Re: [PATCH 4/6] drm: Fix treatment of drm_vblank_offdelay in drm_vblank_on()

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

On Tue, Feb 09, 2016 at 11:06:18AM +0100, Daniel Vetter wrote:
> On Mon, Feb 08, 2016 at 02:13:27AM +0100, Mario Kleiner wrote:
> > drm_vblank_offdelay can have three different types of values:
> > 
> > < 0 is to be always treated the same as dev->vblank_disable_immediate
> > = 0 is to be treated as "never disable vblanks"
> > > 0 is to be treated as disable immediate if kms driver wants it
> >     that way via dev->vblank_disable_immediate. Otherwise it is
> >     a disable timeout in msecs.
> > 
> > This got broken in Linux 3.18+ for the implementation of
> > drm_vblank_on. If the user specified a value of zero which should
> > always reenable vblank irqs in this function, a kms driver could
> > override the users choice by setting vblank_disable_immediate
> > to true. This patch fixes the regression and keeps the user in
> > control.
> > 
> > Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
> > Cc: <stable@vger.kernel.org> # 3.18+
> > Cc: michel@daenzer.net
> > Cc: vbabka@suse.cz
> > Cc: ville.syrjala@linux.intel.com
> > Cc: daniel.vetter@ffwll.ch
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: alexander.deucher@amd.com
> > Cc: christian.koenig@amd.com
> > ---
> >  drivers/gpu/drm/drm_irq.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> > index 5c27ad3..fb17c45 100644
> > --- a/drivers/gpu/drm/drm_irq.c
> > +++ b/drivers/gpu/drm/drm_irq.c
> > @@ -1492,8 +1492,8 @@ void drm_vblank_on(struct drm_device *dev, unsigned int pipe)
> >  	 * re-enable interrupts if there are users left, or the
> >  	 * user wishes vblank interrupts to be enabled all the time.
> >  	 */
> > -	if (atomic_read(&vblank->refcount) != 0 ||
> > -	    (!dev->vblank_disable_immediate && drm_vblank_offdelay == 0))
> > +	if (atomic_read(&vblank->refcount) != 0 || drm_vblank_offdelay == 0 ||
> > +	    (!dev->vblank_disable_immediate && drm_vblank_offdelay > 0))
> 
> Hm, shouldn't we change this to only enable the vblank irq if we need it,
> i.e. offdelay == 0? For delayed disabling there's kinda no need to enable
> it superflously after a modeset, if userspace didn't yet ask for vblank
> timestamps. But then is was specifically added by Ville in cd19e52aee922,
> so I guess someone really wants this.

IIRC what I wanted was to just re-enable the interrupt for the offdelay==0
case. I think it just ended up as a mess due to changing some of the
semantics of offdelay<0 vs. offdelay==0 vs. disable_immediate during the 
review of the series. So yeah, given how drm_vblank_put() works now, I'd
just make this check for offdelay==0.

> 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> >  		WARN_ON(drm_vblank_enable(dev, pipe));
> >  	spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
> >  }
> > -- 
> > 1.9.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Ville Syrjälä
Intel OTC