linux-kernel - Re: [PATCH 1/3] dt-bindings: display: ssd1307fb: Change "solomon,page-offset" default value

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Message-ID: <20231120-granola-sturdy-83d054f1b6fa@spud>
Date:   Mon, 20 Nov 2023 15:50:20 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Sahaj Sarup <sahaj.sarup@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 1/3] dt-bindings: display: ssd1307fb: Change
 "solomon,page-offset" default value

On Thu, Nov 16, 2023 at 07:07:37PM +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.
> 
> In fact, using a default value of 1 leads to the display not working when
> the fbdev is attached to the framebuffer console.
> 
> Reported-by: Sahaj Sarup <sahaj.sarup@...aro.org>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> ---
> 
>  .../devicetree/bindings/display/solomon,ssd1307fb.yaml          | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> index 3afbb52d1b7f..badd81459aaa 100644
> --- a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> +++ b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> @@ -35,7 +35,7 @@ properties:
>  
>    solomon,page-offset:
>      $ref: /schemas/types.yaml#/definitions/uint32
> -    default: 1
> +    default: 0

I think I saw it pointed out by Maxime elsewhere that this breaks the
ABI. It would be nice if DT defaults matched the hardware's, but I don't
really think it is worth breaking the ABI here. Expanding the property
description to explain the impact of the particular values might help
with incorrect usage.

Thanks,
Conor.

>      description:
>        Offset of pages (band of 8 pixels) that the screen is mapped to
>  
> -- 
> 2.41.0
> 

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