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

[Alexander Shishkin]

On 18 March 2010 14:32, Mathieu Desnoyers <mathieu.desnoyers at polymtl.ca> wrote:
> * Alexander Shishkin (virtuoso at slind.org) wrote:
>> On systems where ldrexh/strexh are not available, use a generic local
>> version.
>>
>> Signed-off-by: Alexander Shishkin <virtuoso at slind.org>
>> CC: linux-arm-kernel-bounces at lists.infradead.org
>> CC: Imre Deak <imre.deak at nokia.com>
>> CC: Mathieu Desnoyers <mathieu.desnoyers at polymtl.ca>
>> ---
>>  arch/arm/include/asm/system.h |   61 ++++++++++++++++++++++++++++++++++-------
>>  1 files changed, 51 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
>> index d65b2f5..a2ee41f 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 +295,22 @@ 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:
>> +             ret = __xchg_local_generic(x, ptr, 2);
>> +             break;
>
> Calling __xchg_local_generic as an underlying xchg() implementation on
> SMP is buggy. I think we have to surround this by "#ifndef CONFIG_SMP",
> so it generates a "__bad_xchg(ptr, size), ret = 0;" error for short xchg
> on SMP. That's odd though, as a program that builds on UP will break on
> SMP.

Point taken, thanks.

Regards,
--
Alex