ARC: Emulate one-byte cmpxchg

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. ]
[ paulmck: Apply feedback from Vineet Gupta. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Palmer Dabbelt <palmer@rivosinc.com>
Cc: <linux-snps-arc@lists.infradead.org>
Acked-by: Vineet Gupta <vgupta@kernel.org>

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

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;							\
@@ -65,8 +69,6 @@
 	__typeof__(*(ptr)) _prev_;					\
 	unsigned long __flags;						\
 									\
-	BUILD_BUG_ON(sizeof(_p_) != 4);					\
-									\
 	/*								\
 	 * spin lock/unlock provide the needed smp_mb() before/after	\
 	 */								\