Messages in this thread | | | From | Kevin Hilman <> | Subject | Re: [RFC][PATCH 1/3] PM / sleep: Flag to speed up suspend-resume of runtime-suspended devices | Date | Fri, 09 May 2014 15:48:21 -0700 |
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"Rafael J. Wysocki" <rjw@rjwysocki.net> writes:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > > Currently, some subsystems (e.g. PCI and the ACPI PM domain) have to > resume all runtime-suspended devices during system suspend, mostly > because those devices may need to be reprogrammed due to different > wakeup settings for system sleep and for runtime PM. > > For some devices, though, it's OK to remain in runtime suspend > throughout a complete system suspend/resume cycle (if the device was in > runtime suspend at the start of the cycle). We would like to do this > whenever possible, to avoid the overhead of extra power-up and power-down > events. > > However, problems may arise because the device's descendants may require > it to be at full power at various points during the cycle. Therefore the > most straightforward way to do this safely is if the device and all its > descendants can remain runtime suspended until the resume stage of system > resume. > > To this end, introduce dev->power.leave_runtime_suspended. > If a subsystem or driver sets this flag during the ->prepare() callback, > and if the flag is set in all of the device's descendants, and if the > device is still in runtime suspend at the beginning of the ->suspend() > callback, that callback is allowed to return 0 without clearing > power.leave_runtime_suspended and without changing the state of the > device, unless the current state of the device is not appropriate for > the upcoming system sleep state (for example, the device is supposed to > wake up the system from that state and its current wakeup settings are > not suitable for that). Then, the PM core will not invoke the device's > ->suspend_late(), ->suspend_irq(), ->resume_irq(), ->resume_early(), or > ->resume() callbacks.
Up to here, this sounds great.
> Instead, it will invoke ->runtime_resume() during the device resume > stage of system resume.
But this part I'm not fully following...
> By leaving this flag set after ->suspend(), a driver or subsystem tells > the PM core that the device is runtime suspended, it is in a suitable > state for system suspend (for example, the wakeup setting does not > need to be changed), and it does not need to return to full > power until the resume stage.
But taking this "leave runtime suspended" idea the next logical step, why would/should a device need to return to full power at the ->resume() stage? especially when it wasn't at full power when ->suspend() happened?
IOW, why doesn't "leave runtime suspended" mean "leave runtime suspended until runtime resumed on demand."
Forcing ->runtime_resume() during device resume means that in most cases, a device will be forcibly runtime resumed, only to have nothing to do but go idle and runtime suspend again, resulting in a(nother) unnessary power-up, power-down cycle this patch is trying to avoid during ->suspend().
Hmm, but wait a minute...
[...]
> @@ -735,6 +735,11 @@ static int device_resume(struct device * > if (dev->power.syscore) > goto Complete; > > + if (pm_leave_runtime_suspended(dev)) { > + pm_runtime_resume(dev); > + goto Complete; > + }
... maybe I'm forgetting how this works (since it's Friday and my brain is already shutting down for the week) but after pm_runtime_resume() is called, won't the device remain runtime active until pm_runtime_suspend() is called, or until a pm_runtime_get()/pm_runtime_put() cycle happens?
That means that on device resume, the device is forced into full power state (even though it was runtime suspended when ->suspend() happened) and will stay there until its used again.
That seems like a rather unpleasant (and non-intuitive) side-effect of "leave runtime suspended".
Kevin
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