Hi, Geert,

On Mon, Jul 26, 2021 at 4:36 PM Geert Uytterhoeven [off-list ref] wrote:

Hi Huacai,

On Sat, Jul 24, 2021 at 2:36 PM Huacai Chen [off-list ref] wrote:

quoted

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 <redacted>

Thanks for your patch!

quoted

--- 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.

quoted

--- 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