Re: [PATCH] drm/i915: Flush outstanding unpin tasks before pageflipping

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

On Fri, Sep 28, 2012 at 01:07:59PM +0100, Chris Wilson wrote:
> On Fri, 28 Sep 2012 15:05:01 +0300, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > On Fri, Sep 28, 2012 at 12:29:56PM +0100, Chris Wilson wrote:
> > > If we accumulate unpin tasks because we are pageflipping faster than the
> > > system can schedule its workers, we can effectively create a
> > > pin-leak. The solution taken here is to limit the number of unpin tasks
> > > we have per-crtc and to flush those outstanding tasks if we accumulate
> > > too many. This should prevent any jitter in the normal case, and also
> > > prevent the hang if we should run too fast.
> > > 
> > > Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=46991
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > ---
> > >  drivers/gpu/drm/i915/intel_display.c |   20 +++++++++++++++-----
> > >  drivers/gpu/drm/i915/intel_drv.h     |    4 +++-
> > >  2 files changed, 18 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > > index 04407fd..14f1b51 100644
> > > --- a/drivers/gpu/drm/i915/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/intel_display.c
> > > @@ -6310,14 +6310,19 @@ static void intel_unpin_work_fn(struct work_struct *__work)
> > >  {
> > >  	struct intel_unpin_work *work =
> > >  		container_of(__work, struct intel_unpin_work, work);
> > > +	struct drm_device *dev = work->crtc->dev;
> > >  
> > > -	mutex_lock(&work->dev->struct_mutex);
> > > +	mutex_lock(&dev->struct_mutex);
> > >  	intel_unpin_fb_obj(work->old_fb_obj);
> > >  	drm_gem_object_unreference(&work->pending_flip_obj->base);
> > >  	drm_gem_object_unreference(&work->old_fb_obj->base);
> > >  
> > > -	intel_update_fbc(work->dev);
> > > -	mutex_unlock(&work->dev->struct_mutex);
> > > +	intel_update_fbc(dev);
> > > +	mutex_unlock(&dev->struct_mutex);
> > > +
> > > +	BUG_ON(atomic_read(&to_intel_crtc(work->crtc)->unpin_work_count) == 0);
> > > +	atomic_dec(&to_intel_crtc(work->crtc)->unpin_work_count);
> > 
> > AFAICS you always have struct_mutex locked in the relevant functions,
> > so no need for an atomic variable.
> 
> It's not in every case, since we need to do the flush without holding
> the lock, we have the choice of making this variable atomic, or taking
> and dropping the lock. Obviously I choose the former.

Ah right. I missed the conditional flush.

-- 
Ville Syrjälä
Intel OTC