Re: [PATCH] drm/i915: Flush the pending flips on the CRTC before modification

[Chris Wilson <chris@chris-wilson.co.uk>]

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On Fri, 28 Sep 2012 13:05:51 +0300, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> On Thu, Sep 27, 2012 at 09:25:58PM +0100, Chris Wilson wrote:
> > This was meant to be the purpose of the
> > intel_crtc_wait_for_pending_flips() function which is called whilst
> > preparing the CRTC for a modeset or before disabling. However, as Ville
> > Syrjala pointed out, we set the pending flip notification on the old
> > framebuffer that is no longer attached to the CRTC by the time we come
> > to flush the pending operations. Instead, we can simply wait on the
> > pending unpin work to be finished on this CRTC, knowning that the
> > hardware has therefore finished modifying the registers, before proceeding
> > with our direct access.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/intel_display.c |   24 ++++++++++++++++++++++--
> >  1 file changed, 22 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index a262326..39df185 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -2896,15 +2896,36 @@ static void ironlake_fdi_disable(struct drm_crtc *crtc)
> >  	udelay(100);
> >  }
> >  
> > +static bool intel_crtc_has_pending_flip(struct drm_crtc *crtc)
> > +{
> > +	struct drm_device *dev = crtc->dev;
> > +	struct drm_i915_private *dev_priv = dev->dev_private;
> > +	unsigned long flags;
> > +	bool pending;
> > +
> > +	if (atomic_read(&dev_priv->mm.wedged))
> > +		return false;
> > +
> > +	spin_lock_irqsave(&dev->event_lock, flags);
> > +	pending = to_intel_crtc(crtc)->unpin_work != NULL;
> > +	spin_unlock_irqrestore(&dev->event_lock, flags);
> 
> The locking looks pointless here.

It does rather. Being pedagogical we should probably leave a mb of some
sort in there...

  pending = to_intel_crtc(crtc)->unpin_work != NULL;
  smp_rmb();

with the existing spin_lock providing the necessary barriers before the
wake_up();
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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