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

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

On Thu, 1 Nov 2012 08:07:59 -0700, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
> On Thu,  1 Nov 2012 09:26:26 +0000
> Chris Wilson <chris@chris-wilson.co.uk> 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
> > Reported-and-tested-by: Tvrtko Ursulin <tvrtko.ursulin@onelan.co.uk>
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/intel_display.c |   22 ++++++++++++++++------
> >  drivers/gpu/drm/i915/intel_drv.h     |    4 +++-
> >  2 files changed, 19 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index 69b1739..800b195 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -6908,14 +6908,19 @@ 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);
> > +
> >  	kfree(work);
> >  }
> >  
> > @@ -6963,9 +6968,9 @@ static void do_intel_finish_page_flip(struct drm_device *dev,
> >  
> >  	atomic_clear_mask(1 << intel_crtc->plane,
> >  			  &obj->pending_flip.counter);
> > -
> >  	wake_up(&dev_priv->pending_flip_queue);
> > -	schedule_work(&work->work);
> > +
> > +	queue_work(dev_priv->wq, &work->work);
> >  
> >  	trace_i915_flip_complete(intel_crtc->plane, work->pending_flip_obj);
> >  }
> > @@ -7266,7 +7271,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
> >  		return -ENOMEM;
> >  
> >  	work->event = event;
> > -	work->dev = crtc->dev;
> > +	work->crtc = crtc;
> >  	intel_fb = to_intel_framebuffer(crtc->fb);
> >  	work->old_fb_obj = intel_fb->obj;
> >  	INIT_WORK(&work->work, intel_unpin_work_fn);
> > @@ -7291,6 +7296,9 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
> >  	intel_fb = to_intel_framebuffer(fb);
> >  	obj = intel_fb->obj;
> >  
> > +	if (atomic_read(&intel_crtc->unpin_work_count) >= 2)
> > +		flush_workqueue(dev_priv->wq);
> > +
> 
> Have you by chance tested this with the async flip patch?  I wonder if
> in that case whether 2 is too small, and something like 100 might be
> better (though really async flips are for cases where we can't keep up
> with refresh, so a small number shouldn't hurt too much there either).

The limit on 2 is due to the limited resolution of pincount. Hence my
earlier fear for your async flip patch.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre