Subject: [PATCH 1/1] [RFC] arm: add half-word __xchg

From: [email-masked] (Mathieu Desnoyers)
Date: Thu, 18 Mar 2010 09:50:08 -0400

* Alexander Shishkin (virtuoso at slind.org) wrote:
> On systems where ldrexh/strexh are not available, use a generic local
> version or __bad_xchg() on SMP.
> 
> Signed-off-by: Alexander Shishkin <[email-masked]>
> CC: linux-arm-kernel-bounces at lists.infradead.org
> CC: Imre Deak <[email-masked]>
> CC: Mathieu Desnoyers <[email-masked]>
> ---
>  arch/arm/include/asm/system.h |   65 ++++++++++++++++++++++++++++++++++------
>  1 files changed, 55 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
> index d65b2f5..82248ae 100644
> --- a/arch/arm/include/asm/system.h
> +++ b/arch/arm/include/asm/system.h
> @@ -218,6 +218,39 @@ do {									\
>  	last = __switch_to(prev,task_thread_info(prev), task_thread_info(next));	\
>  } while (0)
>  
> +static inline unsigned long __xchg_local_generic(unsigned long x,
> +						 volatile void *ptr, int size)
> +{
> +	extern void __bad_xchg(volatile void *, int);
> +	unsigned long ret;
> +	unsigned long flags;
> +
> +	switch (size) {
> +	case 1:
> +		raw_local_irq_save(flags);
> +		ret = *(volatile unsigned char *)ptr;
> +		*(volatile unsigned char *)ptr = x;
> +		raw_local_irq_restore(flags);
> +		break;
> +
> +	case 2:
> +		raw_local_irq_save(flags);
> +		ret = *(volatile unsigned short *)ptr;
> +		*(volatile unsigned short *)ptr = x;
> +		raw_local_irq_restore(flags);
> +		break;
> +
> +	case 4:
> +		raw_local_irq_save(flags);
> +		ret = *(volatile unsigned long *)ptr;
> +		*(volatile unsigned long *)ptr = x;
> +		raw_local_irq_restore(flags);
> +		break;
> +	}
> +
> +	return ret;
> +}
> +
>  #if defined(CONFIG_CPU_SA1100) || defined(CONFIG_CPU_SA110)
>  /*
>   * On the StrongARM, "swp" is terminally broken since it bypasses the
> @@ -262,6 +295,26 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
>  			: "r" (x), "r" (ptr)
>  			: "memory", "cc");
>  		break;
> +#ifdef CONFIG_CPU_32v6K
> +	case 2:
> +		asm volatile("@	__xchg2\n"
> +		"1:	ldrexh	%0, [%3]\n"
> +		"	strexh	%1, %2, [%3]\n"
> +		"	teq	%1, #0\n"
> +		"	bne	1b"
> +			: "=&r" (ret), "=&r" (tmp)
> +			: "r" (x), "r" (ptr)
> +			: "memory", "cc");
> +		break;
> +#else
> +	case 2:
> +#ifdef CONFIG_SMP
> +		__bad_xchg(ptr, size), ret = 0;

You don't need to put this one explicitly. See the default case of the
switch.

But.. thinking about it. It's bad to have a 2-byte xchg primitive that
only works on UP and breaks the build on SMP. We should instead
implement a workaround based on __cmpxchg4 to perform the 2-byte xchg().

Thanks,

Mathieu

> +#else
> +		ret = __xchg_local_generic(x, ptr, 2);
> +#endif
> +		break;
> +#endif
>  	case 4:
>  		asm volatile("@	__xchg4\n"
>  		"1:	ldrex	%0, [%3]\n"
> @@ -277,17 +330,9 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
>  #error SMP is not supported on this platform
>  #endif
>  	case 1:
> -		raw_local_irq_save(flags);
> -		ret = *(volatile unsigned char *)ptr;
> -		*(volatile unsigned char *)ptr = x;
> -		raw_local_irq_restore(flags);
> -		break;
> -
> +	case 2:
>  	case 4:
> -		raw_local_irq_save(flags);
> -		ret = *(volatile unsigned long *)ptr;
> -		*(volatile unsigned long *)ptr = x;
> -		raw_local_irq_restore(flags);
> +		ret = __xchg_local_generic(x, ptr, size);
>  		break;
>  #else
>  	case 1:
> -- 
> 1.6.3.3
> 

-- 
Mathieu Desnoyers
Operating System Efficiency Consultant
EfficiOS Inc.
http://www.efficios.com