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

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Message-ID: <feb3545e650c04c8cc21c40dad36dbf4076b3ebc.camel@icenowy.me>
Date: Wed, 18 Dec 2024 13:47:46 +0800
From: Icenowy Zheng <uwu@...nowy.me>
To: Sui Jingfeng <sui.jingfeng@...ux.dev>, 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
Subject: Re: [PATCH v2] loongarch/mm: disable WUC for pgprot_writecombine as
 same as ioremap_wc

在 2024-12-18星期三的 11:05 +0800，Sui Jingfeng写道：
> Hi,
> 
> 
> On 2024/12/18 07:44, Icenowy Zheng wrote:
> > 在 2024-12-03星期二的 00:23 +0800，Sui Jingfeng写道：
> > > Hi,
> > > 
> > > On 10/10/23 20:26, Xi Ruoyao wrote:
> > > > On Tue, 2023-10-10 at 11:02 +0800, Sui Jingfeng wrote:
> > > > 
> > > > > On LoongArch, cached mapping and uncached mappings are DMA-
> > > > > coherent and guaranteed by
> > > > > the hardware. While WC mappings is *NOT* DMA-coherent when 3D
> > > > > GPU
> > > > > is involved. Therefore,
> > > > > On downstream kernel, We disable write combine(WC) mappings
> > > > > at
> > > > > the drm drivers side.
> > > > Why it's only an issue when 3D GPU is involved?
> > > No one saying that only 3D GPU is suffer from this kind of issue,
> > > I just meant that the issue is there at least for GPU
> > > 
> > > > What's the difference between 3D GPUs and other devices?  Is it
> > > > possible that the other
> > > > devices (say neural accelerators) start to perform DMA accesses
> > > > in
> > > > a
> > > > similar way and then suddenly broken?
> > > You, the patch contributor or the maintainer or whatever stuff
> > > should carry on the test, right?
> > Well doing some test on PCIe peripherals need some professional
> > tool,
> > then I assume who raises the idea should do it, because not
> > everyone
> > can do.
> 
> 
> You are posting the patch, and then you are acclaiming that you are
> not even able to do sufficient test? right?

What I can test is that w/o this patch things break and w/ this patch
things work.

If you are against this patch, you need to provide more effective
evidence / a better solution than my approach.

Trying to make the problem more complex without providing extra
evidence isn't constructive.

> 
>