For memory accesses with write-combining attributes (e.g. those returned
by ioremap_wc()), the CPU may wait for prior accesses to be merged with
subsequent ones. But in some situation, such wait is bad for the
performance.
We introduce io_stop_wc() to prevent the merging of write-combining
memory accesses before this macro with those after it.
We add implementation for ARM64 using DGH instruction and provide NOP
implementation for other architectures.
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
Suggested-by: Will Deacon <will@xxxxxxxxxx>
Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
---
v1->v2: change 'Normal-Non Cacheable' to 'write-combining'
---
Documentation/memory-barriers.txt | 8 ++++++++
arch/arm64/include/asm/barrier.h | 9 +++++++++
include/asm-generic/barrier.h | 11 +++++++++++
3 files changed, 28 insertions(+)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 7367ada13208..b12df9137e1c 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1950,6 +1950,14 @@ There are some more advanced barrier functions:
For load from persistent memory, existing read memory barriers are sufficient
to ensure read ordering.
+ (*) io_stop_wc();
+
+ For memory accesses with write-combining attributes (e.g. those returned
+ by ioremap_wc(), the CPU may wait for prior accesses to be merged with
+ subsequent ones. io_stop_wc() can be used to prevent the merging of
+ write-combining memory accesses before this macro with those after it when
+ such wait has performance implications.
+
===============================
IMPLICIT KERNEL MEMORY BARRIERS
===============================
diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 1c5a00598458..62217be36217 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -26,6 +26,14 @@
#define __tsb_csync() asm volatile("hint #18" : : : "memory")
#define csdb() asm volatile("hint #20" : : : "memory")
+/*
+ * Data Gathering Hint:
+ * This instruction prevents merging memory accesses with Normal-NC or
+ * Device-GRE attributes before the hint instruction with any memory accesses
+ * appearing after the hint instruction.
+ */
+#define dgh() asm volatile("hint #6" : : : "memory")
+
#ifdef CONFIG_ARM64_PSEUDO_NMI
#define pmr_sync() \
do { \
@@ -46,6 +54,7 @@
#define dma_rmb() dmb(oshld)
#define dma_wmb() dmb(oshst)
+#define io_stop_wc() dgh()
#define tsb_csync() \
do { \
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 640f09479bdf..4c2c1b830344 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -251,5 +251,16 @@ do { \
#define pmem_wmb() wmb()
#endif
+/*
+ * ioremap_wc() maps I/O memory as memory with write-combining attributes. For
+ * this kind of memory accesses, the CPU may wait for prior accesses to be
+ * merged with subsequent ones. In some situation, such wait is bad for the
+ * performance. io_stop_wc() can be used to prevent the merging of
+ * write-combining memory accesses before this macro with those after it.
+ */
+#ifndef io_stop_wc
+#define io_stop_wc do { } while (0)
+#endif
+
#endif /* !__ASSEMBLY__ */
#endif /* __ASM_GENERIC_BARRIER_H */
--
2.20.1
