Hi Cyril,
Was your change to modify librttest.h as follows :
/* atomic_add - add integer to atomic variable and returns a value.
* i: integer value to add
* v: pointer of type atomic_t
*/
static inline void atomic_add( int i, atomic_t *v )
{
(void)__sync_add_and_fetch(&v->counter, i);
}
/* atomic_inc: atomically increment the integer passed by reference
*/
static inline int atomic_inc(atomic_t *v)
{
return (int)__sync_add_and_fetch(&v->counter, 1);
}
Cheers,
~Pev
On 27 October 2010 13:10, Cyril Hrubis <[email protected]> wrote:
> Hi!
>> >> > The realtime tests aren't compiled for ARM currently as there's no
>> >> > implementation of atomic_add() in :
>> >> > testcases/realtime/include/librttest.h
>> >> >
>> >> > As a side note - in this function can we change the #error to not be
>> >> > blank and read something along the lines of :
>> >> > #error ERROR: atomic_add: Architecture not supported for compilation
>> >> >
>> >> > Anyway, the existing function here supports three architectures and
>> >> > lifts the implementations as follows :
>> >> > __x86_64__ || __i386__
>> >> > arch/x86/include/asm/atomic_32.h
>> >> > __powerpc__
>> >> > arch/powerpc/include/asm/atomic.h (might need review!)
>> >> > __sh__
>> >> > arch/sh/include/asm/atomic-llsc.h (might need review!)
>> >> >
>> >> > Logically we can add an ARM case following the same logic. Looking in :
>> >> > arch/arm/include/asm/atomic.h
>> >> >
>> >> > We can implement this easily for v6+ architectures. The case for
>> >> > atomic_add in librttest.h would look something like :
>> >> > #elif defined(__arm__) && (__LINUX_ARM_ARCH__ >= 6)
>> >> > unsigned long tmp;
>> >> > int result;
>> >> >
>> >> > __asm__ __volatile__(
>> >> > "1: ldrex %0, [%2]\n"
>> >> > " add %0, %0, %3\n"
>> >> > " strex %1, %0, [%2]\n"
>> >> > " teq %1, #0\n"
>> >> > " bne 1b"
>> >> > : "=&r" (result), "=&r" (tmp)
>> >> > : "r" (&v->counter), "Ir" (i)
>> >> > : "cc");
>> >> >
>> >> > return result;
>> >> >
>> >> > For < v6 architectures this isn't easy as they don't implement
>> >> > ldrex/strex. The atomic.h implementation won't work as it's a kernel
>> >> > only implementation using local_irq_save() and local_irq_restore() so
>> >> > obviously no use for userspace atomic add.
>> >> >
>> >> > After reading up for a while I've come across a very cunning trick in
>> >> > the kernel where an atomic compare/exchange is implemented in
>> >> > __kuser_cmpxchg. This is implemented in :
>> >> > arch/arm/kernel/entry-armv.S
>> >> >
>> >> > This tantalisingly comments :
>> >> > /*
>> >> > * For example, a user space atomic_add implementation could look
>> >> > like this:
>> >> > *
>> >> > * #define atomic_add(ptr, val) \
>> >> > * ({ register unsigned int *__ptr asm("r2") = (ptr); \
>> >> > * register unsigned int __result asm("r1"); \
>> >> > * asm volatile ( \
>> >> > * "1: @ atomic_add\n\t" \
>> >> > * "ldr r0, [r2]\n\t" \
>> >> > * "mov r3, #0xffff0fff\n\t" \
>> >> > * "add lr, pc, #4\n\t" \
>> >> > * "add r1, r0, %2\n\t" \
>> >> > * "add pc, r3, #(0xffff0fc0 - 0xffff0fff)\n\t" \
>> >> > * "bcc 1b" \
>> >> > * : "=&r" (__result) \
>> >> > * : "r" (__ptr), "rIL" (val) \
>> >> > * : "r0","r3","ip","lr","cc","memory" ); \
>> >> > * __result; })
>> >> > */
>> >> >
>> >> > So translating this into librttest.h format gives :
>> >> > #elif defined(__arm__) && (__LINUX_ARM_ARCH__ < 6)
>> >> > register unsigned int result asm("r1");
>> >> >
>> >> > /* See, arm/kernel/entry-armv.S:__kernel_cmpxchg() */
>> >> > __asm__ __volatile__ (
>> >> > "1: ldr r0, [r2]\n\t"
>> >> > " mov r3, #0xffff0fff\n\t"
>> >> > " add lr, pc, #4\n\t"
>> >> > " add r1, r0, %2\n\t"
>> >> > " add pc, r3, #(0xffff0fc0 - 0xffff0fff)\n\t"
>> >> > " bcc 1b"
>> >> > : "=&r" (result)
>> >> > : "r" (&v->counter), "rIL" (i)
>> >> > : "r0","r3","ip","lr","cc","memory" );
>> >> >
>> >> > return result;
>> >> >
>> >> > Does this look reasonable to you guys?
>> >>
>> >> CCing Gowrishankar from IBM.
>> >
>> > We recently agreed to use gcc buildins as the inline assembler was
>> > broken anyway. But it seems that this patch hasn't made it into git.
>> >
>> > Garret could you please commit patch from Gowrishankar from thread
>> > "[PATCH] fix realtime atomic_add() inline asm". It basically removes all
>> > the
>> > inline assembler in favor of __sync_add_and_fetch().
>>
>> Hmmm... did we reach closure on that issue? I thought it was still
>> outstanding...
>
> Well it seems, at least to me, that everybody agreed for that change. I
> personally checked that this change works for me and gave my ack.
>
> --
> Cyril Hrubis
> [email protected]
>
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