Re: [PATCH v2] loongarch/mm: disable WUC for pgprot_writecombine as same as ioremap_wc

[Icenowy Zheng]

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.
> 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.

But in general writecombining is broken (if it's broken in one place,
it's broken and not totally right, and needs to be handled).

I said in another mail that if you want to keep this performance, you
should add another pgprot_ type instead of using pgprot_writecombine
(which breaks other drivers).

> 
> 
> > >