linux-kernel - Re: [PATCH V2] LoongArch: Make WriteCombine configurable for ioremap()

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Date:   Wed, 15 Mar 2023 12:33:19 +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 V2] LoongArch: Make WriteCombine configurable for
 ioremap()

On Wed, 2023-03-15 at 08:44 +0800, Huacai Chen wrote:

/* snip */

> +config ARCH_WRITECOMBINE
> +       bool "Enable WriteCombine (WUC) for ioremap()"
> +       help
> +         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.

'You can override this setting via writecombine=yes/no boot parameter.'

/* snip */

> +static int __init setup_writecombine(char *p)
> +{
> +       if (!strcmp(p, "on"))
> +               pgprot_wc = PAGE_KERNEL_WUC;
> +       else if (!strcmp(p, "off"))
> +               pgprot_wc = PAGE_KERNEL_SUC;

else
	pr_warn("Unknown writecombine setting \"%s\".\n", p);

Otherwise LGTM.

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