On Tue, Dec 18, 2012 at 10:04:56PM +0100, Juergen Beisert wrote:
> 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.
>
Yes. Try:
printf("%d\n", -10 / 2U);
I am amazed. Apparently the compiler converts an expression to unsigned if the
divisor is unsigned. Which of course completely defeats the purpose of the
operation.
I'll try to come up with a fix. Let me know if you have an idea. Obviously this
does not only affect the "0" case, but all operations with negative dividend and
unsigned divisor.
Guenter
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