linux-kernel - Re: [RFC][PATCH 2/3] PM / sleep: Mechanism to avoid resuming runtime-suspended devices unnecessarily

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Date:	Tue, 13 May 2014 12:19:14 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
cc:	Linux PM list <linux-pm@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Aaron Lu <aaron.lu@...el.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...aro.org>,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [RFC][PATCH 2/3] PM / sleep: Mechanism to avoid resuming
 runtime-suspended devices unnecessarily

On Tue, 13 May 2014, Rafael J. Wysocki wrote:

> > Maybe the call to __pm_runtime_disable() should be moved from
> > __device_suspend_late() to __device_suspend(), after the callback has
> > been invoked (or skipped, as the case may be).  Then after runtime PM
> > has been disabled, you can check the device's status has changed and go
> > back to invoke the callback if necessary.
> 
> We moved __pm_runtime_disable() to __device_suspend_late() to be able to
> use pm_runtime_resume() in __device_suspend() (and we actually do that in
> some places now).
> 
> But, in principle, we can do __pm_runtime_disable() temporarily in some place
> between ->prepare() and ->suspend(), it doesn't matter if that's in
> device_prepare() in __device_suspend() really.

It should be as late as possible, to allow for detecting wakeup 
requests.

>  Then, we can check the device's
> runtime PM status (that'd need to be done carefully to take the disabling into
> account) and
> (1) if the device is runtime-suspended, set direct_complete for it without
>     enabling runtime PM, or
> (2) if the device is not runtime-suspended, clear direct_complete for it
>     and re-enable runtime PM.
> and in case of (1) we would re-enable runtime PM in device_complete().
> 
> That should work I suppose?

Yes; it's similar to what I proposed.  Note that this can be skipped if 
direct_complete is already clear.

> Of course, question is what ->prepare() is supposed to do then if it needs
> to check the state of the device before deciding whether or not to return 1.
> I guess it would need to disable runtime PM around that check too.

It would be surprising if ->prepare() needed to make any difficult
checks.  This would imply that the device could have multiple
runtime-suspend states, some of which are appropriate for system
suspend while others aren't.  Not impossible, but I wouldn't expect it
to come up often.

Besides, as I mentioned before, we never have to worry about status 
changes.  If one occurs while ->prepare() is running or afterward, it 
means the device is runtime-resumed and therefore the setting of 
direct_complete doesn't matter.

Alan Stern

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