[RFC PATCH v4 10/40] bitops: Document the difference in indexing between fls() and __fls()
From: Srivatsa S. Bhat <hidden>
Date: 2013-09-25 23:20:18
Also in:
linux-mm, lkml
Subsystem:
bitops api, generic include/asm header files, the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Yury Norov, Arnd Bergmann, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
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 <redacted> --- 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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