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.
Signed-off-by: Robert P. J. Day <[EMAIL PROTECTED]>
---
some brief tests indicate that these routines return the correct
answer. i'm not sure if i've officially submitted this before or if
it's already been accepted in a tree somewhere.
diff --git a/include/linux/log2.h b/include/linux/log2.h
index 1b8a2c1..ff8835b 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 */
--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA
http://crashcourse.ca
========================================================================
-
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