Free Electrons

Embedded Linux Experts

 * Copyright (C) 1999 Cort Dougan <>

 * Memory barrier.
 * The sync instruction guarantees that all memory accesses initiated
 * by this processor have been performed (with respect to all other
 * mechanisms that access memory).  The eieio instruction is a barrier
 * providing an ordering (separately) for (a) cacheable stores and (b)
 * loads and stores to non-cacheable memory (e.g. I/O devices).
 * mb() prevents loads and stores being reordered across this point.
 * rmb() prevents loads being reordered across this point.
 * wmb() prevents stores being reordered across this point.
 * read_barrier_depends() prevents data-dependent loads being reordered
 *	across this point (nop on PPC).
 * *mb() variants without smp_ prefix must order all types of memory
 * operations with one another. sync is the only instruction sufficient
 * to do this.
 * For the smp_ barriers, ordering is for cacheable memory operations
 * only. We have to use the sync instruction for smp_mb(), since lwsync
 * doesn't order loads with respect to previous stores.  Lwsync can be
 * used for smp_rmb() and smp_wmb().
 * However, on CPUs that don't support lwsync, lwsync actually maps to a
 * heavy-weight sync, so smp_wmb() can be a lighter-weight eieio.
#define mb()   __asm__ __volatile__ ("sync" : : : "memory")
#define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
#define wmb()  __asm__ __volatile__ ("sync" : : : "memory")

#    define SMPWMB      LWSYNC
#    define SMPWMB      eieio

#define __lwsync()	__asm__ __volatile__ (stringify_in_c(LWSYNC) : : :"memory")
#define dma_rmb()	__lwsync()
#define dma_wmb()	__asm__ __volatile__ (stringify_in_c(SMPWMB) : : :"memory")

#define __smp_lwsync()	__lwsync()

#define __smp_mb()	mb()
#define __smp_rmb()	__lwsync()
#define __smp_wmb()	__asm__ __volatile__ (stringify_in_c(SMPWMB) : : :"memory")

 * This is a barrier which prevents following instructions from being
 * started until the value of the argument x is known.  For example, if
 * x is a variable loaded from memory, this prevents following
 * instructions from being executed until the load has been performed.
#define data_barrier(x)	\
	asm volatile("twi 0,%0,0; isync" : : "r" (x) : "memory");

#define __smp_store_release(p, v)						\
do {									\
	compiletime_assert_atomic_type(*p);				\
	__smp_lwsync();							\
	WRITE_ONCE(*p, v);						\
} while (0)

#define __smp_load_acquire(p)						\
({									\
	typeof(*p) ___p1 = READ_ONCE(*p);				\
	compiletime_assert_atomic_type(*p);				\
	__smp_lwsync();							\
	___p1;								\

 * This must resolve to hwsync on SMP for the context switch path.
 * See _switch, and core scheduler context switch memory ordering
 * comments.
#define smp_mb__before_spinlock()   smp_mb()

#include <asm-generic/barrier.h>