[PATCH v5 1/3] ARC: Emulate two-byte cmpxchg

[Bradley Morgan <brads@mainlining.org>]

ARC has no two-byte atomic compare and swap, so the
arch_cmpxchg_relaxed() macro switch lets case 2 fall through to
BUILD_BUG() via default, and a two-byte cmpxchg() fails the build.

Route case 2 through the new cmpxchg_emu_u16(), which narrows the
old and new values itself, so the dispatch keeps the (uintptr_t)
casts the case 1 path already uses.

The sizeof(*(_p_)) fix that v4 folded in here landed separately
through Vineet's f050c3e61d2a, so this patch is now only the
case 2 wiring.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/arc/include/asm/cmpxchg.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index 66b3743c9400..b30b20933f32 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -50,6 +50,9 @@
 	case 1:								\
 		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
 		break;							\
+	case 2:								\
+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *__force)_p_, (unsigned long)_o_, (unsigned long)_n_);	\
+		break;							\
 	case 4:								\
 		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
 		break;							\
-- 
2.53.0