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Subject[PATCH v2 4/7] arch/sparc: Introduce cmpxchg_u8 SPARC
Date
SPARC supports 32 bit and 64 bit cmpxchg right now. Add support
for 8 bit (1 byte) cmpxchg. This is required to support queued
rwlocks feature which uses 1 byte cmpxchg.

The function __cmpxchg_u8 here uses the 4 byte cas instruction with a
byte manipulation to achieve 1 byte cmpxchg.

Signed-off-by: Babu Moger <babu.moger@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Reviewed-by: Jane Chu <jane.chu@oracle.com>
Reviewed-by: Vijay Kumar <vijay.ac.kumar@oracle.com>
---
arch/sparc/include/asm/cmpxchg_64.h | 29 +++++++++++++++++++++++++++++
1 files changed, 29 insertions(+), 0 deletions(-)

diff --git a/arch/sparc/include/asm/cmpxchg_64.h b/arch/sparc/include/asm/cmpxchg_64.h
index faa2f61..000f7d7 100644
--- a/arch/sparc/include/asm/cmpxchg_64.h
+++ b/arch/sparc/include/asm/cmpxchg_64.h
@@ -87,6 +87,33 @@ static inline unsigned long __xchg(unsigned long x, __volatile__ void * ptr,
return new;
}

+/*
+ * Use 4 byte cas instruction to achieve 1 byte cmpxchg. Main logic
+ * here is to get the bit shift of the byte we are interested in.
+ * The XOR is handy for reversing the bits for big-endian byte order
+ */
+static inline unsigned long
+__cmpxchg_u8(volatile unsigned char *m, unsigned char old, unsigned char new)
+{
+ unsigned long maddr = (unsigned long)m;
+ int bit_shift = (((unsigned long)m & 3) ^ 3) << 3;
+ unsigned int mask = 0xff << bit_shift;
+ unsigned int *ptr = (unsigned int *) (maddr & ~3);
+ unsigned int old32, new32, load;
+ unsigned int load32 = *ptr;
+
+ do {
+ new32 = (load32 & ~mask) | (new << bit_shift);
+ old32 = (load32 & ~mask) | (old << bit_shift);
+ load32 = __cmpxchg_u32(ptr, old32, new32);
+ if (load32 == old32)
+ return old;
+ load = (load32 & mask) >> bit_shift;
+ } while (load == old);
+
+ return load;
+}
+
/* This function doesn't exist, so you'll get a linker error
if something tries to do an invalid cmpxchg(). */
void __cmpxchg_called_with_bad_pointer(void);
@@ -95,6 +122,8 @@ static inline unsigned long __xchg(unsigned long x, __volatile__ void * ptr,
__cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
{
switch (size) {
+ case 1:
+ return __cmpxchg_u8(ptr, old, new);
case 4:
return __cmpxchg_u32(ptr, old, new);
case 8:
--
1.7.1
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