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

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Date:   Mon, 20 Nov 2023 20:37:25 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Conor Dooley <conor@...nel.org>
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

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

Hello Connor,

> 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


Yes, Maxime also agrees with you as you mentioned. It's fair to say that
this may affect potential users even when I honestly think that's unlikely.

I'll just discard these patches and point out to people reporting the same
problem than Sahaj, that they need to add a "solomon,page-offset" property.

> description to explain the impact of the particular values might help
> with incorrect usage.
>

I'm unsure how much that would help to be honest but I might post a patch.

> Thanks,
> Conor.
>
 
-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat