Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754785Ab1BDJzb (ORCPT ); Fri, 4 Feb 2011 04:55:31 -0500 Received: from mail-pz0-f46.google.com ([209.85.210.46]:43926 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753434Ab1BDJz2 (ORCPT ); Fri, 4 Feb 2011 04:55:28 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; b=F8320u7K5ge6cXl6ORFbjMYq48HqbXBMZigI5A4HLxy4yyJuZkTilyRo+heT584umF OAlypWEndOrsrsQ4ff0HSx3Sz9E/cd8gWyGGaQh4FppQ2boo+SDcNJh48QHrvTRpwVZe 9/63KZ0dsQd2lnrJjmQDr0hiSo4sksoS3Ttss= Date: Fri, 4 Feb 2011 17:54:24 +0800 From: Richard Zhao To: Russell King - ARM Linux Cc: Stephen Boyd , Nicolas Pitre , Dima Zavin , Lorenzo Pieralisi , Vincent Guittot , linux-sh@vger.kernel.org, Ben Herrenschmidt , Sascha Hauer , Paul Mundt , linux-kernel@vger.kernel.org, Saravana Kannan , Ben Dooks , Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= , Jeremy Kerr , linux-arm-kernel@lists.infradead.org Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare Message-ID: <20110204095424.GB2347@richard-laptop> References: <201102011711.31258.jeremy.kerr@canonical.com> <20110201105449.GY1147@pengutronix.de> <20110201131512.GH31216@n2100.arm.linux.org.uk> <20110201141837.GA1147@pengutronix.de> <20110201143932.GK31216@n2100.arm.linux.org.uk> <20110201151846.GD1147@pengutronix.de> <20110201152458.GP31216@n2100.arm.linux.org.uk> <4D48741F.8060006@codeaurora.org> <20110201212409.GU31216@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110201212409.GU31216@n2100.arm.linux.org.uk> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2549 Lines: 60 On Tue, Feb 01, 2011 at 09:24:09PM +0000, Russell King - ARM Linux wrote: > On Tue, Feb 01, 2011 at 12:59:11PM -0800, Stephen Boyd wrote: > > On 02/01/2011 07:24 AM, Russell King - ARM Linux wrote: > > > I'd also be tempted at this stage to build-in a no-op dummy clock, > > > that being the NULL clk: > > > > > > int clk_prepare(struct clk *clk) > > > { > > > int ret = 0; > > > > > > if (clk) { > > > mutex_lock(&clk->mutex); > > > if (clk->prepared == 0) > > > ret = clk->ops->prepare(clk); > > > if (ret == 0) > > > clk->prepared++; > > > mutex_unlock(&clk->mutex); > > > } > > > > > > return ret; > > > } > > > > I'm afraid this will hide enable/disable imbalances on some targets and > > then expose them on others. Maybe its not a big problem though since > > this also elegantly handles the root(s) of the tree. > > You can't catch enable/disable imbalances in the prepare code, and you > can't really catch them in the unprepare code either. > > Consider two drivers sharing the same struct clk. When the second driver > prepares the clock, the enable count could well be non-zero, caused by > the first driver. Ditto for when the second driver is removed, and it > calls unprepare - the enable count may well be non-zero. > > The only thing you can check is that when the prepare count is zero, > the enable count is also zero. You can also check in clk_enable() and > clk_disable() that the prepare count is non-zero. but how can we check prepare count without mutex lock? Even if prepare count is atomic_t, it can not guarantee the clock is actually prepared or unprepared. So it's important for driver writer to maintain the call sequence. Thanks Richard > > If you want tigher checking than that, you need to somehow identify and > match up the clk_prepare/clk_enable/clk_disable/clk_unprepare calls from > a particular driver instance. Addresses of the functions don't work as > you can't be certain that driver code will be co-located within a certain > range. Adding an additional argument to these functions which is driver > instance specific seems to be horrible too. > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel -- 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/