Messages in this thread Patch in this message | | | Date | Tue, 18 Dec 2012 17:45:56 -0800 | From | Guenter Roeck <> | Subject | Re: Strange results of DIV_ROUND_CLOSEST |
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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! >
Juergen,
Can you test the following patch ?
Thanks, Guenter
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)); \ } \ -- 1.7.9.7
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