linux-kernel - Re: Strange results of DIV_ROUND

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Message-Id: <201212182204.56523.jbe@pengutronix.de>
Date:	Tue, 18 Dec 2012 22:04:56 +0100
From:	Juergen Beisert <jbe@...gutronix.de>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Strange results of DIV_ROUND_CLOSEST

Hi Guenter,

Guenter Roeck wrote:
> On Tue, Dec 18, 2012 at 04:03:41PM +0100, Juergen Beisert wrote:
> > commit 263a523d18bca306016d75f5c8d5c57c37fe52fb changes the code of
> > DIV_ROUND_CLOSEST in include/linux/kernel.h to fix a compile time
> > warning.
> >
> > But now feeding in a zero into this macro results into 4198403. Tested
> > with gcc 4.4.3 and 4.7.2, on arch x86 and ARM.
> >
> > I can reproduce this behaviour, when my ADC delivers a '0' value in the
> > driver drivers/hwmon/s3c-hwmon.c in function s3c_hwmon_ch_show() with a
> > current 3.7.1 kernel. The value is correct again, when the ADC delivers
> > at least a '1'.
> >
> > Any ideas how to fix it correctly?
>
> Odd one. I ran the macro through a large number of values and divisors as
> well as various optimization options, with different compilers, and always
> get correct results.
>
> What are your compile options, and what are the channel multiplier and
> dividers set to ?

Refer the lines 177 to 182 in drivers/hwmon/s3c-hwmon.c. "cfg->mult" is '3300' 
in my case, and "cfg->div" is '1023'. And whenever s3c_hwmon_read_ch() 
returns '0' line 184 returns '4198403' since Linux-3.6. checked with my 
gcc-4.6.2 cross compiler for Linux-3.6 and with gcc-4.6.2 for Linux-3.7.

I did a quick test with this macro on my host with gcc-4.4.3 and a simple 
userland program and surprise, surprise:

result = DIV_ROUND_CLOSEST(0, 1023);

works as expected (result is 0), but

int x = 0;
unsigned y = 1023;
result = DIV_ROUND_CLOSEST(x, y);

gives me result = 4198403!

Strange.

Regards,
Juergen

-- 
Pengutronix e.K.                              | Juergen Beisert             |
Linux Solutions for Science and Industry      | http://www.pengutronix.de/  |
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