From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757338AbZEHMTY (ORCPT ); Fri, 8 May 2009 08:19:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752434AbZEHMTP (ORCPT ); Fri, 8 May 2009 08:19:15 -0400 Received: from science.horizon.com ([192.35.100.1]:28611 "HELO science.horizon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752304AbZEHMTP (ORCPT ); Fri, 8 May 2009 08:19:15 -0400 Date: 8 May 2009 08:19:13 -0400 Message-ID: <20090508121913.9039.qmail@science.horizon.com> From: "George Spelvin" To: johnstul@us.ibm.com, linux@horizon.com Subject: Re: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration variance Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, tglx@linutronix.de, ulrich.windl@rz.uni-regensburg.de, williams@redhat.com, zippel@linux-m68k.org In-Reply-To: <1241742647.7518.108.camel@localhost.localdomain> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > To mitigate this, I wanted to provide a tsc_khz= boot option. This would > allow users to set the tsc_khz value at boot-up, assuming they are > within 1Mhz of the calibrated value (to protect against bad values). > Once the tsc_khz value is set in grub, the box will always boot with the > same value, so the NTP drift value prior to reboot will still be correct > after rebooting. A run-time adjustable would be more convenient, but this is simple and works. The 1 MHz tolerance, however, isn't implemented right. I can't quote figure out what you were trying to do; was that (x + (x/2))/1000 trying to round the /1000 properly? That should be (x + 1000/2)/1000 in that case, which I normally write as (x/500 + 1)/2; But in any case, no equality comparison can possibly work; there's always a case where the measured value rounds to k and the correct value rounds to k+1. Also, 1 MHz is a pretty wide tolerance on sub-GHz processors. I'd suggest the following: diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index d57de05..d7ab640 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -825,15 +825,42 @@ static void __init init_tsc_clocksource(void) clocksource_register(&clocksource_tsc); } +unsigned long tsc_khz_specified; +static int __init tsc_khz_specified_setup(char *str) +{ + tsc_khz_specified = simple_strtoul(str, NULL, 0); + return 1; +} + +__setup("tsc_khz=", tsc_khz_specified_setup); + + void __init tsc_init(void) { u64 lpj; int cpu; + long difference; if (!cpu_has_tsc) return; tsc_khz = calibrate_tsc(); + + /* * * If the calibrated TSC freq and user specified TSC freq * * are close enough, pick the what the user told us. Reject * * obviously bogus values to make the option safe to use. + */ + difference = tsc_khz - tsc_khz_specified; + if (difference < 0) + difference = -difference; + if (difference <= tsc_khz >> 10) { /* 1/1024 = 976 ppm */ + printk(KERN_INFO "Using user defined TSC freq: %lu.%03lu MHz\n", + tsc_khz_specified/1000, + tsc_khz_specified%1000); + tsc_khz = tsc_khz_specified; + } + cpu_khz = tsc_khz; if (!tsc_khz) {