linux-kernel - Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND

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Date:	Thu, 20 Dec 2012 11:30:38 +0100
From:	Juergen Beisert <jbe@...gutronix.de>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org
Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

Hi Jean,

Jean Delvare wrote:
> On Wed, 19 Dec 2012 14:41:22 -0800, Guenter Roeck wrote:
> > On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> > > The changelog didn't describe the end-user visible effects of the bug.
> > > Please always include this information.  Because...
> >
> > One observed effect is that the s2c_hwmon driver reports a value of
> > 4198403 instead of 0 if the ADC reads 0.
> >
> > Other impact is unpredictable. Problem is seen if the divisor is an
> > unsigned variable or constant and the dividend is less than (divisor/2).
>
> Really? In my own testing, the problem only shows with dividend == 0,
> and even then, only when dividend is signed and divisor is not.
> DIV_ROUND_CLOSEST(5, 20U) returns 0 as expected, and so do
> DIV_ROUND_CLOSEST(0 / 20), DIV_ROUND_CLOSEST(0U / 20) and
> DIV_ROUND_CLOSEST(0U / 20U).
>
> Are your observations different?

I tried it with this simple user-land program to get an idea what's going 
wrong in the s3c_hwmon.c ADC driver:

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

int main(int argc, char *argv[])
{
	int x;
	unsigned y;

	printf("Constants\n");

	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(-1, 2));
	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(-1, 1023));
	printf("0 -> %d\n", DIV_ROUND_CLOSEST(0, 1023));
	printf("0 -> %d\n", DIV_ROUND_CLOSEST(0, 2));
	printf("1 -> %d\n", DIV_ROUND_CLOSEST(1, 2));
	printf("1 -> %d\n", DIV_ROUND_CLOSEST(3300, 1023));
	printf("2 -> %d\n", DIV_ROUND_CLOSEST(6600, 1023));

	printf("Variables\n");

	x = -1; y = 2;
	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(x, y));
	x = -1; y = 1023;
	printf("-1 -> %d\n", DIV_ROUND_CLOSEST(x, y));
	x = 0; y = 1023;
	printf("0 -> %d\n", DIV_ROUND_CLOSEST(x, y));
	x = 3300; y = 1023;
	printf("3300 -> %d\n", DIV_ROUND_CLOSEST(3300, 1023));
	x = 6600; y = 1023;
	printf("6600 -> %d\n", DIV_ROUND_CLOSEST(6600, 1023));

	return 0;
}

Result is on my x86 host (same on my ARM target):

Constants
-1 -> -1
-1 -> 0
0 -> 0
0 -> 0
1 -> 1
1 -> 3
2 -> 6
Variables
-1 -> 2147483647
-1 -> 4198403
0 -> 4198403
3300 -> 3
6600 -> 6

Regards,
Juergen

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