linux-kernel - Re: [PATCH V4] LoongArch: Make WriteCombine configurable for ioremap()

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Date:   Mon, 20 Mar 2023 15:57:24 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     Huacai Chen <chenhuacai@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH V4] LoongArch: Make WriteCombine configurable for
 ioremap()

On Thu, 2023-03-16 at 14:41 +0800, Huacai Chen wrote:
> LoongArch maintains cache coherency in hardware, but when paired with
> LS7A chipsets the WUC attribute (Weak-ordered UnCached, which is similar
> to WriteCombine) is out of the scope of cache coherency machanism for
> PCIe devices (this is a PCIe protocol violation, which may be fixed in
> newer chipsets).
> 
> This means WUC can only used for write-only memory regions now, so this
> option is disabled by default, making WUC silently fallback to SUC for
> ioremap(). You can enable this option if the kernel is ensured to run on
> hardware without this bug.
> 
> Kernel parameter writecombine=on/off can be used to override the Kconfig
> option.
> 
> Suggested-by: WANG Xuerui <kernel@...0n.name>
> Reviewed-by: WANG Xuerui <kernel@...0n.name>
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>

LGTM.

I still prefer an automatic way, but anyway we can implement it later
after a bug-free LS7A successor is launched.

Should we Cc: stable@...r.kernel.org and make a PR for 6.3 as well?  To
me it's a "bug fix" and needed for stable releases, but I'm not sure.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University