sh: support 1 and 2 byte xchg

This completes the xchg implementation for sh architecture.  Note: The
llsc variant is tricky since this only supports 4 byte atomics, the
existing implementation of 1 byte xchg is wrong: we need to do a 4 byte
cmpxchg and retry if any bytes changed meanwhile.

Write this in C for clarity.
Suggested-by: Rich Felker <dalias@libc.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

arch/sh/include/asm/cmpxchg-grb.h

@@ -23,6 +23,28 @@ static inline unsigned long xchg_u32(volatile u32 *m, unsigned long val)
 	return retval;
 }
 
+static inline unsigned long xchg_u16(volatile u16 *m, unsigned long val)
+{
+	unsigned long retval;
+
+	__asm__ __volatile__ (
+		"   .align  2             \n\t"
+		"   mova    1f,   r0      \n\t" /* r0 = end point */
+		"   mov    r15,   r1      \n\t" /* r1 = saved sp */
+		"   mov    #-6,   r15     \n\t" /* LOGIN */
+		"   mov.w  @%1,   %0      \n\t" /* load  old value */
+		"   extu.w  %0,   %0      \n\t" /* extend as unsigned */
+		"   mov.w   %2,   @%1     \n\t" /* store new value */
+		"1: mov     r1,   r15     \n\t" /* LOGOUT */
+		: "=&r" (retval),
+		  "+r"  (m),
+		  "+r"  (val)		/* inhibit r15 overloading */
+		:
+		: "memory" , "r0", "r1");
+
+	return retval;
+}
+
 static inline unsigned long xchg_u8(volatile u8 *m, unsigned long val)
 {
 	unsigned long retval;

arch/sh/include/asm/cmpxchg-irq.h

@@ -14,6 +14,17 @@ static inline unsigned long xchg_u32(volatile u32 *m, unsigned long val)
 	return retval;
 }
 
+static inline unsigned long xchg_u16(volatile u16 *m, unsigned long val)
+{
+	unsigned long flags, retval;
+
+	local_irq_save(flags);
+	retval = *m;
+	*m = val;
+	local_irq_restore(flags);
+	return retval;
+}
+
 static inline unsigned long xchg_u8(volatile u8 *m, unsigned long val)
 {
 	unsigned long flags, retval;

arch/sh/include/asm/cmpxchg-llsc.h

 #ifndef __ASM_SH_CMPXCHG_LLSC_H
 #define __ASM_SH_CMPXCHG_LLSC_H
 
+#include <linux/bitops.h>
+#include <asm/byteorder.h>
+
 static inline unsigned long xchg_u32(volatile u32 *m, unsigned long val)
 {
 	unsigned long retval;
@@ -22,29 +25,8 @@ static inline unsigned long xchg_u32(volatile u32 *m, unsigned long val)
 	return retval;
 }
 
-static inline unsigned long xchg_u8(volatile u8 *m, unsigned long val)
-{
-	unsigned long retval;
-	unsigned long tmp;
-
-	__asm__ __volatile__ (
-		"1:					\n\t"
-		"movli.l	@%2, %0	! xchg_u8	\n\t"
-		"mov		%0, %1			\n\t"
-		"mov		%3, %0			\n\t"
-		"movco.l	%0, @%2			\n\t"
-		"bf		1b			\n\t"
-		"synco					\n\t"
-		: "=&z"(tmp), "=&r" (retval)
-		: "r" (m), "r" (val & 0xff)
-		: "t", "memory"
-	);
-
-	return retval;
-}
-
 static inline unsigned long
-__cmpxchg_u32(volatile int *m, unsigned long old, unsigned long new)
+__cmpxchg_u32(volatile u32 *m, unsigned long old, unsigned long new)
 {
 	unsigned long retval;
 	unsigned long tmp;
@@ -68,4 +50,36 @@ __cmpxchg_u32(volatile int *m, unsigned long old, unsigned long new)
 	return retval;
 }
 
+static inline u32 __xchg_cmpxchg(volatile void *ptr, u32 x, int size)
+{
+	int off = (unsigned long)ptr % sizeof(u32);
+	volatile u32 *p = ptr - off;
+#ifdef __BIG_ENDIAN
+	int bitoff = (sizeof(u32) - 1 - off) * BITS_PER_BYTE;
+#else
+	int bitoff = off * BITS_PER_BYTE;
+#endif
+	u32 bitmask = ((0x1 << size * BITS_PER_BYTE) - 1) << bitoff;
+	u32 oldv, newv;
+	u32 ret;
+
+	do {
+		oldv = READ_ONCE(*p);
+		ret = (oldv & bitmask) >> bitoff;
+		newv = (oldv & ~bitmask) | (x << bitoff);
+	} while (__cmpxchg_u32(p, oldv, newv) != oldv);
+
+	return ret;
+}
+
+static inline unsigned long xchg_u16(volatile u16 *m, unsigned long val)
+{
+	return __xchg_cmpxchg(m, val, sizeof *m);
+}
+
+static inline unsigned long xchg_u8(volatile u8 *m, unsigned long val)
+{
+	return __xchg_cmpxchg(m, val, sizeof *m);
+}
+
 #endif /* __ASM_SH_CMPXCHG_LLSC_H */

arch/sh/include/asm/cmpxchg.h

@@ -27,6 +27,9 @@ extern void __xchg_called_with_bad_pointer(void);
 	case 4:						\
 		__xchg__res = xchg_u32(__xchg_ptr, x);	\
 		break;					\
+	case 2:						\
+		__xchg__res = xchg_u16(__xchg_ptr, x);	\
+		break;					\
 	case 1:						\
 		__xchg__res = xchg_u8(__xchg_ptr, x);	\
 		break;					\