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

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Date:   Tue, 3 Oct 2023 01:29:15 +0000
From:   Shawn Sung (宋孝謙) 
        <Shawn.Sung@...iatek.com>
To:     CK Hu (胡俊光) <ck.hu@...iatek.com>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.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

Hi Angelo,

On Thu, 2023-09-28 at 12:24 +0200, AngeloGioacchino Del Regno wrote:
> 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.
> 

Thank you for the suggestions.
I'll implement it in the next version.

Thanks,
Shawn