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

From: [email-masked] (Imre Deak)
Date: Thu, 18 Mar 2010 18:33:15 +0200

On Thu, Mar 18, 2010 at 02:50:08PM +0100, ext Mathieu Desnoyers wrote:
> * 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;
> 

On a related note, why can't we have __bad_xchg as an undefined function
for detecting the error already in build time? That seems to be the
approach in similar places.

--Imre