[PATCH] arm: Adding support for atomic half word exchange

[PATCH] arm: Adding support for atomic half word exchange

[Sarbojit Ganguly]

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Since 16 bit half word exchange was not there and MCS based qspinlock by Waiman's xchg_tail() requires an atomic exchange on a half word,
here is a small modification to __xchg() code to support the exchange.
ARMv6 and lower does not have support for LDREXH, so we need to make sure things do not break when we're compiling on ARMv6.

Signed-off-by: Sarbojit Ganguly <ganguly.s@samsung.com>
---
 arch/arm/include/asm/cmpxchg.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
index 1692a05..547101d 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -50,6 +50,24 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
 			: "r" (x), "r" (ptr)
 			: "memory", "cc");
 		break;
+#if !defined (CONFIG_CPU_V6)
+		/*
+		 * Halfword exclusive exchange
+		 * This is new implementation as qspinlock
+		 * wants 16 bit atomic CAS.
+		 * This is not supported on ARMv6.
+		 */
+	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;
+#endif
 	case 4:
 		asm volatile("@	__xchg4\n"
 		"1:	ldrex	%0, [%3]\n"
-- 

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[PATCH] arm: Adding support for atomic half word exchange

[Sarbojit Ganguly]

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<Ping>

Since 16 bit half word exchange was not there and MCS based qspinlock by Waiman's xchg_tail() requires an atomic exchange on a half word, here is a small modification to __xchg() code to support the exchange.
ARMv6 and lower does not have support for LDREXH, so we need to make sure things do not break when we're compiling on ARMv6.

Signed-off-by: Sarbojit Ganguly <ganguly.s@samsung.com>
---
 arch/arm/include/asm/cmpxchg.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h index 1692a05..547101d 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -50,6 +50,24 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
 			: "r" (x), "r" (ptr)
 			: "memory", "cc");
 		break;
+#if !defined (CONFIG_CPU_V6)
+		/*
+		 * Halfword exclusive exchange
+		 * This is new implementation as qspinlock
+		 * wants 16 bit atomic CAS.
+		 * This is not supported on ARMv6.
+		 */
+	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;
+#endif
 	case 4:
 		asm volatile("@	__xchg4\n"
 		"1:	ldrex	%0, [%3]\n"
-- 
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Re: [PATCH] arm: Adding support for atomic half word exchange

[Catalin Marinas]

On Tue, Aug 18, 2015 at 09:17:53AM +0100, Sarbojit Ganguly wrote:
> <Ping>
> 
> Since 16 bit half word exchange was not there and MCS based qspinlock by Waiman's xchg_tail() requires an atomic exchange on a half word, here is a small modification to __xchg() code to support the exchange.
> ARMv6 and lower does not have support for LDREXH, so we need to make sure things do not break when we're compiling on ARMv6.

First of all, please wrap the text appropriately.

Secondly, you need to cc the relevant maintainer and mailing list (try
running ./scripts/get_maintainer.pl on this patch to get some hints).

-- 
Catalin