From mboxrd@z Thu Jan 1 00:00:00 1970 From: Saravana Kannan Subject: Re: [PATCH v2] clk: Fix race condition between clk_set_parent and clk_enable() Date: Thu, 16 May 2013 14:31:23 -0700 Message-ID: <5195502B.3070003@codeaurora.org> References: <1367383328-25700-1-git-send-email-skannan@codeaurora.org> <1368677244-7279-1-git-send-email-skannan@codeaurora.org> <20130516204455.12127.75296@quantum> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from wolverine01.qualcomm.com ([199.106.114.254]:25735 "EHLO wolverine01.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753346Ab3EPVbZ (ORCPT ); Thu, 16 May 2013 17:31:25 -0400 In-Reply-To: <20130516204455.12127.75296@quantum> Sender: linux-arm-msm-owner@vger.kernel.org List-Id: linux-arm-msm@vger.kernel.org To: Mike Turquette Cc: linux-arm-kernel@lists.infradead.org, Paul Walmsley , Shawn Guo , Sascha Hauer , Rob Herring , Mark Brown , Russell King , Ulf Hansson , Tomasz Figa , linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Andrew Lunn , Jeremy Kerr , Thomas Gleixner , Arnd Bergman , Jamie Iles , Richard Zhao , Magnus Damm , Linus Walleij , Stephen Boyd , Amit Kucheria , Deepak Saxena , Grant Likely On 05/16/2013 01:44 PM, Mike Turquette wrote: > Quoting Saravana Kannan (2013-05-15 21:07:24) >> Without this patch, the following race condition is possible. >> * clk-A has two parents - clk-X and clk-Y. >> * All three are disabled and clk-X is current parent. >> * Thread A: clk_set_parent(clk-A, clk-Y). >> * Thread A: >> * Thread A: Grabs enable lock. >> * Thread A: Sees enable count of clk-A is 0, so doesn't enable clk-Y. >> * Thread A: Updates clk-A SW parent to clk-Y >> * Thread A: Releases enable lock. >> * Thread B: clk_enable(clk-A). >> * Thread B: clk_enable() enables clk-Y, then enabled clk-A and returns. >> >> clk-A is now enabled in software, but not clocking in hardware since the >> hardware parent is still clk-X. >> >> The only way to avoid race conditions between clk_set_parent() and >> clk_enable/disable() is to ensure that clk_enable/disable() calls don't >> require changes to hardware enable state between changes to software clock >> topology and hardware clock topology. >> >> The options to achieve the above are: >> 1. Grab the enable lock before changing software/hardware topology and >> release it afterwards. >> 2. Keep the clock enabled for the duration of software/hardware topology >> change so that any additional enable/disable calls don't try to change >> the hardware state. Once the topology change is complete, the clock can >> be put back in its original enable state. >> >> Option (1) is not an acceptable solution since the set_parent() ops might >> need to sleep. >> >> Therefore, this patch implements option (2). >> >> This patch doesn't violate any API semantics. clk_disable() doesn't >> guarantee that the clock is actually disabled. So, no clients of a clock >> can assume that a clock is disabled after their last call to clk_disable(). >> So, enabling the clock during a parent change is not a violation of any API >> semantics. >> >> This also has the nice side effect of simplifying the error handling code. >> >> Signed-off-by: Saravana Kannan > > Updated to this version in clk-next. > Thanks Mike. I forgot to add the Ack by Ulf. Would be nice if you can put that in. Btw, I did send this email to the list. But looks like this mail is wedged in the series of tubes in the arm mailing list. -Saravana -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation