linux-kernel - Re: [PATCH V3] LoongArch: Make WriteCombine configurable for ioremap()

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Date:   Thu, 16 Mar 2023 12:34:51 +0800
From:   WANG Xuerui <kernel@...0n.name>
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>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH V3] LoongArch: Make WriteCombine configurable for
 ioremap()

On 3/15/23 16:53, 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
"can only be used for". Same for the Kconfig change.
> 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.
We normally want to document this in 
Documentation/admin-guide/kernel-parameters too. This can be done later 
(by you or me).
>
> Suggested-by: WANG Xuerui <kernel@...0n.name>
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
> V2: Add kernel prameter and update commit messages.
> V3: Add a warning message for invalid kernel parameters.
>
>   arch/loongarch/Kconfig          | 16 ++++++++++++++++
>   arch/loongarch/include/asm/io.h |  4 +++-
>   arch/loongarch/kernel/setup.c   | 21 +++++++++++++++++++++
>   3 files changed, 40 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index 7fd51257e0ed..3ddde336e6a5 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -447,6 +447,22 @@ config ARCH_IOREMAP
>   	  protection support. However, you can enable LoongArch DMW-based
>   	  ioremap() for better performance.
>   
> +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=on/off boot parameter.
via the ...
> +
>   config ARCH_STRICT_ALIGN
>   	bool "Enable -mstrict-align to prevent unaligned accesses" if EXPERT
>   	default y
> diff --git a/arch/loongarch/include/asm/io.h b/arch/loongarch/include/asm/io.h
> index 402a7d9e3a53..545e2708fbf7 100644
> --- a/arch/loongarch/include/asm/io.h
> +++ b/arch/loongarch/include/asm/io.h
> @@ -54,8 +54,10 @@ static inline void __iomem *ioremap_prot(phys_addr_t offset, unsigned long size,
>    * @offset:    bus address of the memory
>    * @size:      size of the resource to map
>    */
> +extern pgprot_t pgprot_wc;
> +
>   #define ioremap_wc(offset, size)	\
> -	ioremap_prot((offset), (size), pgprot_val(PAGE_KERNEL_WUC))
> +	ioremap_prot((offset), (size), pgprot_val(pgprot_wc))
>   
>   #define ioremap_cache(offset, size)	\
>   	ioremap_prot((offset), (size), pgprot_val(PAGE_KERNEL))
> diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
> index bae84ccf6d36..27f71f9531e1 100644
> --- a/arch/loongarch/kernel/setup.c
> +++ b/arch/loongarch/kernel/setup.c
> @@ -160,6 +160,27 @@ static void __init smbios_parse(void)
>   	dmi_walk(find_tokens, NULL);
>   }
>   
> +#ifdef CONFIG_ARCH_WRITECOMBINE
> +pgprot_t pgprot_wc = PAGE_KERNEL_WUC;
> +#else
> +pgprot_t pgprot_wc = PAGE_KERNEL_SUC;
> +#endif
> +
> +EXPORT_SYMBOL(pgprot_wc);
> +
> +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);
> +
> +	return 0;
> +}
> +early_param("writecombine", setup_writecombine);
> +
>   static int usermem __initdata;
>   
>   static int __init early_parse_mem(char *p)

With the nits addressed:

Reviewed-by: WANG Xuerui <git@...0n.name>

-- 
WANG "xen0n" Xuerui

Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/