Re: [Update][PATCH 6/10] PM / Domains: System-wide transitions support for generic domains (v5)

[Rafael J. Wysocki]

On Friday, July 08, 2011, Rafael J. Wysocki wrote:
> On Friday, July 08, 2011, Kevin Hilman wrote:
> > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes:
> > 
> > > On Fri, 8 Jul 2011, Rafael J. Wysocki wrote:
> > >
> > >> On Friday, July 08, 2011, Kevin Hilman wrote:
> > >> > "Rafael J. Wysocki" <rjw@xxxxxxx> writes:
> > >> > 
> > >> > > From: Rafael J. Wysocki <rjw@xxxxxxx>
> > >> > >
> > >> > > 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.
> > 
> > Which on OMAP, is *all* devices, so that's a pretty major limitation.  :)
> 
> So we'll need to avoid it _for_ _OMAP_.  Any of the platforms that will use
> this code in near future doesn't have this limitation, AFAICS, so I don't
> really see a point coding for it right now.
> 
> Your point seems to be that the whole thing has to be ready for OMAP
> in advance and my point is that we can make it ready for OMAP later.
> Which I believe is more pragmatic, because if every platform had adopted
> the other viewpoint, we wouldn't have made any progress at all.
> 
> > >> 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.
> > 
> > Like Alan's comment above for (1), I think the same applies for (3)
> > since runtime PM transitions can still happen between .prepare() and
> > .suspend()
> 
> Not with the current PM domains code (and please see my reply to Alan).

Well, in fact, since we already have .active_wakeup(), what about the
following patch (on top of https://lkml.org/lkml/2011/7/8/223):

---
 drivers/base/power/domain.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux-2.6/drivers/base/power/domain.c
===================================================================
--- linux-2.6.orig/drivers/base/power/domain.c
+++ linux-2.6/drivers/base/power/domain.c
@@ -519,6 +519,10 @@ static int pm_genpd_prepare(struct devic
 		return -EBUSY;
 	}
 
+	if (device_may_wakeup(dev)
+	    && !(genpd->active_wakeup && genpd->active_wakeup(dev)))
+		pm_runtime_resume(dev);
+
 	genpd_acquire_lock(genpd);
 
 	if (genpd->prepared_count++ == 0)
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