From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?utf-8?B?U8O2cmVu?= Brinkmann Subject: Re: [PATCH v2 4/9] clocksource/cadence_ttc: Use enable/disable_irq Date: Fri, 6 Dec 2013 14:47:32 -0800 Message-ID: References: <1385514296-26702-1-git-send-email-soren.brinkmann@xilinx.com> <1385514296-26702-5-git-send-email-soren.brinkmann@xilinx.com> <52972F3A.9090103@linaro.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Thomas Gleixner Cc: Daniel Lezcano , Rob Herring , Pawel Moll , Mark Rutland , Stephen Warren , Ian Campbell , Russell King , Michal Simek , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org List-Id: devicetree@vger.kernel.org Hi Thomas, sorry for the delay, but I couldn't find time any earlier. On Thu, Nov 28, 2013 at 08:07:10PM +0100, Thomas Gleixner wrote: > On Thu, 28 Nov 2013, S=C3=B6ren Brinkmann wrote: > > On Thu, Nov 28, 2013 at 03:18:50PM +0100, Thomas Gleixner wrote: > > > Now the problem with this device is that it is not a per cpu > > > device. It's a global device, so this update can conflict with a > > > parallel access on the other CPU. Now the disable_irq() only prev= ents > > > that the other CPU can handle a device interrupt from that timer.= But > > > it does not prevent any parallel access from e.g. the idle code p= ath > > > which will try to reprogram it. > > > > Does that mean interrupts need to be disabled globally? Also, does = the >=20 > Globally disabling interrupts is not going to work, except you want t= o > use stomp_machine(). But that would be overkill. >=20 > > cpuidle path depend on interrupts or can it interfere no matter wha= t? >=20 > It can interfere no matter what. The broadcast is modified for the cp= u > which loses its per cpu timer due to the idle state via >=20 > clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, ...); > =20 > > > Soren, is that timer used as the broadcast device ? > > > > Yes, this is the only broadcast capable timer on Zynq, AFAIK. Other= than > > the TTC we only have the arm_global_timer and smp_twd timers, which= both > > are per_cpu devices and thus not broadcast capable. >=20 > There is a solution to this. We can identify the broadcast device in > the core and serialize all callers including interrupts on a differen= t > cpu against the update. So no need for the disable/enable_irq() dance= =2E IIUC, and please correct me if I'm wrong, with the patch I'd simply cal= l 'clockevents_update_freq() without having to disable IRQs. But I'm not sure whether periodic mode is covered. I found, that I had to reprogram the timer interval in my clock notifier callback when the timer frequency changes. I think 'clockevents_update_freq()' only handles oneshot mode. For that reason I call 'ttc_set_interval()' in the clock notifier in case the timer is in periodic mode. For that call we'd stil= l have possible races. I guess the best solution would be to move that functionality into 'clockevents_update_freq()'? S=C3=B6ren