding vblank timestamp if high-precision query
>  	 * available. Skip this step if query unsupported or failed. Will
>  	 * reinitialize delayed at next vblank interrupt in that case.
> -- 
> 1.8.5.5
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch