MIPS: Fix microMIPS LL/SC immediate offsets

In the microMIPS encoding some memory access instructions have their
immediate offset reduced to 12 bits only.  That does not match the GCC
`R' constraint we use in some places to satisfy the requirement,
resulting in build failures like this:

{standard input}: Assembler messages:
{standard input}:720: Error: macro used $at after ".set noat"
{standard input}:720: Warning: macro instruction expanded into multiple instructions

Fix the problem by defining a macro, `GCC_OFF12_ASM', that expands to
the right constraint depending on whether microMIPS or standard MIPS
code is produced.  Also apply the fix to where `m' is used as in the
worst case this change does nothing, e.g. where the pointer was already
in a register such as a function argument and no further offset was
requested, and in the best case it avoids an extraneous sequence of up
to two instructions to load the high 20 bits of the address in the LL/SC
loop.  This reduces the risk of lock contention that is the higher the
more instructions there are in the critical section between LL and SC.

Strictly speaking we could just bulk-replace `R' with `ZC' as the latter
constraint adjusts automatically depending on the ISA selected.
However it was only introduced with GCC 4.9 and we keep supporing older
compilers for the standard MIPS configuration, hence the slightly more
complicated approach I chose.

The choice of a zero-argument function-like rather than an object-like
macro was made so that it does not look like a function call taking the
C expression used for the constraint as an argument.  This is so as not
to confuse the reader or formatting checkers like `checkpatch.pl' and
follows previous practice.

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8482/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 15 insertions, 12 deletions

diff --git a/arch/mips/include/asm/cmpxchg.h b/arch/mips/include/asm/cmpxchg.h
index eefcaa3..28b1edf 100644
--- a/arch/mips/include/asm/cmpxchg.h
+++ b/arch/mips/include/asm/cmpxchg.h
@@ -10,6 +10,7 @@
 
 #include <linux/bug.h>
 #include <linux/irqflags.h>
+#include <asm/compiler.h>
 #include <asm/war.h>
 
 static inline unsigned long __xchg_u32(volatile int * m, unsigned int val)
@@ -30,8 +31,8 @@ static inline unsigned long __xchg_u32(volatile int * m, unsigned int val)
 		"	sc	%2, %1					\n"
 		"	beqzl	%2, 1b					\n"
 		"	.set	mips0					\n"
-		: "=&r" (retval), "=m" (*m), "=&r" (dummy)
-		: "R" (*m), "Jr" (val)
+		: "=&r" (retval), "=" GCC_OFF12_ASM() (*m), "=&r" (dummy)
+		: GCC_OFF12_ASM() (*m), "Jr" (val)
 		: "memory");
 	} else if (kernel_uses_llsc) {
 		unsigned long dummy;
@@ -45,8 +46,9 @@ static inline unsigned long __xchg_u32(volatile int * m, unsigned int val)
 			"	.set	arch=r4000			\n"
 			"	sc	%2, %1				\n"
 			"	.set	mips0				\n"
-			: "=&r" (retval), "=m" (*m), "=&r" (dummy)
-			: "R" (*m), "Jr" (val)
+			: "=&r" (retval), "=" GCC_OFF12_ASM() (*m),
+			  "=&r" (dummy)
+			: GCC_OFF12_ASM() (*m), "Jr" (val)
 			: "memory");
 		} while (unlikely(!dummy));
 	} else {
@@ -80,8 +82,8 @@ static inline __u64 __xchg_u64(volatile __u64 * m, __u64 val)
 		"	scd	%2, %1					\n"
 		"	beqzl	%2, 1b					\n"
 		"	.set	mips0					\n"
-		: "=&r" (retval), "=m" (*m), "=&r" (dummy)
-		: "R" (*m), "Jr" (val)
+		: "=&r" (retval), "=" GCC_OFF12_ASM() (*m), "=&r" (dummy)
+		: GCC_OFF12_ASM() (*m), "Jr" (val)
 		: "memory");
 	} else if (kernel_uses_llsc) {
 		unsigned long dummy;
@@ -93,8 +95,9 @@ static inline __u64 __xchg_u64(volatile __u64 * m, __u64 val)
 			"	move	%2, %z4				\n"
 			"	scd	%2, %1				\n"
 			"	.set	mips0				\n"
-			: "=&r" (retval), "=m" (*m), "=&r" (dummy)
-			: "R" (*m), "Jr" (val)
+			: "=&r" (retval), "=" GCC_OFF12_ASM() (*m),
+			  "=&r" (dummy)
+			: GCC_OFF12_ASM() (*m), "Jr" (val)
 			: "memory");
 		} while (unlikely(!dummy));
 	} else {
@@ -155,8 +158,8 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz
 		"	beqzl	$1, 1b				\n"	\
 		"2:						\n"	\
 		"	.set	pop				\n"	\
-		: "=&r" (__ret), "=R" (*m)				\
-		: "R" (*m), "Jr" (old), "Jr" (new)			\
+		: "=&r" (__ret), "=" GCC_OFF12_ASM() (*m)		\
+		: GCC_OFF12_ASM() (*m), "Jr" (old), "Jr" (new)		\
 		: "memory");						\
 	} else if (kernel_uses_llsc) {					\
 		__asm__ __volatile__(					\
@@ -172,8 +175,8 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz
 		"	beqz	$1, 1b				\n"	\
 		"	.set	pop				\n"	\
 		"2:						\n"	\
-		: "=&r" (__ret), "=R" (*m)				\
-		: "R" (*m), "Jr" (old), "Jr" (new)			\
+		: "=&r" (__ret), "=" GCC_OFF12_ASM() (*m)		\
+		: GCC_OFF12_ASM() (*m), "Jr" (old), "Jr" (new)		\
 		: "memory");						\
 	} else {							\
 		unsigned long __flags;					\