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Date:   Mon, 20 Nov 2023 15:50:20 +0000
From:   Conor Dooley <conor@kernel.org>
To:     Javier Martinez Canillas <javierm@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Sahaj Sarup <sahaj.sarup@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@gmail.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        devicetree@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH 1/3] dt-bindings: display: ssd1307fb: Change
 "solomon,page-offset" default value
Message-ID: <20231120-granola-sturdy-83d054f1b6fa@spud>
References: <20231116180743.2763021-1-javierm@redhat.com>
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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.
>=20
> 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.
>=20
> 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.
>=20
> In fact, using a default value of 1 leads to the display not working when
> the fbdev is attached to the framebuffer console.
>=20
> Reported-by: Sahaj Sarup <sahaj.sarup@linaro.org>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> ---
>=20
>  .../devicetree/bindings/display/solomon,ssd1307fb.yaml          | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>=20
> 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:
> =20
>    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
> =20
> --=20
> 2.41.0
>=20

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