Re: [Intel-gfx] [PATCH 17/19] drm/i915: Clear .last vblank count before drm_vblank_off() when sanitizing crtc state

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

On Wed, Aug 06, 2014 at 03:30:17PM +0200, Daniel Vetter wrote:
> On Wed, Aug 06, 2014 at 02:50:00PM +0300, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > We call drm_vblank_off() during crtc sanitation to make sure the
> > software and hardware states agree. However drm_vblank_off() will
> > try to update the vblank timestamp and sequence number which lands
> > us in some trouble.
> > 
> > As the pipe is disabled the hardware frame counter query will
> > return 0. If the .last doesn't match the code will try to add the
> > difference to the user visible sequence number. During driver init
> > that's OK as .last == 0, but during resume .last may be anything.
> > So we should make sure we don't try to apply the diff here by zeroing
> > .last beforehand. Otherwise there maybe be a random jump in the user
> > visible sequence number across suspend.
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_display.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index ae5f20d..c00bcd0 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -12918,8 +12918,11 @@ static void intel_sanitize_crtc(struct intel_crtc *crtc)
> >  	/* restore vblank interrupts to correct state */
> >  	if (crtc->active)
> >  		drm_vblank_on(dev, crtc->pipe);
> > -	else
> > +	else {
> > +		/* avoid random jumps in seq/ts */
> > +		dev->vblank[crtc->pipe].last = 0;
> 
> Should we have a drm_vblank_reset for this? I guess other drivers will
> have similar issues with "sorry, just lost it all" kind of transitions.
> 
> Also this case should only happen when we enter the system resume with the
> pipes in dpms off state. In that case we should have sampled something in
> drm_vblank_off already and resampling doesn't look like a good idea.
> 
> Do we instead need some protection in drm_vblank_off to avoid re-sampling?

Yeah, I suppose I could try to fix it in drm vblank code somehow. Just
resetting .last=0 in drm_vblank_off() would avoid the seq jump but would
still leave the vblank counter query in place. Admittedly the resulting
debug spam about vblank counter queries on disabled crtcs is rather
annoying.

> -Daniel
> 
> >  		drm_vblank_off(dev, crtc->pipe);
> > +	}
> >  
> >  	/* We need to sanitize the plane -> pipe mapping first because this will
> >  	 * disable the crtc (and hence change the state) if it is wrong. Note
> > -- 
> > 1.8.5.5
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

-- 
Ville Syrjälä
Intel OTC