Gitweb:     
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b311e921b385b5a848f0806df16276aca1ecae07
Commit:     b311e921b385b5a848f0806df16276aca1ecae07
Parent:     d9aacccf457d07d729cfc7bf20ff0c7428357a9f
Author:     Robert P. J. Day <[EMAIL PROTECTED]>
AuthorDate: Tue Oct 16 23:29:32 2007 -0700
Committer:  Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Wed Oct 17 08:42:56 2007 -0700

    Add a "rounddown_pow_of_two" routine to log2.h
    
    To go along with the existing "roundup_pow_of_two" routine, add one for
    rounding down since that operation appears to crop up on a regular basis in
    the source tree.
    
    [EMAIL PROTECTED]: fix unbalanced parentheses]
    Signed-off-by: Robert P. J. Day <[EMAIL PROTECTED]>
    Signed-off-by: Mariusz Kozlowski <[EMAIL PROTECTED]>
    Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
---
 include/linux/log2.h |   25 +++++++++++++++++++++++++
 1 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index 1b8a2c1..c8cf5e8 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -63,6 +63,15 @@ unsigned long __roundup_pow_of_two(unsigned long n)
        return 1UL << fls_long(n - 1);
 }
 
+/*
+ * round down to nearest power of two
+ */
+static inline __attribute__((const))
+unsigned long __rounddown_pow_of_two(unsigned long n)
+{
+       return 1UL << (fls_long(n) - 1);
+}
+
 /**
  * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
  * @n - parameter
@@ -165,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
        __roundup_pow_of_two(n)                 \
  )
 
+/**
+ * rounddown_pow_of_two - round the given value down to nearest power of two
+ * @n - parameter
+ *
+ * round the given value down to the nearest power of two
+ * - the result is undefined when n == 0
+ * - this can be used to initialise global variables from constant data
+ */
+#define rounddown_pow_of_two(n)                        \
+(                                              \
+       __builtin_constant_p(n) ? (             \
+               (n == 1) ? 0 :                  \
+               (1UL << ilog2(n))) :            \
+       __rounddown_pow_of_two(n)               \
+ )
+
 #endif /* _LINUX_LOG2_H */
-
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