Received-SPF: pass (google.com: domain of linux-kernel+bounces-94293-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Date: Wed, 6 Mar 2024 17:50:09 +0100
From: Sebastian Wick <sebastian.wick@redhat.com>
To: Maxime Ripard <mripard@kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	Jonathan Corbet <corbet@lwn.net>, dri-devel@lists.freedesktop.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm: Document requirements for driver-specific KMS props
 in new drivers
Message-ID: <20240306165009.GB11561@toolbox>
References: <20240229202833.198691-1-sebastian.wick@redhat.com>
 <20240306-hulking-funky-fox-b9581b@houat>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240306-hulking-funky-fox-b9581b@houat>

On Wed, Mar 06, 2024 at 03:14:15PM +0100, Maxime Ripard wrote:
> Hi,
> 
> On Thu, Feb 29, 2024 at 09:28:31PM +0100, Sebastian Wick wrote:
> > When extending support for a driver-specific KMS property to additional
> > drivers, we should apply all the requirements for new properties and
> > make sure the semantics are the same and documented.
> > 
> > Signed-off-by: Sebastian Wick <sebastian.wick@redhat.com>
> > ---
> >  Documentation/gpu/drm-kms.rst | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> > index 13d3627d8bc0..afa10a28035f 100644
> > --- a/Documentation/gpu/drm-kms.rst
> > +++ b/Documentation/gpu/drm-kms.rst
> > @@ -496,6 +496,11 @@ addition to the one mentioned above:
> >  
> >  * An IGT test must be submitted where reasonable.
> >  
> > +For historical reasons, non-standard, driver-specific properties exist. If a KMS
> > +driver wants to add support for one of those properties, the requirements for
> > +new properties apply where possible. Additionally, the documented behavior must
> > +match the de facto semantics of the existing property to ensure compatibility.
> > +
> 
> I'm conflicted about this one, really.
> 
> On one hand, yeah, it's definitely reasonable and something we would
> want on the long run.
> 
> But on the other hand, a driver getting its technical debt worked on for
> free by anyone but its developpers doesn't seem fair to me.

Most of the work would have to be done for a new property as well. The
only additional work is then documenting the de-facto semantics and
moving the existing driver-specific code to the core.

Would it help if we spell out that the developers of the driver-specific
property shall help with both tasks?

> Also, I assume this is in reaction to the discussion we had on the
> Broadcast RGB property. We used in vc4 precisely because there was some
> userspace code to deal with it and we could just reuse it, and it was
> documented. So the requirements were met already.

It was not in drm core and the behavior was not documented properly at
least.

Either way, with Broadcast RGB we were already in a bad situation
because it was implemented by 2 drivers independently. This is what I
want to avoid in the first place. The cleanup afterwards (thank you!)
just exposed the problem.

> Maxime