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

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Apr 15, 2015 at 09:17:03AM +0100, Chris Wilson wrote:
> 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?

Writers are synchronized with vblank_time_lock, so there shouldn't be any
races. Mario also has a patch to clear the ts slot if we don't have
anything to set it too (that one will conflict ofc).

Or what exactly do you mean?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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