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

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Date: Tue, 18 Jun 2024 13:18:10 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
 Jani Nikula <jani.nikula@...ux.intel.com>
Cc: 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

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:

/**
  * @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.
  */