linux-kernel - Re: [RFC PATCH v2 9/9] drm: mali-dp: Add writeback out-fence support

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Date:   Thu, 27 Oct 2016 16:11:21 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Gustavo Padovan <gustavo@...ovan.org>,
        Brian Starkey <brian.starkey@....com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org
Subject: Re: [RFC PATCH v2 9/9] drm: mali-dp: Add writeback out-fence support

On Thu, Oct 27, 2016 at 09:25:19AM -0200, Gustavo Padovan wrote:
> 2016-10-27 Brian Starkey <brian.starkey@....com>:
> 
> > On Wed, Oct 26, 2016 at 07:43:57PM -0200, Gustavo Padovan wrote:
> > > 2016-10-26 Brian Starkey <brian.starkey@....com>:
> > > 
> > > > If userspace has asked for an out-fence for the writeback, we add a
> > > > fence to malidp_mw_job, to be signaled when the writeback job has
> > > > completed.
> > > > 
> > > > Signed-off-by: Brian Starkey <brian.starkey@....com>
> > > > ---
> > > >  drivers/gpu/drm/arm/malidp_hw.c |    5 ++++-
> > > >  drivers/gpu/drm/arm/malidp_mw.c |   18 +++++++++++++++++-
> > > >  drivers/gpu/drm/arm/malidp_mw.h |    3 +++
> > > >  3 files changed, 24 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/arm/malidp_hw.c b/drivers/gpu/drm/arm/malidp_hw.c
> > > > index 1689547..3032226 100644
> > > > --- a/drivers/gpu/drm/arm/malidp_hw.c
> > > > +++ b/drivers/gpu/drm/arm/malidp_hw.c
> > > > @@ -707,8 +707,11 @@ static irqreturn_t malidp_se_irq(int irq, void *arg)
> > > >  		unsigned long irqflags;
> > > >  		/*
> > > >  		 * We can't unreference the framebuffer here, so we queue it
> > > > -		 * up on our threaded handler.
> > > > +		 * up on our threaded handler. However, signal the fence
> > > > +		 * as soon as possible
> > > >  		 */
> > > > +		malidp_mw_job_signal(drm, malidp->current_mw, 0);
> > > 
> > > Drivers should not deal with fences directly. We need some sort of
> > > drm_writeback_finished() that will do the signalling for you.
> > > 
> > 
> > With a signature like this?
> > 	drm_writeback_finished(struct drm_connector_state *state);
> > 
> > I'll have to think about how to achieve that. The state isn't
> > refcounted and the driver isn't in charge of it's lifetime. I'm not
> > sure how/where to ensure the state doesn't get destroyed before its
> > been signaled.
> 
> Maybe we should do something similar to the crtc vblank handlers and
> even hide the connector_state. Those helpers only take the crtc.
> They are able to hold ref to the event as well.

I guess we could reuse the drm_event stuff, but not sure that's too much
overkill. It would at least be a consistent driver interface, and drivers
could reuse stuff like arm_event and similar functions. Which might be
rather beneficial.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch