linux-kernel - [PATCH] include/log2.h: Fix rounddown_pow_of

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Date:	Mon, 12 Dec 2011 10:02:33 -0800
From:	Dmitry Torokhov <dtor@...are.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	pv-drivers@...are.com, Andrei Warkentin <andreiw@...are.com>,
	stable@...nel.org, Dmitry Torokhov <dtor@...are.com>
Subject: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)

From: Andrei Warkentin <andreiw@...are.com>

1 is a power of two, therefore rounddown_pow_of_two(1) should return 1.
It does in case the argument is a variable but in case it's a constant
it behaves incorrectly and returns 0. Probably nobody ever did it so
this was never noticed, however drivers/net/vmxnet3 with latest GCC does
and breaks on unicpu systems.

This is similar to Rolf's patch to roundup_pow_of_two(1).

Signed-off-by: Andrei Warkentin <andreiw@...are.com>
Reviewed-by: Jesper Juhl <jj@...osbits.net>
Reviewed-by: Rolf Eike Beer <eike-kernel@...tec.de>
Cc: stable@...nel.org
Signed-off-by: Dmitry Torokhov <dtor@...are.com>
---
 include/linux/log2.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index 25b8086..ccda848 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -185,7 +185,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 #define rounddown_pow_of_two(n)			\
 (						\
 	__builtin_constant_p(n) ? (		\
-		(n == 1) ? 0 :			\
+		(n == 1) ? 1 :			\
 		(1UL << ilog2(n))) :		\
 	__rounddown_pow_of_two(n)		\
  )
-- 
1.7.4.1

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