From: Linus Torvalds Date: Tue, 13 Dec 2011 06:06:55 +0000 (-0800) Subject: linux/log2.h: Fix rounddown_pow_of_two(1) X-Git-Tag: v3.2-rc6~23 X-Git-Url: https://git.openpandora.org/cgi-bin/gitweb.cgi?p=pandora-kernel.git;a=commitdiff_plain;h=13c07b0286d340275f2d97adf085cecda37ede37;hp=12870da5c3ace563902babdfa10d824fdbd4ce88;ds=sidebyside linux/log2.h: Fix rounddown_pow_of_two(1) Exactly like roundup_pow_of_two(1), the rounddown version was buggy for the case of a compile-time constant '1' argument. Probably because it originated from the same code, sharing history with the roundup version from before the bugfix (for that one, see commit 1a06a52ee1b0: "Fix roundup_pow_of_two(1)"). However, unlike the roundup version, the fix for rounddown is to just remove the broken special case entirely. It's simply not needed - the generic code 1UL << ilog2(n) does the right thing for the constant '1' argment too. The only reason roundup needed that special case was because rounding up does so by subtracting one from the argument (and then adding one to the result) causing the obvious problems with "ilog2(0)". But rounddown doesn't do any of that, since ilog2() naturally truncates (ie "rounds down") to the right rounded down value. And without the ilog2(0) case, there's no reason for the special case that had the wrong value. tl;dr: rounddown_pow_of_two(1) should be 1, not 0. Acked-by: Dmitry Torokhov Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- diff --git a/include/linux/log2.h b/include/linux/log2.h index 25b808631cd9..fd7ff3d91e6a 100644 --- a/include/linux/log2.h +++ b/include/linux/log2.h @@ -185,7 +185,6 @@ unsigned long __rounddown_pow_of_two(unsigned long n) #define rounddown_pow_of_two(n) \ ( \ __builtin_constant_p(n) ? ( \ - (n == 1) ? 0 : \ (1UL << ilog2(n))) : \ __rounddown_pow_of_two(n) \ )