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

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Message-ID: <ffd9ab359d2234e7ed680f161152d95c449e96bd.camel@icenowy.me>
Date: Thu, 19 Dec 2024 12:49:03 +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 2024-12-19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, 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.

> 
>