[PATCH v2] asm-generic: introduce io_stop_wc() and add implementation for ARM64

[Xiongfeng Wang]

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 at huawei.com>
Suggested-by: Will Deacon <will at kernel.org>
Suggested-by: Catalin Marinas <catalin.marinas at arm.com>
---
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