linux-kernel - Re: [PATCH v2] loongarch/mm: disable WUC for pgprot_writecombine as same as ioremap

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Message-ID: <8d38328adea0d752b09a91f69e7a94d4804ac725.camel@icenowy.me>
Date: Wed, 18 Dec 2024 20:37:42 +0800
From: Icenowy Zheng <uwu@...nowy.me>
To: Sui Jingfeng <sui.jingfeng@...ux.dev>, Shuah <shuah@...nel.org>, Xi
 Ruoyao <xry111@...111.site>, WANG Xuerui <kernel@...0n.name>, Huacai Chen
 <chenhuacai@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Mike Rapoport (IBM)"
 <rppt@...nel.org>, Baoquan He <bhe@...hat.com>, "Matthew Wilcox (Oracle)"
 <willy@...radead.org>, David Hildenbrand <david@...hat.com>, Zhen Lei
 <thunder.leizhen@...wei.com>, Thomas Gleixner <tglx@...utronix.de>, Zhihong
 Dong <donmor3000@...mail.com>, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org,  "conduct@...nel.org" <conduct@...nel.org>
Subject: Re: [PATCH v2] loongarch/mm: disable WUC for pgprot_writecombine as
 same as ioremap_wc

在 2024-12-18星期三的 18:05 +0800，Sui Jingfeng写道：
> Hi,
> 
> 
> On 2024/12/18 14:23, Icenowy Zheng wrote:
> > 在 2024-12-18星期三的 11:24 +0800，Sui Jingfeng写道：
> > > Hi,
> > > 
> > > 
> > > On 2024/12/18 02:18, Shuah wrote:
> > > > On 12/2/24 09:23, Sui Jingfeng wrote:
> > > > > Hi,
> > > > > 
> > > > > > IIUC this is a hardware bug of 7A1000 and 7A2000, so the
> > > > > > proper
> > > > > > location
> > > > > > of the workaround is in the bridge chip driver.  Or am I
> > > > > > misunderstanding something?
> > > > > > 
> > > > > You are misunderstanding everything and ranting like a dog.
> > > > > 
> > > > Sui Jingfeng,
> > > > 
> > > > This is not the way to work with your fellow developers in the
> > > > community to express disagreements.
> > > 
> > > I'm not expressing disagreements, but argue that the contributor
> > > and/or talker should provide *sufficient* hardware details and
> > > tests. Instead of pointless *ranting* in order to get harmful
> > > patch merged.
> > I don't think the original patch is harmful.
> > 
> 
> I have told you countless times, that is, disabling Write-Combine
> made the performance of BMC graphics(drm/ast) decrease dramatically.

So do the ast driver have any kind of command queues that needs to be
read by the card and executed? If it has, I assume it will break like
amdgpu/radeon if this is utilized (I remember ast has some 2D accel);
if it does not have, then maybe you can add an architecture-specific
pgprot type (e.g. pgprot_wuc), see how pgprot_noncached_wc is defined
for PowerPC or pgprot_device is defined for ARM64 -- in fact you may
find another architecture that have some page attribute that behave
like WUC on LongArch; and then utilize the newly introduced pgprot type
in drm/ast, instead of using WUC as pgprot_writecombine, which do not
exactly match (usual driver expect an ARM-like behavior of
pgprot_combine, I assume).

> And downstream Loongson developer really dislike related patch,
> and asking for solutions to me.
> 
> 
> > > The discussion completely not make scene at all.
> > > 
> > > 
> > > > I would recommend following up with an apology.
> > > > 
> > > I will not apology to indecent contributors and/or maintainers
> > > like this, never.
> > > 
> > > 
> > > > thanks,
> > > > -- Shuah
> > > > 
> > > > 
> > > > 
> > > > 
> > > >