From 1077fa36f23e259858caf6f269a47393a5aff523 Mon Sep 17 00:00:00 2001 From: Alexander Duyck Date: Thu, 11 Dec 2014 15:02:06 -0800 Subject: arch: Add lightweight memory barriers dma_rmb() and dma_wmb() There are a number of situations where the mandatory barriers rmb() and wmb() are used to order memory/memory operations in the device drivers and those barriers are much heavier than they actually need to be. For example in the case of PowerPC wmb() calls the heavy-weight sync instruction when for coherent memory operations all that is really needed is an lsync or eieio instruction. This commit adds a coherent only version of the mandatory memory barriers rmb() and wmb(). In most cases this should result in the barrier being the same as the SMP barriers for the SMP case, however in some cases we use a barrier that is somewhere in between rmb() and smp_rmb(). For example on ARM the rmb barriers break down as follows: Barrier Call Explanation --------- -------- ---------------------------------- rmb() dsb() Data synchronization barrier - system dma_rmb() dmb(osh) data memory barrier - outer sharable smp_rmb() dmb(ish) data memory barrier - inner sharable These new barriers are not as safe as the standard rmb() and wmb(). Specifically they do not guarantee ordering between coherent and incoherent memories. The primary use case for these would be to enforce ordering of reads and writes when accessing coherent memory that is shared between the CPU and a device. It may also be noted that there is no dma_mb(). Most architectures don't provide a good mechanism for performing a coherent only full barrier without resorting to the same mechanism used in mb(). As such there isn't much to be gained in trying to define such a function. Cc: Frederic Weisbecker Cc: Mathieu Desnoyers Cc: Michael Ellerman Cc: Michael Neuling Cc: Russell King Cc: Geert Uytterhoeven Cc: Heiko Carstens Cc: Linus Torvalds Cc: Martin Schwidefsky Cc: Tony Luck Cc: Oleg Nesterov Cc: "Paul E. McKenney" Cc: Peter Zijlstra Cc: Ingo Molnar Cc: David Miller Acked-by: Benjamin Herrenschmidt Acked-by: Will Deacon Signed-off-by: Alexander Duyck Signed-off-by: David S. Miller --- arch/s390/include/asm/barrier.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'arch/s390') diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h index 33d191d295e4..8d724718ec21 100644 --- a/arch/s390/include/asm/barrier.h +++ b/arch/s390/include/asm/barrier.h @@ -24,6 +24,8 @@ #define rmb() mb() #define wmb() mb() +#define dma_rmb() rmb() +#define dma_wmb() wmb() #define smp_mb() mb() #define smp_rmb() rmb() #define smp_wmb() wmb() -- cgit v1.2.3