linux-kernel - Re: [RESEND PATCH v6 17/20] drm/mediatek: Support MT8188 Padding in display driver

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Message-ID: <5d1669c0-64c3-8a5d-6f4e-88f72d4b40b4@collabora.com>
Date:   Thu, 28 Sep 2023 12:24:34 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Shawn Sung (宋孝謙) 
        <Shawn.Sung@...iatek.com>,
        CK Hu (胡俊光) <ck.hu@...iatek.com>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        Singo Chang (張興國) 
        <Singo.Chang@...iatek.com>,
        Jason-JH Lin (林睿祥) 
        <Jason-JH.Lin@...iatek.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Nancy Lin (林欣螢) <Nancy.Lin@...iatek.com>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "conor+dt@...nel.org" <conor+dt@...nel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Project_Global_Chrome_Upstream_Group 
        <Project_Global_Chrome_Upstream_Group@...iatek.com>,
        "airlied@...il.com" <airlied@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "nfraprado@...labora.com" <nfraprado@...labora.com>
Subject: Re: [RESEND PATCH v6 17/20] drm/mediatek: Support MT8188 Padding in
 display driver

Il 28/09/23 05:39, Shawn Sung (宋孝謙) ha scritto:
> Hi CK,
> 
> On Thu, 2023-09-28 at 03:05 +0000, CK Hu (胡俊光) wrote:
>> Hi, Hsiao-chien:
>>
>> On Mon, 2023-09-11 at 15:42 +0800, Hsiao Chien Sung wrote:
>>> Padding is a new display module on MT8188, it 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.
>>
>> You just set padding to bypass mode and not clear settings to 0. Any
>> thing wrong?
>>
> 
> Since the deafult value of all the registers in Padding is zero, and
> we are not using Padding currently, it's fine if we just set padding to
> bypass mode witout clearing other registers.
> 
> The comment is just a reminder in case we forget it in the future.

Do *not* rely on default register values, because you don't know what booted
Linux in the first place: you shall *not* expect a clean state and you shall
*not* expect a clean boot.

Besides, what I see is that you're setting GENMASK(1, 0) without explaining
why in the code: you have to add at least the definitions for PADDING_EN and
PADDING_BYPASS.

I also don't see why you shouldn't add at least basic handling for this block,
as it looks easy enough: after all, you anyway have to make sure that the
registers are cleared - might as well just add a little more effort on top
and actually set them to meaningful values? That's ultimately your choice, but
I don't want to see any GENMASK(31,0) write even for register clearing.

Please make this driver proper.

Thanks,
Angelo

> 
> Regards,
> Hsiao Chien Sung