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

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Message-ID: <CAAhV-H6j1iCujsXFRCic0LS=cwkwSDz+8_YMWz9ZO+tZ4y3K3w@mail.gmail.com>
Date:   Wed, 15 Mar 2023 14:24:32 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Xi Ruoyao <xry111@...111.site>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>, 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, Mar 15, 2023 at 12:33 PM Xi Ruoyao <xry111@...111.site> wrote:
>
> 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.'
OK, thanks.

>
> /* 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);
OK, thanks.

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