[RFC] __ffs64()

[Steven Whitehouse <swhiteho@redhat.com>]

Hi,

I'd like to add a new bitop, __ffs64() which I need in order to fix a
bug in GFS2. The question is, where should it go?

On 64 bit arches, __ffs64() would be a synonym for __ffs(), but on 32
bit arches it degenerates to a conditional plus a call to __ffs(). I'm
assuming that there would not be a lot of point in optimising this
operation on 32 bit arches even if such an instruction was available, so
that I should do something like the below patch.

Does that seem reasonable, or should I give it a separate header file
under asm-generic/bitops/ like some of the similar operations? It looks
like I'd have to touch a lot of other files if I were to go that route,

Steve.

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 6182913..d15255f 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -112,6 +112,23 @@ static inline unsigned fls_long(unsigned long l)
 	return fls64(l);
 }
 
+/**
+ * __ffs64 - find first set bit in a 64 bit word
+ * @word: The 64 bit word
+ *
+ * On 64 bit arches this is a synomyn for __ffs
+ */
+static inline unsigned long __ffs64(u64 word)
+{
+#if BITS_PER_LONG == 32
+	if (((u32)word) == 0UL)
+		return __ffs((u32)(word >> 32)) + 32;
+#elif BITS_PER_LONG != 64
+#error BITS_PER_LONG not 32 or 64
+#endif
+	return __ffs((unsigned long)word);
+}
+
 #ifdef __KERNEL__
 #ifdef CONFIG_GENERIC_FIND_FIRST_BIT