__div64_32(): make it overridable at compile time

Some architectures may want to override the default implementation
at compile time to do things inline.  For example, ARM uses a
non-standard calling convention for better efficiency in this case.
Signed-off-by: Nicolas Pitre <nico@linaro.org>

include/asm-generic/div64.h

@@ -194,7 +194,9 @@ static inline uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
 }
 #endif
 
+#ifndef __div64_32
 extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
+#endif
 
 /* The unnecessary pointer compare is there
  * to check for type safety (n must be 64bit)

lib/div64.c

@@ -13,7 +13,8 @@
  *
  * Code generated for this function might be very inefficient
  * for some CPUs. __div64_32() can be overridden by linking arch-specific
- * assembly versions such as arch/ppc/lib/div64.S and arch/sh/lib/div64.S.
+ * assembly versions such as arch/ppc/lib/div64.S and arch/sh/lib/div64.S
+ * or by defining a preprocessor macro in arch/include/asm/div64.h.
  */
 
 #include <linux/export.h>
@@ -23,6 +24,7 @@
 /* Not needed on 64bit architectures */
 #if BITS_PER_LONG == 32
 
+#ifndef __div64_32
 uint32_t __attribute__((weak)) __div64_32(uint64_t *n, uint32_t base)
 {
 	uint64_t rem = *n;
@@ -55,8 +57,8 @@ uint32_t __attribute__((weak)) __div64_32(uint64_t *n, uint32_t base)
 	*n = res;
 	return rem;
 }
-
 EXPORT_SYMBOL(__div64_32);
+#endif
 
 #ifndef div_s64_rem
 s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder)