MIPS: cmpxchg: Implement 1 byte & 2 byte cmpxchg()

Implement support for 1 & 2 byte cmpxchg() using read-modify-write atop
a 4 byte cmpxchg(). This allows us to support these atomic operations
despite the MIPS ISA only providing 4 & 8 byte atomic operations.

This is required in order to support queued rwlocks (qrwlock) in a later
patch, since these make use of a 1 byte cmpxchg() in their slow path.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16355/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/include/asm/cmpxchg.h b/arch/mips/include/asm/cmpxchg.h
index a633bf8..a633f61 100644 (file)
--- a/arch/mips/include/asm/cmpxchg.h
+++ b/arch/mips/include/asm/cmpxchg.h
@@ -142,10 +142,17 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz
        __ret;                                                          \
 })
 
+extern unsigned long __cmpxchg_small(volatile void *ptr, unsigned long old,
+                                    unsigned long new, unsigned int size);
+
 static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
                                      unsigned long new, unsigned int size)
 {
        switch (size) {
+       case 1:
+       case 2:
+               return __cmpxchg_small(ptr, old, new, size);
+
        case 4:
                return __cmpxchg_asm("ll", "sc", (volatile u32 *)ptr, old, new);
 

diff --git a/arch/mips/kernel/cmpxchg.c b/arch/mips/kernel/cmpxchg.c
index 5acfbf9..7730f1d 100644 (file)
--- a/arch/mips/kernel/cmpxchg.c
+++ b/arch/mips/kernel/cmpxchg.c
@@ -50,3 +50,60 @@ unsigned long __xchg_small(volatile void *ptr, unsigned long val, unsigned int s
 
        return (load32 & mask) >> shift;
 }
+
+unsigned long __cmpxchg_small(volatile void *ptr, unsigned long old,
+                             unsigned long new, unsigned int size)
+{
+       u32 mask, old32, new32, load32;
+       volatile u32 *ptr32;
+       unsigned int shift;
+       u8 load;
+
+       /* Check that ptr is naturally aligned */
+       WARN_ON((unsigned long)ptr & (size - 1));
+
+       /* Mask inputs to the correct size. */
+       mask = GENMASK((size * BITS_PER_BYTE) - 1, 0);
+       old &= mask;
+       new &= mask;
+
+       /*
+        * Calculate a shift & mask that correspond to the value we wish to
+        * compare & exchange within the naturally aligned 4 byte integer
+        * that includes it.
+        */
+       shift = (unsigned long)ptr & 0x3;
+       if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))
+               shift ^= sizeof(u32) - size;
+       shift *= BITS_PER_BYTE;
+       mask <<= shift;
+
+       /*
+        * Calculate a pointer to the naturally aligned 4 byte integer that
+        * includes our byte of interest, and load its value.
+        */
+       ptr32 = (volatile u32 *)((unsigned long)ptr & ~0x3);
+       load32 = *ptr32;
+
+       while (true) {
+               /*
+                * Ensure the byte we want to exchange matches the expected
+                * old value, and if not then bail.
+                */
+               load = (load32 & mask) >> shift;
+               if (load != old)
+                       return load;
+
+               /*
+                * Calculate the old & new values of the naturally aligned
+                * 4 byte integer that include the byte we want to exchange.
+                * Attempt to exchange the old value for the new value, and
+                * return if we succeed.
+                */
+               old32 = (load32 & ~mask) | (old << shift);
+               new32 = (load32 & ~mask) | (new << shift);
+               load32 = cmpxchg(ptr32, old32, new32);
+               if (load32 == old32)
+                       return old;
+       }
+}