Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752883Ab3C0OZd (ORCPT ); Wed, 27 Mar 2013 10:25:33 -0400 Received: from mail-pd0-f170.google.com ([209.85.192.170]:57179 "EHLO mail-pd0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751310Ab3C0OZc convert rfc822-to-8bit (ORCPT ); Wed, 27 Mar 2013 10:25:32 -0400 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8BIT To: Thomas Gleixner , Ulf Hansson From: Mike Turquette In-Reply-To: Cc: Viresh Kumar , linaro-kernel@lists.linaro.org, patches@linaro.org, linux-kernel@vger.kernel.org, Laurent Pinchart , David Brown , linux-arm-kernel@lists.infradead.org References: <1364368183-24420-1-git-send-email-mturquette@linaro.org> Message-ID: <20130327142519.4014.82657@quantum> User-Agent: alot/0.3.3+ Subject: Re: [PATCH v4] clk: allow reentrant calls into the clk framework Date: Wed, 27 Mar 2013 07:25:19 -0700 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2959 Lines: 81 Quoting Thomas Gleixner (2013-03-27 04:09:17) > On Wed, 27 Mar 2013, Ulf Hansson wrote: > > On 27 March 2013 10:55, Viresh Kumar wrote: > > > On 27 March 2013 15:10, Thomas Gleixner wrote: > > >> On Wed, 27 Mar 2013, Mike Turquette wrote: > > >> > > >>> Reentrancy into the clock framework from the clk.h api is necessary > > >>> for clocks that are prepared and unprepared via i2c_transfer (which > > >>> includes many PMICs and discrete audio chips) as well as for several > > >>> other use cases. > > >> > > >> That explanation sucks. > > >> > > >> Why does an i2c clock need reentrancy? Just because it's i2c or what? > > > > > > I am noway connected to this development but was just going through > > > your mail and i think i might know the answer why is this required. > > > > > > Consider an example where an external chip has clock controller and has > > > bits which can be programmed to enable/disable clock. And this chip is > > > connected via spi/i2c to SoC. > > > > > > clk_prepare(peripheral on external chip) > > > -> i2c_xfer(to write to external chips register) > > > -> clk_enable(i2c controller) > > > ->controller-xfer-routine.. and finally we enable clk here... > > Which does not explain the whole issue: > > clk_prepare() takes the mutex > clk_enable() takes the spinlock > > That works today. > > The issue arises, if you need to call clk_prepare(i2c) in the xfer > routine. > The issue arises any time a clk_ops callback calls a function that unwittingly re-enters the clock framework. I think the easiest example to understand and perhaps the most common in practice is a clock which is controlled via an i2c transfer. Viresh's example makes the mistake of calling clk_enable(i2c_controller), but it must also call clk_prepare(i2c_controller) which is missing in the call graph above. That nested call to clk_prepare is where the reentrancy comes from. This has nothing to do with the prepare/enable locking split and leaves that relationship intact. > > > > > > Sorry if i am on the wrong side :) > > Only slightly :) > > > I agree with you Viresh. I guess Mike should update the commit message. > > > > I would also like add another reason to why this is needed. For some > > clks you would like to do pinctrl operations from a clk hw. But since > > a pinctrl driver likely requires a clk to be prepared|enabled, we run > > into a clk reentrant issue. > > Fair enough. This all wants to go into the changelog, so we can > understand why we have this business. > I'll submit a v5 which I hope will end the pain and suffering this patch has caused you. Regards, Mike > Thanks, > > tglx -- 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/