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

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Message-ID: <c9fd9cdc-4116-4181-a1b5-2b5dd1e37f56@linux.dev>
Date: Thu, 19 Dec 2024 13:49:02 +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 2024/12/19 12:49, Icenowy Zheng wrote:
> 在 2024-12-19星期四的 10:54 +0800，Sui Jingfeng写道：
>> On 2024/12/18 20:43, Icenowy Zheng wrote:
>>> For the fact of drm/ast's dramatical drop, it's because write to
>>> the
>>> framebuffer can no longer be reordered.
>>
>> No, your understanding is wrong, very very wrong and a big wrong.
>>
>> It's not because it can't reorder the write. Rather, it's because
>> that the CPU can't do write gathering and can't do burst write any
>> more.
> Write gathering is a kind of write reordering,


No, your understanding is broken.

Write gathering *isn't* a kind of write reordering.
Its doesn't have to reorder, it just cache the write operation with
the CPU's write buffer.


> comparing to strongly
> ordered writing (which is literally one byte per write).
>
>> So do you still think your patch is harmless?
> Well, I said that performance w/o correctness is meaningless.


The point is that Write-Combine on drm/ast will get both correctness and performance.


>>
-- 
Best regards,
Sui