[RFC 4/5] powerpc: atomic: implement xchg_* and atomic{, 64}_xchg_* variants

[Boqun Feng <boqun.feng@gmail.com>]

Implement xchg_relaxed and define atomic{,64}_xchg_* as xchg_relaxed,
based on these _relaxed variants, release/acquire variants can be built.

Note that xchg_relaxed and atomic_{,64}_xchg_relaxed are not compiler
barriers.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 arch/powerpc/include/asm/atomic.h  |  2 ++
 arch/powerpc/include/asm/cmpxchg.h | 64 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/arch/powerpc/include/asm/atomic.h b/arch/powerpc/include/asm/atomic.h
index 806ce50..4965dcf 100644
--- a/arch/powerpc/include/asm/atomic.h
+++ b/arch/powerpc/include/asm/atomic.h
@@ -193,6 +193,7 @@ static __inline__ int atomic_dec_return(atomic_t *v)
 
 #define atomic_cmpxchg(v, o, n) (cmpxchg(&((v)->counter), (o), (n)))
 #define atomic_xchg(v, new) (xchg(&((v)->counter), new))
+#define atomic_xchg_relaxed(v, new) xchg_relaxed(&((v)->counter), (new))
 
 /**
  * __atomic_add_unless - add unless the number is a given value
@@ -461,6 +462,7 @@ static __inline__ long atomic64_dec_if_positive(atomic64_t *v)
 
 #define atomic64_cmpxchg(v, o, n) (cmpxchg(&((v)->counter), (o), (n)))
 #define atomic64_xchg(v, new) (xchg(&((v)->counter), new))
+#define atomic64_xchg_relaxed(v, new) xchg_relaxed(&((v)->counter), (new))
 
 /**
  * atomic64_add_unless - add unless the number is a given value
diff --git a/arch/powerpc/include/asm/cmpxchg.h b/arch/powerpc/include/asm/cmpxchg.h
index ad6263c..66374f4 100644
--- a/arch/powerpc/include/asm/cmpxchg.h
+++ b/arch/powerpc/include/asm/cmpxchg.h
@@ -54,6 +54,32 @@ __xchg_u32_local(volatile void *p, unsigned long val)
 	return prev;
 }
 
+/*
+ * Atomic exchange relaxed
+ *
+ * Changes the memory location '*p' to be val and returns
+ * the previous value stored there.
+ *
+ * Note that this is not a compiler barrier, there is no order
+ * guarantee around.
+ */
+static __always_inline unsigned long
+__xchg_u32_relaxed(u32 *p, unsigned long val)
+{
+	unsigned long prev;
+
+	__asm__ __volatile__(
+"1:	lwarx	%0,0,%2\n"
+	PPC405_ERR77(0, %2)
+"	stwcx.	%3,0,%2\n"
+"	bne-	1b"
+	: "=&r" (prev), "+m" (*p)
+	: "r" (p), "r" (val)
+	: "cc");
+
+	return prev;
+}
+
 #ifdef CONFIG_PPC64
 static __always_inline unsigned long
 __xchg_u64(volatile void *p, unsigned long val)
@@ -90,6 +116,23 @@ __xchg_u64_local(volatile void *p, unsigned long val)
 
 	return prev;
 }
+
+static __always_inline unsigned long
+__xchg_u64_relaxed(u64 *p, unsigned long val)
+{
+	unsigned long prev;
+
+	__asm__ __volatile__(
+"1:	ldarx	%0,0,%2\n"
+	PPC405_ERR77(0, %2)
+"	stdcx.	%3,0,%2\n"
+"	bne-	1b"
+	: "=&r" (prev), "+m" (*p)
+	: "r" (p), "r" (val)
+	: "cc");
+
+	return prev;
+}
 #endif
 
 /*
@@ -127,6 +170,21 @@ __xchg_local(volatile void *ptr, unsigned long x, unsigned int size)
 	__xchg_called_with_bad_pointer();
 	return x;
 }
+
+static __always_inline unsigned long
+__xchg_relaxed(void *ptr, unsigned long x, unsigned int size)
+{
+	switch (size) {
+	case 4:
+		return __xchg_u32_relaxed(ptr, x);
+#ifdef CONFIG_PPC64
+	case 8:
+		return __xchg_u64_relaxed(ptr, x);
+#endif
+	}
+	__xchg_called_with_bad_pointer();
+	return x;
+}
 #define xchg(ptr,x)							     \
   ({									     \
      __typeof__(*(ptr)) _x_ = (x);					     \
@@ -140,6 +198,12 @@ __xchg_local(volatile void *ptr, unsigned long x, unsigned int size)
      		(unsigned long)_x_, sizeof(*(ptr))); 			     \
   })
 
+#define xchg_relaxed(ptr, x)						\
+({									\
+	__typeof__(*(ptr)) _x_ = (x);					\
+	(__typeof__(*(ptr))) __xchg_relaxed((ptr),			\
+			(unsigned long)_x_, sizeof(*(ptr)));		\
+})
 /*
  * Compare and exchange - if *p == old, set it to new,
  * and return the old value of *p.
-- 
2.5.0