linux-kernel - Re: [PATCH v2 1/2] linux/kernel.h: Introduce IDIV_ROUND

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Message-Id: <20120830161532.0db909c9.akpm@linux-foundation.org>
Date:	Thu, 30 Aug 2012 16:15:32 -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 v2 1/2] linux/kernel.h: Introduce IDIV_ROUND_CLOSEST

On Tue, 28 Aug 2012 09:30:55 -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, it is sometimes used on integers which can be negative (such as
> temperatures). Introduce new macro IDIV_ROUND_CLOSEST which also supports
> negative dividends.
> 

Can't we just fix DIV_ROUND_CLOSEST?  That will make it a bit slower
but it's not exactly a speed demon right now.  And fixing
DIV_ROUND_CLOSEST() might just fix other bugs that we don't know about
yet.

Also, the name IDIV_ROUND_CLOSEST doesn't communicate much at all.


> +#define IDIV_ROUND_CLOSEST(x, divisor)(			\
> +{							\
> +	typeof(x) __x = x;				\
> +	typeof(divisor) __d = divisor;			\
> +	(((typeof(x))-1) >= 0 || (__x) >= 0) ?		\
> +		DIV_ROUND_CLOSEST((__x), (__d)) :	\
> +		(((__x) - ((__d) / 2)) / (__d));	\
> +}							\
> +)

And it doesn't help that the new "function" is undocumented.  Yes, we
screwed up with DIV_ROUND_CLOSEST(), but that doesn't mean we need to
keep screwing up!
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