From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755405Ab2LRVH5 (ORCPT ); Tue, 18 Dec 2012 16:07:57 -0500 Received: from metis.ext.pengutronix.de ([92.198.50.35]:58132 "EHLO metis.ext.pengutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755361Ab2LRVHv (ORCPT ); Tue, 18 Dec 2012 16:07:51 -0500 From: Juergen Beisert Organization: Pengutronix - Linux Solutions for Science and Industry To: Guenter Roeck Subject: Re: Strange results of DIV_ROUND_CLOSEST Date: Tue, 18 Dec 2012 22:04:56 +0100 User-Agent: KMail/1.9.10 Cc: linux-kernel@vger.kernel.org References: <201212181603.42094.jbe@pengutronix.de> <20121218171620.GA19972@roeck-us.net> In-Reply-To: <20121218171620.GA19972@roeck-us.net> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <201212182204.56523.jbe@pengutronix.de> X-SA-Exim-Connect-IP: 2001:6f8:1178:4:5054:ff:fe8d:eefb X-SA-Exim-Mail-From: jbe@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-kernel@vger.kernel.org Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Regards, Juergen -- Pengutronix e.K. | Juergen Beisert | Linux Solutions for Science and Industry | http://www.pengutronix.de/ |