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asm-generic: introduce io_stop_wc() and add implementation for ARM64

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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@huawei.com>
Suggested-by: Will Deacon <will@kernel.org>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20211221035556.60346-1-wangxiongfeng2@huawei.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Xiongfeng Wang 2021-12-21 11:55:56 +08:00 committed by Catalin Marinas
parent c2c529b27c
commit d5624bb29f
3 changed files with 28 additions and 0 deletions
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8
Documentation/memory-barriers.txt
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@ -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
===============================

9
arch/arm64/include/asm/barrier.h
View File

@ -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 { \

11
include/asm-generic/barrier.h
View File

@ -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 */