linux-kernel - Re: [PATCH 3/3] drm/ssd130x: Change "solomon,page-offset" property default value

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Message-ID: <87bkbs21sz.fsf@minerva.mail-host-address-is-not-set>
Date:   Fri, 17 Nov 2023 09:59:08 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Maxime Ripard <mripard@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Sahaj Sarup <sahaj.sarup@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 3/3] drm/ssd130x: Change "solomon,page-offset" property
 default value

Maxime Ripard <mripard@...nel.org> writes:

Hello Maxime,

> On Thu, Nov 16, 2023 at 07:07:39PM +0100, Javier Martinez Canillas wrote:
>> This is used to specify the page start address offset of the display RAM.
>> 
>> The value is used as offset when setting the page start address with the
>> SSD130X_SET_PAGE_RANGE command, and the driver currently sets its value to
>> 1 if the property is not present in the Device Tree.
>> 
>> But the datasheet mentions that the value on reset for the page start is a
>> 0, so it makes more sense to also have 0 as the default value for the page
>> offset if the property is not present.
>
> I can see the argument, but that's a DT ABI breaking change.
>

Yes, I know it's a DT ABI breaking change but what I'm trying to argue is
that the DT binding schema isn't correct to start with. Even the RPi DTBO
for this device (which I guess is used by most people with a SSD1306) has
a property to explicitly set this to 0:

  ssd1306: oled@3c{
    ...
    solomon,page-offset = <0>;
    ...
  };

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

>> In fact, using a default value of 1 leads to the display not working when
>> the emulated fbdev is attached to the framebuffer console.
>
> Could we fix that one instead? What is the issue about, exactly
>

This is the issue that Sahaj reported:

https://twitter.com/sahajsarup/status/1725088484736766364

I can try to figure out how to make the fbcon to work with a page-offset=1
but didn't investigate since thought that 0 is a much better default. Just
like the maximum resolution is the default if no width and height are set.

> Maxime

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat