Messages in this thread | | | From | Juergen Beisert <> | Subject | Re: Strange results of DIV_ROUND_CLOSEST | Date | Wed, 19 Dec 2012 08:32:53 +0100 |
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Hi Guenter,
Guenter Roeck wrote: > On Tue, Dec 18, 2012 at 10:04:56PM +0100, Juergen Beisert wrote: > > 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! > > Can you test the following patch ? > > From d4a639c6ace7cb0de247f59c38abac72d671f135 Mon Sep 17 00:00:00 2001 > From: Guenter Roeck <linux@roeck-us.net> > Date: Tue, 18 Dec 2012 15:48:44 -0800 > Subject: [PATCH] linux/kernel.h: Fix DIV_ROUND_CLOSEST for unsigned > divisors > > Commit 263a523 fixes a warning seen with W=1 due to change in > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations > with unsigned divisors to unsigned, even if the dividend is signed and > negative (-10 / 5U = 858993457). As a result, DIV_ROUND_CLOSEST(0, 2U) > and similar operations now return bad data. > > Fix by checking for the divisor variable type when deciding which operation > to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad data > for negative dividends divided by unsigned divisors. Mark the latter case > as unsupported. > > Reported-by: Juergen Beisert <jbe@pengutronix.de> > Signed-off-by: Guenter Roeck <linux@roeck-us.net> > --- > An alternative would be to always convert divisors to signed: > int __d = (int)divisor; \ > Not sure if that would be better. > > include/linux/kernel.h | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h > index d97ed58..45726dc 100644 > --- a/include/linux/kernel.h > +++ b/include/linux/kernel.h > @@ -77,13 +77,15 @@ > > /* > * Divide positive or negative dividend by positive divisor and round > - * to closest integer. Result is undefined for negative divisors. > + * to closest integer. Result is undefined for negative divisors and > + * for negative dividends if the divisor variable type is unsigned. > */ > #define DIV_ROUND_CLOSEST(x, divisor)( \ > { \ > typeof(x) __x = x; \ > typeof(divisor) __d = divisor; \ > - (((typeof(x))-1) > 0 || (__x) > 0) ? \ > + (((typeof(x))-1) > 0 || \ > + ((typeof(divisor))-1) > 0 || (__x) > 0) ? \ > (((__x) + ((__d) / 2)) / (__d)) : \ > (((__x) - ((__d) / 2)) / (__d)); \ > } \
You can add my "tested-by: Juergen Beisert <jbe@pengutronix.de>". With this patch at least my S3C2440 ADC driver reports correct values again. Keep in mind it should also be fixed in the stable 3.6/3.7 kernel.
Regards, Juergen -- Pengutronix e.K. | Juergen Beisert | Linux Solutions for Science and Industry | http://www.pengutronix.de/ |
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