linux-kernel - Re: [PATCH v7 04/23] dt-bindings: display: mediatek: padding: Add MT8188

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Date:   Fri, 13 Oct 2023 17:26:47 +0100
From:   Daniel Stone <daniel@...ishbar.org>
To:     Hsiao Chien Sung <shawn.sung@...iatek.com>
Cc:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        CK Hu <ck.hu@...iatek.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Chun-Kuang Hu <chunkuang.hu@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
        "Jason-JH . Lin" <jason-jh.lin@...iatek.com>,
        Singo Chang <singo.chang@...iatek.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        "Nancy . Lin" <nancy.lin@...iatek.com>,
        linux-mediatek@...ts.infradead.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v7 04/23] dt-bindings: display: mediatek: padding: Add MT8188

Hi Shawn,

On Fri, 6 Oct 2023 at 08:38, Hsiao Chien Sung <shawn.sung@...iatek.com> wrote:
> +  Padding provides ability to add pixels to width and height of a layer with
> +  specified colors. Due to hardware design, Mixer in VDOSYS1 requires
> +  width of a layer to be 2-pixel-align, or 4-pixel-align when ETHDR is enabled,
> +  we need Padding to deal with odd width.
> +  Please notice that even if the Padding is in bypass mode, settings in
> +  register must be cleared to 0, or undefined behaviors could happen.

If I understand the driver correctly, padding is automatically applied
to compensate for unaligned dimensions. The first/last rows/columns of
the overlay area will be filled with a specified colour (black?) to
preserve the area. This is unfortunately not OK to do transparently.
Userspace must be aware of this policy decision and specifically
request it. If not, the atomic request check should fail and tell
userspace that the requested configuration is not possible to achieve.

Cheers,
Daniel