Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932166AbaBDUkv (ORCPT ); Tue, 4 Feb 2014 15:40:51 -0500 Received: from terminus.zytor.com ([198.137.202.10]:48289 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754644AbaBDUks (ORCPT ); Tue, 4 Feb 2014 15:40:48 -0500 Message-ID: <52F15000.2080102@zytor.com> Date: Tue, 04 Feb 2014 12:39:28 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 To: Geert Uytterhoeven , "Theodore Ts'o" , Stephan Mueller , =?UTF-8?B?SsO2cm4gRW5nZWw=?= , Linux Kernel Developers List , "Maciej W. Rozycki" , Ralf Baechle , dave.taht@gmail.com, John Crispin , andrewmcgr@gmail.com, Thorsten Glaser , sandyinchina@gmail.com Subject: Re: [RFC PATCH 0/5] CPU Jitter RNG References: <2039634.jSmQAS6tdi@myon.chronox.de> <20140204170823.GF12768@thunk.org> <52F13A1C.3040003@zytor.com> <20140204192325.GA11831@thunk.org> In-Reply-To: X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/04/2014 11:39 AM, Geert Uytterhoeven wrote: > On Tue, Feb 4, 2014 at 8:23 PM, wrote: >> However, where a decade ago the ethernet card probably had its own >> independent clock crystal/oscillator, I'm going to guess that these >> days with SOC's and even on laptops, with ethernet device part of the >> chipset, it is probably being driven off the same master oscillator. > > USB typically still has its own crystal. USB and the Ethernet PHY frequently do still have their own crystals, for reasons not entirely clear to me. However, what all of these have in common is that they are way out in the periphery. >> I wonder if there's anyway we can either figure out manually, or >> preferably, automatically at boot time, which devices actually have >> independent clock oscillators. > > You may find this information in the DT on some platforms (if you're > lucky). On most systems today, all the high speed clocks (CPU, memory, etc.) are all fed from a single oscillator. On PCs there used to be a separate 14.31818 MHz oscillator for the PIT, PMTIMER and HPET, but that is increasingly handled by a frequency converter from the main bus clock. Oscillators are expensive, and true asynchronous domains cause problems with metastability. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/