Re: [Update][PATCH 6/10] PM / Domains: System-wide transitions

Alan Stern
2011-07-08T14:37:32Z

On Fri, 8 Jul 2011, Rafael J. Wysocki wrote:

> On Friday, July 08, 2011, Kevin Hilman wrote:
> > "Rafael J. Wysocki" <rjw@sisk.pl> writes:
> >
> > > From: Rafael J. Wysocki <rjw@sisk.pl>
> > >
> > > Make generic PM domains support system-wide power transitions
> > > (system suspend and hibernation). Add suspend, resume, freeze, thaw,
> > > poweroff and restore callbacks to be associated with struct
> > > generic_pm_domain objects and make pm_genpd_init() use them as
> > > appropriate.
> > >
> > > The new callbacks do nothing for devices belonging to power domains
> > > that were powered down at run time (before the transition).
> >
> > Thinking about this some more, how is a driver supposed to reconfigure
> > wakeups during suspend if it has already been runtime suspended?
>
> If the device belongs to a PM domain that has been powered off, it
> won't be notified.
>
> > For example, assume a device where device_may_wakeup() = false. This
> > means wakeups during *suspend* are disabled, but wakeups wakeups are
> > assumed to enabled when it is runtime suspended.
> >
> > So now, assume this device is RPM_SUSPENDED, it has wakeups *enabled*,
> > and then system suspend comes along.
> >
> > With this current patch, the driver will never receive any callbacks, so
> > it can never disable its wakeups.
> >
> > Am I missing something?
>
> As I said above, this only happens with devices that belog to PM domains
> that were powered off before system suspend has started, so the problem
> is limited to devices that wakeup is signaled on behalf of even when they
> have no power.
>
> So this is a limitation, but not affecting all platforms.
>
> There are a few ways to avoid this limitation I can think of:
> (1) Add a "make me operational during system suspend" flag to struct dev_pm_info
> and run pm_runtime_resume() on such devices from the core (either dpm_prepare()
> core, or pm_genpd_prepare()).

What's to prevent the device from being runtime-suspended again before
the wakeup setting can be changed?

> (2) Add a "my .prepare() is safe to run if device is not accessible" flag to
> struct dev_pm_info and make pm_genpd_prepare() execute .prepare() for such
> devices regardless of whether or not their PM domains are off.
> (3) Call .prepare() from all drivers unconditionally during system suspend
> (and probably .complete() too) in the hope they won't access inaccessible
> devices.
> Probably, there's more.

In the PM domain's suspend code, do a runtime resume if the wakeup
setting needs to be changed, rather than simply skipping over the
device.

> In any case I think it's material for future work.

Alan Stern