Re: [PATCH] LoongArch: Make WriteCombine configurable for ioremap()

[Xi Ruoyao <xry111@xry111.site>]

On Tue, 2023-03-14 at 16:54 +0800, Huacai Chen wrote:
> LoongArch maintains cache coherency in hardware, but when works 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, may be fixed in newer
> chipsets).

IIUC all launched LS7A models (7A1000 and 7A2000) suffers this issue?

> This means WUC can only used for write-only memory regions now, so this
> option is disabled by default (means WUC falls back to SUC for ioremap).
> You can enable this option if the kernel is ensured to run on bug-free
> hardwares.

Hmm, is it possible to make a PCI quirk so SUC/WUC will be decided
automatically from the vendor:device ID of the PCI root controller? 
Then we don't need to rely on the user or distro maintainer to select an
option.  I see there is already many architecture-dependant #if
directives in drivers/pci/quirks.c so I guess such a quirk is acceptable
in PCI tree...

If a PCI quirk is not possible, then is it possible to make a kernel
command line option, leaving this CONFIG as the default value of the
option?  I guess in the future many LoongArch users will just install a
binary distro, then it would be much easier to edit grub.cfg than
rebuilding the kernel when they finally buy a compliant PCIe controller.

> Suggested-by: WANG Xuerui <kernel@xen0n.name>
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
>  arch/loongarch/Kconfig          | 14 ++++++++++++++
>  arch/loongarch/include/asm/io.h |  5 +++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index 0d11738a861a..e3f5c422636f 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -446,6 +446,20 @@ 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 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, 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 (means WUC falls back to SUC for
> +         ioremap). You can enable this option if the kernel is ensured to run
> +         on bug-free hardwares.
> +
>  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..079ef897ed1a 100644
> --- a/arch/loongarch/include/asm/io.h
> +++ b/arch/loongarch/include/asm/io.h
> @@ -54,8 +54,13 @@ 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
>   */
> +#ifdef CONFIG_ARCH_WRITECOMBINE
>  #define ioremap_wc(offset, size)       \
>         ioremap_prot((offset), (size), pgprot_val(PAGE_KERNEL_WUC))
> +#else
> +#define ioremap_wc(offset, size)       \
> +       ioremap_prot((offset), (size), pgprot_val(PAGE_KERNEL_SUC))
> +#endif
>  
>  #define ioremap_cache(offset, size)    \
>         ioremap_prot((offset), (size), pgprot_val(PAGE_KERNEL))

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