Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors

Jean Delvare
2012-12-20T11:01:16Z

Hi Juergen,

On Thu, 20 Dec 2012 11:30:38 +0100, Juergen Beisert wrote:
> Hi Jean,
>
> Jean Delvare wrote:
> > On Wed, 19 Dec 2012 14:41:22 -0800, Guenter Roeck wrote:
> > > 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));

This all works properly, because everything is signed here.

> 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));

I don't think variables vs. constants make any difference. What makes a
difference is signed vs. unsigned. You see failures here because y is
unsigned. You'd see the same with the constants above by changing 2 to
2U and 1023 to 1023U.

>
> 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

I see the same here with your test program.

--
Jean Delvare