linux-kernel - Re: [PATCH v4] linux/kernel.h: Fix DIV_ROUND

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Message-Id: <20120831123812.62729e1c.akpm@linux-foundation.org>
Date:	Fri, 31 Aug 2012 12:38:12 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jean Delvare <khali@...ux-fr.org>, lm-sensors@...sensors.org
Subject: Re: [PATCH v4] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support
 negative dividends

On Fri, 31 Aug 2012 08:02:19 -0700
Guenter Roeck <linux@...ck-us.net> wrote:

> DIV_ROUND_CLOSEST returns a bad result for negative dividends:
> 	DIV_ROUND_CLOSEST(-2, 2) = 0
> 
> Most of the time this does not matter. However, in the hardware monitoring
> subsystem, DIV_ROUND_CLOSEST is sometimes used on integers which can be
> negative (such as temperatures).
> 
> ...
>
> +
> +/*
> + * Divide positive or negative dividend by positive divisor and round
> + * to closest integer. Result is undefined for negative divisors.
> + */
>  #define DIV_ROUND_CLOSEST(x, divisor)(			\
>  {							\
> -	typeof(divisor) __divisor = divisor;		\
> -	(((x) + ((__divisor) / 2)) / (__divisor));	\
> +	typeof(x) __x = x;				\
> +	typeof(divisor) __d = divisor;			\
> +	(((typeof(x))-1) >= 0 || (__x) >= 0) ?		\
> +		(((__x) + ((__d) / 2)) / (__d)) :	\
> +		(((__x) - ((__d) / 2)) / (__d));	\
>  }							\

Looks good to me.  The patch causes no change in text size for
kernel/sched/fair.o and drivers/cpuidle/governors/menu.o, so it seems
that the cc trickery is working.

I'll add it to my tree for testing - please merge this via your tree.
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