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authorAlexander Duyck <alexander.h.duyck@redhat.com>2014-12-11 15:02:06 -0800
committerDavid S. Miller <davem@davemloft.net>2014-12-11 21:15:06 -0500
commit1077fa36f23e259858caf6f269a47393a5aff523 (patch)
tree569b84cfd3b409f07ce6a10f0166ca78307705e1 /arch/metag
parent8a449718414ff10b9d5559ed3e8e09c7178774f2 (diff)
downloadlinux-1077fa36f23e259858caf6f269a47393a5aff523.tar.gz
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 <fweisbec@gmail.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Cc: Michael Ellerman <michael@ellerman.id.au> Cc: Michael Neuling <mikey@neuling.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: David Miller <davem@davemloft.net> Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org> Acked-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'arch/metag')
-rw-r--r--arch/metag/include/asm/barrier.h14
1 files changed, 7 insertions, 7 deletions
diff --git a/arch/metag/include/asm/barrier.h b/arch/metag/include/asm/barrier.h
index 6d8b8c9b7c2..d703d8e26a6 100644
--- a/arch/metag/include/asm/barrier.h
+++ b/arch/metag/include/asm/barrier.h
@@ -4,8 +4,6 @@
#include <asm/metag_mem.h>
#define nop() asm volatile ("NOP")
-#define mb() wmb()
-#define rmb() barrier()
#ifdef CONFIG_METAG_META21
@@ -41,11 +39,13 @@ static inline void wr_fence(void)
#endif /* !CONFIG_METAG_META21 */
-static inline void wmb(void)
-{
- /* flush writes through the write combiner */
- wr_fence();
-}
+/* flush writes through the write combiner */
+#define mb() wr_fence()
+#define rmb() barrier()
+#define wmb() mb()
+
+#define dma_rmb() rmb()
+#define dma_wmb() wmb()
#ifndef CONFIG_SMP
#define fence() do { } while (0)