Re: [PATCH 1/9] drm: Always reject drm_vblank_get() after drm_vblank_off()

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

On Mon, May 26, 2014 at 03:21:41PM +0200, Daniel Vetter wrote:
> On Mon, May 26, 2014 at 02:46:24PM +0300, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > Make sure drm_vblank_get() never succeeds when called between
> > drm_vblank_off() and drm_vblank_on(). Borrow a trick from the
> > old drm_vblank_{pre,post}_modeset() functions and just bump
> > the refcount in drm_vblank_off() and drop it in drm_vblank_on().
> > 
> > Hopefully the use of inmodeset won't conflict badly with
> > drm_vblank_{pre,post}_modeset().
> > 
> > For i915 there's a window between drm_vblank_off() and marking the
> > crtc as inactive where the current code still allows drm_vblank_get().
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> Afaics we never check anywhere for inmodeset in drm_vblank_get, so how
> does this work?

If refcount is already >0 and vblank irq is disabled, drm_vblank_get() will
return -EINVAL.

> -Daniel
> 
> > ---
> >  drivers/gpu/drm/drm_irq.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> > index b565372..7b1e08c 100644
> > --- a/drivers/gpu/drm/drm_irq.c
> > +++ b/drivers/gpu/drm/drm_irq.c
> > @@ -1039,6 +1039,15 @@ void drm_vblank_off(struct drm_device *dev, int crtc)
> >  	}
> >  	spin_unlock(&dev->event_lock);
> >  
> > +	/*
> > +	 * Prevent subsequent drm_vblank_get() from re-enabling
> > +	 * the vblank interrupt by bumping the refcount.
> > +	 */
> > +	if (!dev->vblank[crtc].inmodeset) {
> > +		atomic_inc(&dev->vblank[crtc].refcount);
> > +		dev->vblank[crtc].inmodeset = 1;
> > +	}
> > +
> >  	spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
> >  }
> >  EXPORT_SYMBOL(drm_vblank_off);
> > @@ -1079,6 +1088,11 @@ void drm_vblank_on(struct drm_device *dev, int crtc)
> >  	unsigned long irqflags;
> >  
> >  	spin_lock_irqsave(&dev->vbl_lock, irqflags);
> > +	/* Drop our private "prevent drm_vblank_get" refcount */
> > +	if (dev->vblank[crtc].inmodeset) {
> > +		atomic_dec(&dev->vblank[crtc].refcount);
> > +		dev->vblank[crtc].inmodeset = 0;
> > +	}
> >  	/* re-enable interrupts if there's are users left */
> >  	if (atomic_read(&dev->vblank[crtc].refcount) != 0)
> >  		WARN_ON(drm_vblank_enable(dev, crtc));
> > -- 
> > 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