linux-kernel - Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-tmds181: Add TI TMDS181 and SN65DP159 bindings

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Message-ID: <20250819-impeach-prognosis-247bec1a809b@spud>
Date: Tue, 19 Aug 2025 18:22:34 +0100
From: Conor Dooley <conor@...nel.org>
To: Mike Looijmans <mike.looijmans@...ic.nl>
Cc: Krzysztof Kozlowski <krzk@...nel.org>, dri-devel@...ts.freedesktop.org,
	Andrzej Hajda <andrzej.hajda@...el.com>,
	Conor Dooley <conor+dt@...nel.org>,
	David Airlie <airlied@...il.com>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Jonas Karlman <jonas@...boo.se>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Neil Armstrong <neil.armstrong@...aro.org>,
	Rob Herring <robh@...nel.org>, Robert Foss <rfoss@...nel.org>,
	Simona Vetter <simona@...ll.ch>,
	Thomas Zimmermann <tzimmermann@...e.de>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-tmds181: Add TI TMDS181
 and SN65DP159 bindings

On Tue, Aug 19, 2025 at 10:26:15AM +0200, Mike Looijmans wrote:
> On 19-08-2025 09:51, Krzysztof Kozlowski wrote:
> > On 19/08/2025 09:46, Mike Looijmans wrote:
> > > > > +
> > > > > +properties:
> > > > > +  compatible:
> > > > > +    enum:
> > > > > +      - ti,tmds181
> > > > > +      - ti,sn65dp159
> > > > The driver contains:
> > > > +	{ .compatible = "ti,tmds181", },
> > > > +	{ .compatible = "ti,sn65dp159", },
> > > > +	{}
> > > > so why is a fallback compatible not suitable here?
> > > I don't understand the question. The two are slightly different chips,
> > Your driver says they are compatible. No one said the same, but compatible.
> > 
> > > so it makes sense to describe that in the DT.
> > Compatible devices should use fallback. There is plenty of examples (90%
> > of all binding files?) including example-schema describing this.
> 
> Please help me out here, I'm happy to oblige, but I don't understand what
> you're asking.
> 
> To the best of my knowledge "fallback" compatible is when you write
> something like this in the device-tree:
>    compatible = "st,m25p80", "jedec,spi-nor";
> Which means that we can use the "jedec,spi-nor" driver if there's no
> specific match for "st,m25p80", correct?
> 
> I don't understand how that relates to your request, this is the first time
> I ever got this particular feedback. Looking at say the ti,sn65dsi83 driver,
> it does the same thing (supports the ti,sn65dsi83 and ti,sn65dsi84).
> 
> Please explain or point me somewhere where I can find this?

Devices that are supersets of, or functionally identical to, others should
use fallback compatibles. The driver treats these devices as functionally
identical to one another when it comes to match data (as there is none)
so you need to either use a fallback compatible or explain in your
commit message why one is not suitable here.

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