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

[Huacai Chen]

OK, V4 is coming.

Huacai

On Thu, Mar 16, 2023 at 12:35 PM WANG Xuerui <kernel@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxx>
>
> --
> WANG "xen0n" Xuerui
>
> Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/
>
>