From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rafael J. Wysocki" Subject: Re: [PATCH v3 1/6] PM / core: Add LEAVE_SUSPENDED driver flag Date: Fri, 17 Nov 2017 00:07:59 +0100 Message-ID: <2654404.6YT5rmSnae@aspire.rjw.lan> References: <3806130.B2KCK0tvef@aspire.rjw.lan> <5556673.MXo6XDN1f4@aspire.rjw.lan> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Return-path: In-Reply-To: Sender: linux-doc-owner@vger.kernel.org To: Ulf Hansson Cc: Linux PM , Bjorn Helgaas , Alan Stern , Greg Kroah-Hartman , LKML , Linux ACPI , Linux PCI , Linux Documentation , Mika Westerberg , Andy Shevchenko , Kevin Hilman List-Id: linux-acpi@vger.kernel.org On Thursday, November 16, 2017 4:10:16 PM CET Ulf Hansson wrote: > On 12 November 2017 at 01:37, Rafael J. Wysocki wrote: > > From: Rafael J. Wysocki > > > > Define and document a new driver flag, DPM_FLAG_LEAVE_SUSPENDED, to > > instruct the PM core and middle-layer (bus type, PM domain, etc.) > > code that it is desirable to leave the device in runtime suspend > > after system-wide transitions to the working state (for example, > > the device may be slow to resume and it may be better to avoid > > resuming it right away). > > > > Generally, the middle-layer code involved in the handling of the > > device is expected to indicate to the PM core whether or not the > > device may be left in suspend with the help of the device's > > power.may_skip_resume status bit. That has to happen in the "noirq" > > phase of the preceding system suspend (or analogous) transition. > > The middle layer is then responsible for handling the device as > > appropriate in its "noirq" resume callback which is executed > > regardless of whether or not the device may be left suspended, but > > the other resume callbacks (except for ->complete) will be skipped > > automatically by the core if the device really can be left in > > suspend. > > > > The additional power.must_resume status bit introduced for the > > implementation of this mechanisn is used internally by the PM core > > to track the requirement to resume the device (which may depend on > > its children etc). > > > > Signed-off-by: Rafael J. Wysocki > > Acked-by: Greg Kroah-Hartman > > --- > > > > v2 -> v3: Take dev->power.usage_count when updating power.must_resume in > > __device_suspend_noirq(). > > > > --- > > Documentation/driver-api/pm/devices.rst | 24 ++++++++++- > > drivers/base/power/main.c | 66 +++++++++++++++++++++++++++++--- > > drivers/base/power/runtime.c | 9 ++-- > > include/linux/pm.h | 14 +++++- > > include/linux/pm_runtime.h | 9 ++-- > > 5 files changed, 104 insertions(+), 18 deletions(-) > > > > Index: linux-pm/include/linux/pm.h > > =================================================================== > > --- linux-pm.orig/include/linux/pm.h > > +++ linux-pm/include/linux/pm.h > > @@ -559,6 +559,7 @@ struct pm_subsys_data { > > * NEVER_SKIP: Do not skip system suspend/resume callbacks for the device. > > * SMART_PREPARE: Check the return value of the driver's ->prepare callback. > > * SMART_SUSPEND: No need to resume the device from runtime suspend. > > + * LEAVE_SUSPENDED: Avoid resuming the device during system resume if possible. > > * > > * Setting SMART_PREPARE instructs bus types and PM domains which may want > > * system suspend/resume callbacks to be skipped for the device to return 0 from > > @@ -572,10 +573,14 @@ struct pm_subsys_data { > > * necessary from the driver's perspective. It also may cause them to skip > > * invocations of the ->suspend_late and ->suspend_noirq callbacks provided by > > * the driver if they decide to leave the device in runtime suspend. > > + * > > + * Setting LEAVE_SUSPENDED informs the PM core and middle-layer code that the > > + * driver prefers the device to be left in runtime suspend after system resume. > > */ > > Question: Can LEAVE_SUSPENDED and NEVER_SKIP be valid combination? I > guess not!? Should we validate for wrong combinations? Why not? There's no real overlap between them. > > [...] > > > /** > > * __device_suspend_noirq - Execute a "noirq suspend" callback for given device. > > * @dev: Device to handle. > > @@ -1127,10 +1161,28 @@ static int __device_suspend_noirq(struct > > } > > > > error = dpm_run_callback(callback, dev, state, info); > > - if (!error) > > - dev->power.is_noirq_suspended = true; > > - else > > + if (error) { > > async_error = error; > > + goto Complete; > > + } > > + > > + dev->power.is_noirq_suspended = true; > > + > > + if (dev_pm_test_driver_flags(dev, DPM_FLAG_LEAVE_SUSPENDED)) { > > + /* > > + * The only safe strategy here is to require that if the device > > + * may not be left in suspend, resume callbacks must be invoked > > + * for it. > > + */ > > + dev->power.must_resume = dev->power.must_resume || > > + !dev->power.may_skip_resume || > > + atomic_read(&dev->power.usage_count); > > dev->power.usage_count is always > 0 at this point, meaning that > dev->power.must_resume always becomes true. :-) > > You should rather use "atomic_read(&dev->power.usage_count) > 1". Right, thanks. I tend to forget about that. > > + } else { > > + dev->power.must_resume = true; > > + } > > + > > + if (dev->power.must_resume) > > + dpm_superior_set_must_resume(dev); > > > > Complete: > > complete_all(&dev->power.completion); > > @@ -1487,6 +1539,9 @@ static int __device_suspend(struct devic > > dev->power.direct_complete = false; > > } > > > > + dev->power.may_skip_resume = false; > > + dev->power.must_resume = false; > > + > > First, these assignment could be bypassed if the direct_complete path > is used. Perhaps it's more robust to reset these flags already in > device_prepare(). In the direct-complete case may_skip_resume doesn't matter. must_resume should be set to "false", however, so that parents of direct-complete devices may be left in suspend (in case they don't fall under direct-complete themselves), so good catch. But it is sufficient to do that before the power.direct_complete check above. :-) > Second, have you considered setting the default value of > dev->power.may_skip_resume to true? Yes. > That would means the subsystem > instead need to implement an opt-out method. I am thinking that it may > not be an issue, since we anyway at this point, don't have drivers > using the LEAVE_SUSPENDED flag. Opt-out doesn't work because of the need to invoke the "noirq" callbacks. > [...] > > > +However, it may be desirable to leave some devices in runtime suspend after > > +system transitions to the working state and device drivers can use the > > +``DPM_FLAG_LEAVE_SUSPENDED`` flag to indicate to the PM core (and middle-layer > > +code) that this is the case. Whether or not the devices will actually be left > > +in suspend may depend on their state before the given system suspend-resume > > +cycle and on the type of the system transition under way. In particular, > > +devices are not left suspended if that transition is a restore from hibernation, > > +as device states are not guaranteed to be reflected by the information stored in > > +the hibernation image in that case. > > + > > +The middle-layer code involved in the handling of the device has to indicate to > > +the PM core if the device may be left in suspend with the help of its > > +:c:member:`power.may_skip_resume` status bit. That has to happen in the "noirq" > > +phase of the preceding system-wide suspend (or analogous) transition. The > > Does it have to be managed in the "noirq" phase? Wouldn't be perfectly > okay do this in the suspend and suspend_late phases as well? The wording is slightly misleading I think. In fact technically may_skip_resume may be set earlier, but the core checks it in the "noirq" phase only anyway. Thanks, Rafael