powerpc/32: add memset16()

Commit 694fc88ce2 ("powerpc/string: Implement optimized
memset variants") added memset16(), memset32() and memset64()
for the 64 bits PPC.

On 32 bits, memset64() is not relevant, and as shown below,
the generic version of memset32() gives a good code, so only
memset16() is candidate for an optimised version.

000009c0 <memset32>:
 9c0:   2c 05 00 00     cmpwi   r5,0
 9c4:   39 23 ff fc     addi    r9,r3,-4
 9c8:   4d 82 00 20     beqlr
 9cc:   7c a9 03 a6     mtctr   r5
 9d0:   94 89 00 04     stwu    r4,4(r9)
 9d4:   42 00 ff fc     bdnz    9d0 <memset32+0x10>
 9d8:   4e 80 00 20     blr

The last part of memset() handling the not 4-bytes multiples
operates on bytes, making it unsuitable for handling word without
modification. As it would increase memset() complexity, it is
better to implement memset16() from scratch. In addition it
has the advantage of allowing a more optimised memset16() than what
we would have by using the memset() function.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/string.h

@@ -10,6 +10,7 @@
 #define __HAVE_ARCH_MEMMOVE
 #define __HAVE_ARCH_MEMCMP
 #define __HAVE_ARCH_MEMCHR
+#define __HAVE_ARCH_MEMSET16
 
 extern char * strcpy(char *,const char *);
 extern char * strncpy(char *,const char *, __kernel_size_t);
@@ -24,7 +25,6 @@ extern int memcmp(const void *,const void *,__kernel_size_t);
 extern void * memchr(const void *,int,__kernel_size_t);
 
 #ifdef CONFIG_PPC64
-#define __HAVE_ARCH_MEMSET16
 #define __HAVE_ARCH_MEMSET32
 #define __HAVE_ARCH_MEMSET64
 
@@ -46,6 +46,8 @@ static inline void *memset64(uint64_t *p, uint64_t v, __kernel_size_t n)
 {
 	return __memset64(p, v, n * 8);
 }
+#else
+extern void *memset16(uint16_t *, uint16_t, __kernel_size_t);
 #endif
 #endif /* __KERNEL__ */