From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rafael J. Wysocki" Subject: Re: [RFC][PATCH 0/3] PM / sleep: Avoid resuming runtime-suspended devices during system suspend Date: Tue, 13 May 2014 17:46:25 +0200 Message-ID: <4083601.DlzRABTDaL@vostro.rjw.lan> References: Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7Bit Return-path: Received: from v094114.home.net.pl ([79.96.170.134]:50476 "HELO v094114.home.net.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1754071AbaEMP3j (ORCPT ); Tue, 13 May 2014 11:29:39 -0400 In-Reply-To: Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Alan Stern Cc: Linux PM list , ACPI Devel Maling List , Aaron Lu , Mika Westerberg , Linux Kernel Mailing List , Kevin Hilman , Ulf Hansson On Tuesday, May 13, 2014 11:25:07 AM Alan Stern wrote: > Crossing emails again... > > On Tue, 13 May 2014, Rafael J. Wysocki wrote: > > > > There's nothing to prevent a runtime-suspended device from being > > > resumed in between the ->prepare() and ->suspend() callbacks. > > > > I'm moving the barrier from __device_suspend() to device_prepare(), so there > > shouldn't be surprise resumes in that time frame. > > A wakeup request from the hardware can cause a runtime resume, even > if most threads are in the freezer: > > Not all kernel threads get frozen. One of the unfrozen threads > could respond to the wakeup request by calling > pm_runtime_resume(). > > Some runtime PM callbacks are marked as IRQ-safe and can run > directly within an interrupt handler. > > > > Therefore it makes little sense to check the device's runtime status in > > > device_prepare(). The check should be done in __device_suspend(). > > > > If we do the barrier in device_prepare(), then I'm not sure what mechanism > > would cause the device to resume. > > See above. A wakeup request can arrive after the barrier has finished. > > > If there is one, the whole approach is in danger, because ->prepare() has to > > check if devices are runtime-suspended and has to be sure that their status > > won't change after it has returned 1. > > ->prepare() cannot guarantee in all cases that a device will remain in > runtime suspend. Fortunately, it doesn't need to. In fact (as I > mentioned sometime before), it doesn't even need to check whether the > device currently is runtime suspended -- it suffices to know that _if_ > the device is runtime suspended _then_ it has the proper settings for > system suspend. > > Regardless, status changes cannot cause a problem. If the device does > get runtime-resumed after ->prepare(), it will remain that way when > __device_suspend() runs. The device can't be runtime-suspended again, > because device_prepare() does pm_get_noresume(). > > Therefore, if the device is still runtime-suspended when > __device_suspend() runs, we can be sure that its status and state are > still the same as when ->prepare() ran. But if the device is runtime-suspended, we cannot know if it's going to resume a while later. That's the problem. Rafael