linux-kernel - [RFC PATCH v4 10/40] bitops: Document the difference in indexing between fls() and _

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Message-ID: <20130925231554.26184.87472.stgit@srivatsabhat.in.ibm.com>
Date:	Thu, 26 Sep 2013 04:45:56 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	akpm@...ux-foundation.org, mgorman@...e.de, dave@...1.net,
	hannes@...xchg.org, tony.luck@...el.com,
	matthew.garrett@...ula.com, riel@...hat.com, arjan@...ux.intel.com,
	srinivas.pandruvada@...ux.intel.com, willy@...ux.intel.com,
	kamezawa.hiroyu@...fujitsu.com, lenb@...nel.org, rjw@...k.pl
Cc:	gargankita@...il.com, paulmck@...ux.vnet.ibm.com,
	svaidy@...ux.vnet.ibm.com, andi@...stfloor.org,
	isimatu.yasuaki@...fujitsu.com, santosh.shilimkar@...com,
	kosaki.motohiro@...il.com, srivatsa.bhat@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH v4 10/40] bitops: Document the difference in indexing
 between fls() and __fls()

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@...ux.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 41639ce..9186e4a 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -388,6 +388,10 @@ 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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