Message-ID: <20241016093120.GF2712@pendragon.ideasonboard.com>
Date: Wed, 16 Oct 2024 12:31:20 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Rob Herring <robh@...nel.org>
Cc: Krzysztof Kozlowski <krzk@...nel.org>,
	Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Dave Stevenson <dave.stevenson@...pberrypi.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>,
	Martin Kepplinger <martink@...teo.de>,
	Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
	"Paul J. Murphy" <paul.j.murphy@...el.com>,
	Daniele Alessandrelli <daniele.alessandrelli@...el.com>,
	Tommaso Merciai <tomm.merciai@...il.com>,
	Martin Hecht <martin.hecht@...et.eu>,
	Zhi Mao <zhi.mao@...iatek.com>,
	Alain Volmat <alain.volmat@...s.st.com>,
	Mikhail Rudenko <mike.rudenko@...il.com>,
	Ricardo Ribalda <ribalda@...nel.org>,
	Kieran Bingham <kieran.bingham@...asonboard.com>,
	Umang Jain <umang.jain@...asonboard.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Vladimir Zapolskiy <vladimir.zapolskiy@...aro.org>,
	Dongchun Zhu <dongchun.zhu@...iatek.com>,
	Quentin Schulz <quentin.schulz@...obroma-systems.com>,
	Todor Tomov <todor.too@...il.com>, linux-media@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] media: dt-bindings: Use additionalProperties: false
 for endpoint: properties:

Hi Rob,

On Tue, Oct 15, 2024 at 02:44:18PM -0500, Rob Herring wrote:
> On Tue, Oct 15, 2024 at 02:28:06PM +0300, Laurent Pinchart wrote:
> > On Tue, Oct 15, 2024 at 08:11:18AM +0200, Krzysztof Kozlowski wrote:
> > > On 14/10/2024 22:29, Laurent Pinchart wrote:
> > > > On Mon, Oct 14, 2024 at 10:47:31AM +0200, Krzysztof Kozlowski wrote:
> > > >> On 14/10/2024 10:31, Bryan O'Donoghue wrote:
> > > >>> On 14/10/2024 08:45, Krzysztof Kozlowski wrote:
> > > >>>> I do not understand the reasoning behind this change at all. I don't
> > > >>>> think DT maintainers ever suggested it (in fact, rather opposite:
> > > >>>> suggested using unevaluatedProps) and I think is not a consensus of any
> > > >>>> talks.
> > > >>>
> > > >>> No there is not but then, how do you give consistent feedback except 
> > > >>> proposing something to be a baseline.
> > > >>>
> > > >>> On the one hand you have upstream additionalProperties: false and 
> > > >>> unevaluatedProperites: false - it'd be better to have a consistent 
> > > >>> message on which is to be used.
> 
> There are 3 options:
> 
> - no $ref => additionalProperties
> - has a $ref:
>     - additionalProperties and list ref-ed properties
>     - unevaluatedProperties and don't list ref-ed properties
> 
> I do debate (with myself)

Those are the best and worst debates at the same time :-)

> that that is too complicated as many don't 
> understand the difference. We could go back to always using 
> additionalProperties which is what we had before unevaluatedProperties 
> was added. The other option is always use unevaluatedProperties. 2 
> things have stopped me from going that route. I don't care to fix 
> 'additionalProperties' treewide which would be necessary to implement a 
> meta-schema or check that unevaluatedProperties is used. It's not 
> something I want to manually check in reviews. The other reason is just 
> to not change what the rules are again.
> 
> > > >>
> > > >> Well, I am afraid that push towards additionalProps will lead to grow
> > > >> common schema (video-interface-devices or video-interfaces) into huge
> > > >> one-fit-all binding. And that's not good.
> > > >>
> > > >> If a common binding for a group of devices encourages you to list its
> > > >> subset, then it is not that common.
> > > >>
> > > >> Solution is to fix that, e.g. split it per classes of devices.
> > > > 
> > > > I think splitting large schemas per class is a good idea, but the
> > > > problem will still exist. For instance, if we were to move the
> > > > CSI-2-specific properties to a separate schema, that schema would define
> > > > clock-lanes, data-lanes and clock-noncontinuous. The clock-lanes and
> > > > clock-noncontinuous properties do not apply to every device, how would
> > > > we then handle that ? I see three options:
> > > 
> > > Why is this a problem? Why is this a problem here, but not in other
> > > subsystems having exactly the same case?
> > 
> > I won't talk for other subsystems, but I can say I see value in
> > explicitly expressing what properties are valid for a device in DT
> > bindings both to inform DT authors and to perform validation on DT
> > sources. That's the whole point of YAML schemas, and I can't see a good
> > reason not to use the tooling we have developed when it has an easy way
> > to do the job.
> 
> This topic is just one piece of validation. A property being used that's 
> defined, but meaningless for a device is low on the list of what I care 
> about validating. I can't see how it would cause an actual problem? A 
> driver is going to read the property and do what with it? Could it be an 
> ABI issue ever? I can't see how other than a driver failing for some 
> reason if it finds the property, but that seems a bit far fetched.

I agree the risk of issues at runtime is quite low. My personal take on
this is that the additional complexity of specifying "$prop: true" in
the bindings is low (for me at least), and the increased correctness in
DT sources to avoid confusion for DT readers is worth it. I also like
how more explicit bindings cleary show in a single place what properties
are expected, making it easier for DT authors. That's a personal opinion
though.

-- 
Regards,

Laurent Pinchart

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