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

[Sui Jingfeng]

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