linux-kernel - Re: [PATCH v7 2/9] drm: Support per-plane async flip configuration

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Message-ID: <aflfggx2dc2p3y2a76yecjgmahozmbpnkk2qpekrnkpvviih6i@g2uuxeubozbo>
Date: Tue, 18 Jun 2024 20:43:29 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: André Almeida <andrealmeid@...lia.com>
Cc: Jani Nikula <jani.nikula@...ux.intel.com>, 
	dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, nouveau@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org, 
	kernel-dev@...lia.com, Melissa Wen <mwen@...lia.com>, alexander.deucher@....com, 
	christian.koenig@....com, Simon Ser <contact@...rsion.fr>, 
	Pekka Paalanen <ppaalanen@...il.com>, daniel@...ll.ch, Daniel Stone <daniel@...ishbar.org>, 
	Marek Olšák <maraeo@...il.com>, Dave Airlie <airlied@...il.com>, ville.syrjala@...ux.intel.com, 
	Xaver Hugl <xaver.hugl@...il.com>, Joshua Ashton <joshua@...ggi.es>, 
	Michel Dänzer <michel.daenzer@...lbox.org>, Sam Ravnborg <sam@...nborg.org>, 
	Boris Brezillon <bbrezillon@...nel.org>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	Nicolas Ferre <nicolas.ferre@...rochip.com>, Alexandre Belloni <alexandre.belloni@...tlin.com>, 
	Claudiu Beznea <claudiu.beznea@...on.dev>, Rodrigo Vivi <rodrigo.vivi@...el.com>, 
	Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>
Subject: Re: [PATCH v7 2/9] drm: Support per-plane async flip configuration

On Tue, Jun 18, 2024 at 01:18:10PM GMT, André Almeida wrote:
> Em 18/06/2024 07:07, Dmitry Baryshkov escreveu:
> > On Tue, 18 Jun 2024 at 12:38, Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> > > 
> > > On Tue, 18 Jun 2024, André Almeida <andrealmeid@...lia.com> wrote:
> > > > Drivers have different capabilities on what plane types they can or
> > > > cannot perform async flips. Create a plane::async_flip field so each
> > > > driver can choose which planes they allow doing async flips.
> > > > 
> > > > Signed-off-by: André Almeida <andrealmeid@...lia.com>
> > > > ---
> > > >   include/drm/drm_plane.h | 5 +++++
> > > >   1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h
> > > > index 9507542121fa..0bebc72af5c3 100644
> > > > --- a/include/drm/drm_plane.h
> > > > +++ b/include/drm/drm_plane.h
> > > > @@ -786,6 +786,11 @@ struct drm_plane {
> > > >         * @kmsg_panic: Used to register a panic notifier for this plane
> > > >         */
> > > >        struct kmsg_dumper kmsg_panic;
> > > > +
> > > > +     /**
> > > > +      * @async_flip: indicates if a plane can do async flips
> > > > +      */
> > > 
> > > When is it okay to set or change the value of this member?
> > > 
> > > If you don't document it, people will find creative uses for this.
> > 
> > Maybe it's better to have a callback instead of a static field? This
> > way it becomes clear that it's only relevant at the time of the
> > atomic_check().
> > 
> 
> So we would have something like bool (*async_flip) for struct
> drm_plane_funcs I suppose. Then each driver will implement this function and
> check on runtime if it should flip or not, right?
> 
> I agree that it makes more clear, but as far as I can see this is not
> something that is subject to being changed at runtime at all, so it seems a
> bit overkill to me to encapsulate a static information like that. I prefer
> to improve the documentation on the struct member to see if this solves the
> problem. What do you think of the following comment:

It looks like I keep on mixing async_flips as handled via the
DRM_MODE_PAGE_FLIP_ASYNC and the plane flips that are governed by
.atomic_async_check / .atomic_async_update / drm_atomic_helper_check()
and which end up being used just for legacy cursor updates.

So, yes, those are two different code paths, but with your changes I
think it becomes even easier to get confused between
atomic_async_check() and .async_flip member.


> /**
>  * @async_flip: indicates if a plane can perform async flips. The
>  * driver should set this true only for planes that the hardware
>  * supports flipping asynchronously. It may not be changed during
>  * runtime. This field is checked inside drm_mode_atomic_ioctl() to
>  * allow only the correct planes to go with DRM_MODE_PAGE_FLIP_ASYNC.
>  */

-- 
With best wishes
Dmitry