linux-kernel - Re: [PATCH 1/5] dt-bindings: display: panel: samsung,atna33xc20: Document ATNA45AF01

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Message-ID: <ZpEBb6A06dDU55RB@linaro.org>
Date: Fri, 12 Jul 2024 12:11:59 +0200
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: Doug Anderson <dianders@...omium.org>
Cc: Neil Armstrong <neil.armstrong@...aro.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Jessica Zhang <quic_jesszhan@...cinc.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, dri-devel@...ts.freedesktop.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, Abel Vesa <abel.vesa@...aro.org>,
	Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH 1/5] dt-bindings: display: panel: samsung,atna33xc20:
 Document ATNA45AF01

On Wed, Jul 10, 2024 at 12:16:58PM -0700, Doug Anderson wrote:
> On Wed, Jul 10, 2024 at 12:03 PM Stephan Gerhold
> <stephan.gerhold@...aro.org> wrote:
> >
> > > 2. In theory you could make your compatible look like this:
> > >
> > > compatible = "samsung,atna45af01", "samsung,atna33xc20"
> > >
> > > ...which would say "I have a 45af01 but if the OS doesn't have
> > > anything special to do that it would be fine to use the 33xc20
> > > driver". That would allow device trees to work without the kernel
> > > changes and would allow you to land the DT changes in parallel with
> > > the driver changes and things would keep working.
> > >
> > > ...and, in fact, that would mean you _didn't_ need to add the new
> > > compatible string to the driver, which is nice.
> > >
> >
> > Yeah, I considered this. I mentioned the reason why I decided against
> > this in patch 2:
> >
> > > While ATNA45AF01 would also work with "samsung,atna33xc20" as a fallback
> > > compatible, the original submission of the compatible in commit
> > > 4bfe6c8f7c23 ("drm/panel-simple: Add Samsung ATNA33XC20") had the timings
> > > and resolution hardcoded. These would not work for ATNA45AF01.
> >
> > Basically, it works with the current driver. But if you would run the
> > kernel at the state of the original submission then it would behave
> > incorrectly. This is why I considered the resolution and timings to be
> > part of the "samsung,atna33xc20" "ABI". The new panel would not be
> > compatible with that.
> 
> Ah, oops. My eyes totally glazed over the description since the patch
> was so simple. :-P Sorry about that.
> 
> IMO I'd still prefer using the fallback compatible, but happy to hear
> other opinions. In the original commit things were pretty broken still
> (sorta like how it's broken for you using "edp-panel") and the
> resolution hasn't been hardcoded for a long while...

I briefly discussed this with Krzysztof on IRC yesterday and we
concluded that a fallback compatible is better. If one considers just
the non-discoverable part of the interface for the binding (i.e. the
non-standard power sequence), then the two panels are indeed compatible.

I will send a v2 with the fallback compatible on Monday. I think this
can also simplify backporting the backlight fix as you mentioned, since
then no driver change is required to make it work.

Thanks,
Stephan