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

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Message-ID: <alpine.LNX.2.00.1111082056510.5763@swampdragon.chaosbits.net>
Date:	Tue, 8 Nov 2011 20:57:27 +0100 (CET)
From:	Jesper Juhl <jj@...osbits.net>
To:	Andrei Warkentin <andreiw@...are.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)

On Tue, 8 Nov 2011, Andrei Warkentin wrote:

> 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
> wrong and returns 0. Probably nobody ever did it so this was never noticed,
> however net/drivers/vmxnet3 with latest GCC does and breaks on unicpu systems.
> 
> Signed-off-by: Andrei Warkentin <andreiw@...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)		\
>   )
> 

For what it's worth; looks good to me.

-- 
Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
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