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Message-ID: <b7ae0feb-d401-43ee-8d5f-ce62ca224638@paulmck-laptop>
Date: Wed, 1 May 2024 22:06:58 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
elver@...gle.com, akpm@...ux-foundation.org, tglx@...utronix.de,
peterz@...radead.org, dianders@...omium.org, pmladek@...e.com,
arnd@...db.de, torvalds@...ux-foundation.org, kernel-team@...a.com,
Andi Shyti <andi.shyti@...ux.intel.com>,
Palmer Dabbelt <palmer@...osinc.com>,
Masami Hiramatsu <mhiramat@...nel.org>, linux-sh@...r.kernel.org
Subject: Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg
On Thu, May 02, 2024 at 06:52:53AM +0200, John Paul Adrian Glaubitz wrote:
> Hi Paul,
>
> On Wed, 2024-05-01 at 16:01 -0700, Paul E. McKenney wrote:
> > Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on sh.
> >
> > [ paulmck: Drop two-byte support per Arnd Bergmann feedback. ]
> > [ paulmck: Apply feedback from Naresh Kamboju. ]
> > [ Apply Geert Uytterhoeven feedback. ]
> >
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Andi Shyti <andi.shyti@...ux.intel.com>
> > Cc: Palmer Dabbelt <palmer@...osinc.com>
> > Cc: Masami Hiramatsu <mhiramat@...nel.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: <linux-sh@...r.kernel.org>
> > ---
> > arch/sh/Kconfig | 1 +
> > arch/sh/include/asm/cmpxchg.h | 3 +++
> > 2 files changed, 4 insertions(+)
> >
> > diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
> > index 2ad3e29f0ebec..f47e9ccf4efd2 100644
> > --- a/arch/sh/Kconfig
> > +++ b/arch/sh/Kconfig
> > @@ -16,6 +16,7 @@ config SUPERH
> > select ARCH_HIBERNATION_POSSIBLE if MMU
> > select ARCH_MIGHT_HAVE_PC_PARPORT
> > select ARCH_WANT_IPC_PARSE_VERSION
> > + select ARCH_NEED_CMPXCHG_1_EMU
> > select CPU_NO_EFFICIENT_FFS
> > select DMA_DECLARE_COHERENT
> > select GENERIC_ATOMIC64
> > diff --git a/arch/sh/include/asm/cmpxchg.h b/arch/sh/include/asm/cmpxchg.h
> > index 5d617b3ef78f7..1e5dc5ccf7bf5 100644
> > --- a/arch/sh/include/asm/cmpxchg.h
> > +++ b/arch/sh/include/asm/cmpxchg.h
> > @@ -9,6 +9,7 @@
> >
> > #include <linux/compiler.h>
> > #include <linux/types.h>
> > +#include <linux/cmpxchg-emu.h>
> >
> > #if defined(CONFIG_GUSA_RB)
> > #include <asm/cmpxchg-grb.h>
> > @@ -56,6 +57,8 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,
> > unsigned long new, int size)
> > {
> > switch (size) {
> > + case 1:
> > + return cmpxchg_emu_u8(ptr, old, new);
> > case 4:
> > return __cmpxchg_u32(ptr, old, new);
> > }
>
> Thanks for the patch. However, I don't quite understand its purpose.
>
> There is already a case for 8-byte cmpxchg in the switch statement below:
>
> case 1: \
> __xchg__res = xchg_u8(__xchg_ptr, x); \
> break;
>
> Does cmpxchg_emu_u8() have any advantages over the native xchg_u8()?
That would be 8-bit xchg() rather than 8-byte cmpxchg(), correct?
Or am I missing something subtle here that makes sh also support one-byte
(8-bit) cmpxchg()?
Thanx, Paul
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