[PATCH] drm/vblank: Fixup and document timestamp update/read barriers

[Chris Wilson]

On Wed, Apr 15, 2015 at 09:17:02AM +0200, Daniel Vetter wrote:
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index c8a34476570a..23bfbc61a494 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -74,6 +74,33 @@ module_param_named(vblankoffdelay, drm_vblank_offdelay, int, 0600);
>  module_param_named(timestamp_precision_usec, drm_timestamp_precision, int, 0600);
>  module_param_named(timestamp_monotonic, drm_timestamp_monotonic, int, 0600);
>  
> +static void store_vblank(struct drm_device *dev, int crtc,
> +			 unsigned vblank_count_inc,
> +			 struct timeval *t_vblank)
> +{
> +	struct drm_vblank_crtc *vblank = &dev->vblank[crtc];
> +	u32 tslot;
> +
> +	assert_spin_locked(&dev->vblank_time_lock);
> +
> +	if (t_vblank) {
> +		tslot = vblank->count + vblank_count_inc;
> +		vblanktimestamp(dev, crtc, tslot) = *t_vblank;
> +	}

It is not obvious this updates the right tslot in all circumstances.
Care to explain?

Otherwise the rest looks consistent with seqlock, using the
vblank->count as the latch.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre