[PATCH] drm/vblank: Avoid storing a timestamp for the same frame twice

[Daniel Vetter]

On Thu, Feb 04, 2021 at 04:04:00AM +0200, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> drm_vblank_restore() exists because certain power saving states
> can clobber the hardware frame counter. The way it does this is
> by guesstimating how many frames were missed purely based on
> the difference between the last stored timestamp vs. a newly
> sampled timestamp.
> 
> If we should call this function before a full frame has
> elapsed since we sampled the last timestamp we would end up
> with a possibly slightly different timestamp value for the
> same frame. Currently we will happily overwrite the already
> stored timestamp for the frame with the new value. This
> could cause userspace to observe two different timestamps
> for the same frame (and the timestamp could even go
> backwards depending on how much error we introduce when
> correcting the timestamp based on the scanout position).
> 
> To avoid that let's not update the stored timestamp unless we're
> also incrementing the sequence counter. We do still want to update
> vblank->last with the freshly sampled hw frame counter value so
> that subsequent vblank irqs/queries can actually use the hw frame
> counter to determine how many frames have elapsed.

Hm I'm not getting the reason for why we store the updated hw vblank
counter?

There's definitely a race when we grab the hw timestamp at a bad time
(which can't happen for the irq handler, realistically), so maybe we
should first adjust that to make sure we never store anything inconsistent
in the vblank state?

And when we have that we should be able to pull the inc == 0 check out
into _restore(), including comment. Which I think should be cleaner.

Or I'm totally off with why you want to store the hw vblank counter?

> 
> Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/drm_vblank.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 893165eeddf3..e127a7db2088 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -176,6 +176,17 @@ static void store_vblank(struct drm_device *dev, unsigned int pipe,
>  
>  	vblank->last = last;
>  
> +	/*
> +	 * drm_vblank_restore() wants to always update
> +	 * vblank->last since we can't trust the frame counter
> +	 * across power saving states. But we don't want to alter
> +	 * the stored timestamp for the same frame number since
> +	 * that would cause userspace to potentially observe two
> +	 * different timestamps for the same frame.
> +	 */
> +	if (vblank_count_inc == 0)
> +		return;
> +
>  	write_seqlock(&vblank->seqlock);
>  	vblank->time = t_vblank;
>  	atomic64_add(vblank_count_inc, &vblank->count);
> -- 
> 2.26.2
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch