Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765680AbZFOUFv (ORCPT ); Mon, 15 Jun 2009 16:05:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756248AbZFOUFo (ORCPT ); Mon, 15 Jun 2009 16:05:44 -0400 Received: from 124x34x33x190.ap124.ftth.ucom.ne.jp ([124.34.33.190]:34507 "EHLO master.linux-sh.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754240AbZFOUFn (ORCPT ); Mon, 15 Jun 2009 16:05:43 -0400 Date: Tue, 16 Jun 2009 05:04:57 +0900 From: Paul Mundt To: john stultz Cc: Magnus Damm , linux-kernel@vger.kernel.org, mingo@elte.hu, tglx@linutronix.de, akpm@linux-foundation.org Subject: Re: [PATCH] clocksource: setup mult_orig in clocksource_enable() Message-ID: <20090615200456.GA31202@linux-sh.org> Mail-Followup-To: Paul Mundt , john stultz , Magnus Damm , linux-kernel@vger.kernel.org, mingo@elte.hu, tglx@linutronix.de, akpm@linux-foundation.org References: <20090501054546.8193.10688.sendpatchset@rx1.opensource.se> <1244667895.8085.20.camel@localhost.localdomain> <1244850967.7231.26.camel@localhost.localdomain> <1245092917.7505.4.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1245092917.7505.4.camel@localhost.localdomain> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2422 Lines: 49 On Mon, Jun 15, 2009 at 12:08:37PM -0700, john stultz wrote: > On Sun, 2009-06-14 at 19:20 +0900, Magnus Damm wrote: > > On Sat, Jun 13, 2009 at 8:56 AM, john stultz wrote: > > > On Thu, 2009-06-11 at 14:51 +0900, Magnus Damm wrote: > > >> I can't think of any way that would work. The clock frequency can be > > >> changed while the clock is disabled. And we can only know the rate > > >> after enabling the clock, see these lines from include/linux/clk.h: > > >> > > >> /** > > >> * clk_get_rate - obtain the current clock rate (in Hz) for a clock source. > > >> * This is only valid once the clock source has been enabled. > > >> * @clk: clock source > > >> */ > > >> unsigned long clk_get_rate(struct clk *clk); > > > > > > Hrmm.. Yuck. > > > > > > Is this really expected behavior that a clk would change frequencies > > > between uses as a clocksource? > > > > Yes, I think so. The clock frequency can change through cpufreq or > > clk_set_rate(). > > But they do not change freq (through cpufreq or anything else) after the > enable() call, right? That would be pretty critical. Otherwise they'd > need to be disqualified like we do the TSC on x86. > This is a bit tricky, the clock needs to be able to adjust its parent divisors/multipliers in order to maintain its current frequency if a parent clock changes frequency. We do not presently prohibit a frequency change that deviates from the current frequency on these clocks, but this would be trivially handled by setting a fixed rate flag for those clocks. This sort of logic is necessary to block frequency changes in the parent clock topology that would throw the child clock's frequency out of sync. Note that in the general case we do not want to disable frequency changes on enabled clocks, enabled clocks only need to know whether they can handle a frequency change or not without destabilizing the system. The only thing the usecount presently disables on an enabled clock is reparenting it. ie, migrating between different parent PLLs while in active use. There is no fundamental limitation as with the TSC, we can have as much or as little flexibility as we like. -- 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/