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Subject[PATCH] Fix order_base_2(0)
Date
The order_base_2() function is either wrongly documented or wrongly
implemented. In the preceding comment, it says that:

ob2(0) = 0

but this is not valid as roundup_pow_of_two()'s documentation asserts that:

* - the result is undefined when n == 0

Fix order_base_2(n) to actually return 0 when n == 0.

However, should we just instead define that the result of order_base_2(0) is
undefined?

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Robert P. J. Day <rpjday@crashcourse.ca>
cc: Andrew Morton <akpm@linux-foundation.org>
---

include/linux/log2.h | 3 +--
1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index fd7ff3d..d6463d8 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -202,7 +202,6 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
* ob2(5) = 3
* ... and so on.
*/
-
-#define order_base_2(n) ilog2(roundup_pow_of_two(n))
+#define order_base_2(n) ((n) == 0 ? 0 : ilog2(roundup_pow_of_two(n)))

#endif /* _LINUX_LOG2_H */


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