[RFC PATCH v3 12/35] bitops: Document the difference in indexing between fls() and __fls()

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

fls() indexes the bits starting with 1, ie., from 1 to BITS_PER_LONG
whereas __fls() uses a zero-based indexing scheme (0 to BITS_PER_LONG - 1).
Add comments to document this important difference.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/include/asm/bitops.h      |    4 ++++
 include/asm-generic/bitops/__fls.h |    5 +++++
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 6dfd019..25e6fdc 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -380,6 +380,10 @@ static inline unsigned long ffz(unsigned long word)
  * @word: The word to search
  *
  * Undefined if no set bit exists, so code should check against 0 first.
+ *
+ * Note: __fls(x) is equivalent to fls(x) - 1. That is, __fls() uses
+ * a zero-based indexing scheme (0 to BITS_PER_LONG - 1), where
+ * __fls(1) = 0, __fls(2) = 1, and so on.
  */
 static inline unsigned long __fls(unsigned long word)
 {
diff --git a/include/asm-generic/bitops/__fls.h b/include/asm-generic/bitops/__fls.h
index a60a7cc..ae908a5 100644
--- a/include/asm-generic/bitops/__fls.h
+++ b/include/asm-generic/bitops/__fls.h
@@ -8,6 +8,11 @@
  * @word: the word to search
  *
  * Undefined if no set bit exists, so code should check against 0 first.
+ *
+ * Note: __fls(x) is equivalent to fls(x) - 1. That is, __fls() uses
+ * a zero-based indexing scheme (0 to BITS_PER_LONG - 1), where
+ * __fls(1) = 0, __fls(2) = 1, and so on.
+ *
  */
 static __always_inline unsigned long __fls(unsigned long word)
 {

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