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

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Message-ID: <1090f735-8252-453f-b6c8-0ec35881ff32@linux.dev>
Date: Wed, 18 Dec 2024 18:29:53 +0800
From: Sui Jingfeng <sui.jingfeng@...ux.dev>
To: Icenowy Zheng <uwu@...nowy.me>, 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


On 2024/12/18 13:47, Icenowy Zheng wrote:
> 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.


I have told you several times, we are asking for hardware details.
Why the Loongarch suffer from the WC bug, like the Loongson Mips?
Why the performance of drm/ast's drop dramatically when your patch
is applied?


>>
-- 
Best regards,
Sui