From: Guenter Roeck <linux@roeck-us.net>
To: Jean Delvare <khali@linux-fr.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.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 v4] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative dividends
Date: Sat, 1 Sep 2012 19:14:10 -0700 [thread overview]
Message-ID: <20120902021410.GA1578@roeck-us.net> (raw)
In-Reply-To: <20120901190254.108e3f6b@endymion.delvare>
On Sat, Sep 01, 2012 at 07:02:54PM +0200, Jean Delvare wrote:
> Hi Andrew, Guenter,
>
> On Fri, 31 Aug 2012 12:38:12 -0700, Andrew Morton wrote:
> > On Fri, 31 Aug 2012 08:02:19 -0700
> > Guenter Roeck <linux@roeck-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.
>
> My testing looks good too.
>
> Acked-by: Jean Delvare <khali@linux-fr.org>
>
Thanks a lot!
> > 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.
>
> Indeed. I looked for test size increase, and for my config, besides
> hwmon, I only spotted the following ones:
>
> drivers/media/rc/mceusb.o
> drivers/gpu/drm/i915/intel_pm.o
> drivers/tty/serial/pch_uart.o
>
> These don't look like hot paths, so I'd say we don't care. Plus the
> intel_pm one can probably be solved by using an unsigned int, I don't
> see why/how ia_freq could be negative (negative frequency anyone?) And
> for mceusb it can be solved easily by changing "1" to "1U".
>
Yes, it helps a lot that the C trickery optimizes the changes away most of the time.
I'll let the patch rest in -next for a few more days before I send it to Linus.
Guenter
next prev parent reply other threads:[~2012-09-02 2:14 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-31 15:02 [PATCH v4] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative dividends Guenter Roeck
2012-08-31 19:38 ` Andrew Morton
2012-08-31 21:05 ` Guenter Roeck
2012-09-01 17:02 ` Jean Delvare
2012-09-02 2:14 ` Guenter Roeck [this message]
2012-09-09 14:57 ` Geert Uytterhoeven
2012-09-09 16:17 ` Guenter Roeck
2012-09-09 20:38 ` Geert Uytterhoeven
2012-09-09 21:30 ` Guenter Roeck
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