[PATCH v3 cmpxchg 1/4] ARC: Emulate one-byte cmpxchg

[Paul E. McKenney]

Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on arc.

[ paulmck: Drop two-byte support per Arnd Bergmann feedback. ]
[ paulmck: Apply feedback from Naresh Kamboju. ]
[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Paul E. McKenney <paulmck at kernel.org>
Cc: Vineet Gupta <vgupta at kernel.org>
Cc: Andi Shyti <andi.shyti at linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda at intel.com>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Palmer Dabbelt <palmer at rivosinc.com>
Cc: <linux-snps-arc at lists.infradead.org>
---
 arch/arc/Kconfig               |  1 +
 arch/arc/include/asm/cmpxchg.h | 33 ++++++++++++++++++++++++---------
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index fd0b0a0d4686a..163608fd49d18 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -13,6 +13,7 @@ config ARC
 	select ARCH_HAS_SETUP_DMA_OPS
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
+	select ARCH_NEED_CMPXCHG_1_EMU
 	select ARCH_SUPPORTS_ATOMIC_RMW if ARC_HAS_LLSC
 	select ARCH_32BIT_OFF_T
 	select BUILDTIME_TABLE_SORT
diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index e138fde067dea..2102ce076f28b 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -8,6 +8,7 @@
 
 #include <linux/build_bug.h>
 #include <linux/types.h>
+#include <linux/cmpxchg-emu.h>
 
 #include <asm/barrier.h>
 #include <asm/smp.h>
@@ -46,6 +47,9 @@
 	__typeof__(*(ptr)) _prev_;					\
 									\
 	switch(sizeof((_p_))) {						\
+	case 1:								\
+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
+		break;							\
 	case 4:								\
 		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
 		break;							\
@@ -65,16 +69,27 @@
 	__typeof__(*(ptr)) _prev_;					\
 	unsigned long __flags;						\
 									\
-	BUILD_BUG_ON(sizeof(_p_) != 4);					\
+	switch(sizeof((_p_))) {						\
+	case 1:								\
+		__flags = cmpxchg_emu_u8((volatile u8 *)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
+		_prev_ = (__typeof__(*(ptr)))__flags;			\
+		break;							\
+		break;							\
+	case 4:								\
+		/*							\
+		 * spin lock/unlock provide the needed smp_mb()		\
+		 * before/after						\
+		 */							\
+		atomic_ops_lock(__flags);				\
+		_prev_ = *_p_;						\
+		if (_prev_ == _o_)					\
+			*_p_ = _n_;					\
+		atomic_ops_unlock(__flags);				\
+		break;							\
+	default:							\
+		BUILD_BUG();						\
+	}								\
 									\
-	/*								\
-	 * spin lock/unlock provide the needed smp_mb() before/after	\
-	 */								\
-	atomic_ops_lock(__flags);					\
-	_prev_ = *_p_;							\
-	if (_prev_ == _o_)						\
-		*_p_ = _n_;						\
-	atomic_ops_unlock(__flags);					\
 	_prev_;								\
 })
 
-- 
2.40.1