From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756648AbZEGSHo (ORCPT ); Thu, 7 May 2009 14:07:44 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757157AbZEGSHe (ORCPT ); Thu, 7 May 2009 14:07:34 -0400 Received: from science.horizon.com ([192.35.100.1]:54038 "HELO science.horizon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1755201AbZEGSHe (ORCPT ); Thu, 7 May 2009 14:07:34 -0400 Date: 7 May 2009 14:07:32 -0400 Message-ID: <20090507180732.27407.qmail@science.horizon.com> From: "George Spelvin" To: linux@horizon.com, ulrich.windl@rz.uni-regensburg.de Subject: Re: [RFC][PATCH] Adjust SHIFT_PLL to improve NTP convergence. Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, williams@redhat.com, zippel@linux-m68k.org In-Reply-To: <4A02C2A7.4033.ED06D54@Ulrich.Windl.rkdvmks1.ngate.uni-regensburg.de> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > IMHO, the "value" of calibration during boot is dubious (unless you just > reboot a "hot" machine): Usually the machine is cold on reboot, and the > oscillators will drift a lot initially. Depending on your environment, > the temperatures inside your box may rise about 10°C or more within a > few minutes. > Maybe an experimental feature "recalibrate anytime (and as long as you'd > like)" would be interesting... Well, a couple of points come to mind: - Rebooting hot is not all that unusual. Less so that with Windows, but some of us install test kernels from time to time. :-) - If you're calibrating the CPU against the PIT, they're often derived via PLL from the same clock source, so the *ratio* is fixed, and thus the calibration won't drift at all. - If there were some mechanism to make the divisor consistent across boots, that would be helpful. Imagine a file in /sys that I could write with an absolute frequency at boot time as long as it was within 1000 ppm (0.1%) of the kernel's measurement. Thinking about it, the third option seems the most useful. It's basically a cleaned-up tickadj.