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

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Message-ID: <bdef7b5dcf7c9b20bfb97ef66ee7e66bfcc304d8.camel@icenowy.me>
Date: Thu, 19 Dec 2024 14:38:36 +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 13:49 +0800，Sui Jingfeng写道：
> 
> 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.

It is, it changes the order "write A -> write B -> write C -> write D"
to "write ABCD concurrently". If one of B/C/D is a register that
triggers latching A, in the former case it will latch A correctly but
in the latter case it will wrongly latch the old value of A instead, so
write gathering is not strongly-ordered.

> 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.
> 
> 
> > >