Re: [PATCH RFC 1/2] arch: Introduce ARCH_HAS_HW_XCHG_SMALL

[Huacai Chen <chenhuacai@gmail.com>]

Hi, Geert,

On Mon, Jul 26, 2021 at 4:36 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Huacai,
>
> On Sat, Jul 24, 2021 at 2:36 PM Huacai Chen <chenhuacai@loongson.cn> wrote:
> > Introduce a new Kconfig option ARCH_HAS_HW_XCHG_SMALL, which means arch
> > has hardware sub-word xchg/cmpxchg support. This option will be used as
> > an indicator to select the bit-field definition in the qspinlock data
> > structure.
> >
> > Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
>
> Thanks for your patch!
>
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -228,6 +228,10 @@ config ARCH_HAS_FORTIFY_SOURCE
> >           An architecture should select this when it can successfully
> >           build and run with CONFIG_FORTIFY_SOURCE.
> >
> > +# Select if arch has hardware sub-word xchg/cmpxchg support
> > +config ARCH_HAS_HW_XCHG_SMALL
>
> What do you mean by "hardware"?
> Does a software fallback count?
This new option is supposed as an indicator to select bit-field
definition of qspinlock, software fallback is not helpful in this
case.

>
> > --- a/arch/m68k/Kconfig
> > +++ b/arch/m68k/Kconfig
> > @@ -5,6 +5,7 @@ config M68K
> >         select ARCH_32BIT_OFF_T
> >         select ARCH_HAS_BINFMT_FLAT
> >         select ARCH_HAS_DMA_PREP_COHERENT if HAS_DMA && MMU && !COLDFIRE
> > +       select ARCH_HAS_HW_XCHG_SMALL
>
> M68k CPUs which support the CAS (Compare And Set) instruction do
> support this on 8-bit, 16-bit, and 32-bit quantities.
> M68k CPUs which lack CAS use a software implementation, which
> supports the same quantities.
>
> As CAS is used only if CONFIG_RMW_INSNS=y, perhaps this needs
> a dependency?
OK, I think this dependency is needed.

Huacai

>
>    select ARCH_HAS_HW_XCHG_SMALL if RMW_INSNS
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds