From: Jean Delvare <khali@linux-fr.org>
To: Guenter Roeck <linux@roeck-us.net>
Cc: linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Pekka Enberg <penberg@kernel.org>, Ingo Molnar <mingo@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
lm-sensors@lm-sensors.org
Subject: Re: [PATCH] kernel.h: Introduce IDIV_ROUND_CLOSEST
Date: Tue, 28 Aug 2012 18:09:26 +0200 [thread overview]
Message-ID: <20120828180926.4d64f99a@endymion.delvare> (raw)
In-Reply-To: <1345855997-9408-1-git-send-email-linux@roeck-us.net>
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@linux-fr.org>
>
> Signed-off-by: Guenter Roeck <linux@roeck-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
next prev parent reply other threads:[~2012-08-28 16:09 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-25 0:53 [PATCH] kernel.h: Introduce IDIV_ROUND_CLOSEST Guenter Roeck
2012-08-28 16:09 ` Jean Delvare [this message]
2012-08-28 16:20 ` Guenter Roeck
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