linux-kernel - Re: [PATCH v3] arm: remove !CPU_V6 and !GENERIC

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Message-ID: <20140121110826.GG30706@mudshark.cambridge.arm.com>
Date:	Tue, 21 Jan 2014 11:08:26 +0000
From:	Will Deacon <will.deacon@....com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"gang.chen@...anux.com" <gang.chen@...anux.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jaccon.bastiaansen@...il.com" <jaccon.bastiaansen@...il.com>
Subject: Re: [PATCH v3] arm: remove !CPU_V6 and !GENERIC_ATOMIC64 build
 dependencies for XEN

On Mon, Jan 20, 2014 at 03:32:48PM +0000, Stefano Stabellini wrote:
> On Thu, 16 Jan 2014, Will Deacon wrote:
> > For the xchg part, yes, that looks a lot better. I don't like the #undef
> > CONFIG_CPU_V6 though, can that be rewritten to use __LINUX_ARM_ARCH__?
> 
> The problem is that the 1 and 2 byte parameter size cases in __cmpxchg
> are ifdef'ed CONFIG_CPU_V6 but drivers/xen/grant-table.c needs them.
> 
> So we can either undef CONFIG_CPU_V6 in grant-table.c or call a
> different function.
> 
> If I switch from ifdef CONFIG_CPU_V6 to if __LINUX_ARM_ARCH__ > 6 in
> __cmpxchg, we still have the problem that if __LINUX_ARM_ARCH__ == 6,
> grant-table.c doesn't compile.
> 
> Maybe the approach taken by the other patch for cmpxchg is better, see
> below.

Yes, I prefer this approach. Minor comment below.

> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index c1f1a7e..ae54ae0 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1881,8 +1881,7 @@ config XEN_DOM0
>  config XEN
>  	bool "Xen guest support on ARM (EXPERIMENTAL)"
>  	depends on ARM && AEABI && OF
> -	depends on CPU_V7 && !CPU_V6
> -	depends on !GENERIC_ATOMIC64
> +	depends on CPU_V7
>  	select ARM_PSCI
>  	select SWIOTLB_XEN
>  	help
> diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
> index df2fbba..cc8a4a2 100644
> --- a/arch/arm/include/asm/cmpxchg.h
> +++ b/arch/arm/include/asm/cmpxchg.h
> @@ -133,6 +133,44 @@ extern void __bad_cmpxchg(volatile void *ptr, int size);
>   * cmpxchg only support 32-bits operands on ARMv6.
>   */
>  
> +static inline unsigned long __cmpxchg8(volatile void *ptr, unsigned long old,
> +				      unsigned long new)
> +{
> +	unsigned long oldval, res;
> +
> +	do {
> +		asm volatile("@ __cmpxchg1\n"
> +		"	ldrexb	%1, [%2]\n"
> +		"	mov	%0, #0\n"
> +		"	teq	%1, %3\n"
> +		"	strexbeq %0, %4, [%2]\n"
> +			: "=&r" (res), "=&r" (oldval)
> +			: "r" (ptr), "Ir" (old), "r" (new)
> +			: "memory", "cc");
> +	} while (res);
> +
> +	return oldval;
> +}
> +
> +static inline unsigned long __cmpxchg16(volatile void *ptr, unsigned long old,
> +				      unsigned long new)
> +{
> +	unsigned long oldval, res;
> +
> +	do {
> +		asm volatile("@ __cmpxchg1\n"

Can you fix this comment while you're here please?

Will
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