linux-kernel - Re: [PATCH v2 2/5] dt-bindings: display: ssd1307fb: Remove default width and height values

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Message-ID: <87r0qj19zs.fsf@minerva.mail-host-address-is-not-set>
Date:   Sat, 10 Jun 2023 19:51:35 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Conor Dooley <conor@...nel.org>
Cc:     devicetree@...r.kernel.org, Conor Dooley <conor+dt@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-kernel@...r.kernel.org, Maxime Ripard <mripard@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 2/5] dt-bindings: display: ssd1307fb: Remove default
 width and height values

Conor Dooley <conor@...nel.org> writes:

> On Fri, Jun 09, 2023 at 07:09:37PM +0200, Javier Martinez Canillas wrote:
>> A default resolution in the ssd130x driver isn't set to an arbitrary 96x16
>> anymore. Instead is set to a width and height that's controller dependent.
>
> Did that change to the driver not break backwards compatibility with
> existing devicetrees that relied on the default values to get 96x16?
>

It would but I don't think it is an issue in pratice. Most users of these
panels use one of the multiple libraries on top of the spidev interface.

For the small userbase that don't, I believe that they will use the rpif
kernel and ssd1306-overlay.dtbo DTB overlay, which defaults to width=128
and height=64 [1]. So those users will have to explicitly set a width and
height for a 96x16 panel anyways.

The intersection of users that have a 96x16 panel, assumed that default
and consider the DTB a stable ABI, and only update their kernel but not
the  DTB should be very small IMO.

[1]: https://github.com/raspberrypi/linux/blob/rpi-6.1.y/arch/arm/boot/dts/overlays/ssd1306-overlay.dts

> Cheers,
> Conor.
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat