linux-kernel - Re: [PATCH] kernel.h: Introduce IDIV_ROUND

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Message-ID: <20120828180926.4d64f99a@endymion.delvare>
Date:	Tue, 28 Aug 2012 18:09:26 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, lm-sensors@...sensors.org
Subject: Re: [PATCH] kernel.h: Introduce IDIV_ROUND_CLOSEST

Hi Guenter,

On Fri, 24 Aug 2012 17:53:17 -0700, Guenter Roeck 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 often used on integers which can be negative (such as
> temperatures). Introduce new macro IDIV_ROUND_CLOSEST which also supports
> negative dividends.

Good catch. It's been broken for years and I never noticed :(

Acked-by: Jean Delvare <khali@...ux-fr.org>

> 
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
> I can take this patch through my hwmon tree, but would like to get an Ack first.
> Alternative would be to put it into include/linux/hwmon.h, but I would prefer
> to avoid that.

I agree it should sit next to DIV_ROUND_CLOSEST in <linux/kernel.h>.

> 
> Also, if someone has an idea for a simpler implementation, I would really like
> to know about it.

I can't think of anything better.

Note that your implementation only supports negative dividend not
divisor. While it won't be a problem for hwmon drivers, and most
probably not a problem for other drivers either, it might be worth
putting in a comment so as to prevent false expectations from the
reader.

> 
>  include/linux/kernel.h |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 6043821..a89483c 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -89,6 +89,15 @@
>  }							\
>  )
>  
> +#define IDIV_ROUND_CLOSEST(x, divisor)(			\
> +{							\
> +	typeof(x) __x = x;				\
> +	typeof(divisor) __d1 = divisor;			\
> +	typeof(divisor) __d2 = (__x) < 0 ? -(__d1) : (__d1);\
> +	(((__x) + ((__d2) / 2)) / (__d1));		\
> +}							\
> +)
> +
>  /*
>   * Multiplies an integer by a fraction, while avoiding unnecessary
>   * overflow or loss of precision.


-- 
Jean Delvare
--
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