[RFC PATCH] arm: asm/cmpxchg.h: Add support half-word xchg()

Pranith Kumar
2015-02-25T15:36:20Z

This patch adds support for a half-word xchg() for ARM using ldrexh/strexh
instructions. It also fixes an asm comment for __cmpxchg2.

Currently using a half-word xchg() results in the following splat on an ARMv7
machine.

[ 45.833303] xchg: bad data size: pc 0xbe806020, ptr 0xeb18deee, size 2
[ 45.833324] ------------[ cut here ]------------
[ 45.837939] kernel BUG at /dvs/git/dirty/git-master_linux/kernel/arch/arm/kernel/traps.c:727!

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
arch/arm/include/asm/cmpxchg.h | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
index abb2c37..9505cca 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -50,6 +50,16 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
: "r" (x), "r" (ptr)
: "memory", "cc");
break;
+ case 2:
+ asm volatile("@ __xchg2\n"
+ "1: ldrexh %0, [%3]\n"
+ " strexh %1, %2, [%3]\n"
+ " teq %1, #0\n"
+ " bne 1b"
+ : "=&r" (ret), "=&r" (tmp)
+ : "r" (x), "r" (ptr)
+ : "memory", "cc");
+ break;
case 4:
asm volatile("@ __xchg4\n"
"1: ldrex %0, [%3]\n"
@@ -93,6 +103,12 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
: "memory", "cc");
break;
#endif
+ case 2:
+ raw_local_irq_save(flags);
+ ret = *(volatile uint16_t *)ptr;
+ *(volatile uint16_t *)ptr = x;
+ raw_local_irq_restore(flags);
+ break;
default:
__bad_xchg(ptr, size), ret = 0;
break;
@@ -158,7 +174,7 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
break;
case 2:
do {
- asm volatile("@ __cmpxchg1\n"
+ asm volatile("@ __cmpxchg2\n"
" ldrexh %1, [%2]\n"
" mov %0, #0\n"
" teq %1, %3\n"
--
1.9.1