[PATCH v3 0/9] PM / Domains: Fix race conditions during boot

[Rafael J. Wysocki]

On Tuesday, November 04, 2014 01:05:21 AM Dmitry Torokhov wrote:
> On Tue, Nov 04, 2014 at 09:54:19AM +0100, Ulf Hansson wrote:
> > [...]
> > 
> > > Generally, there are two or even three levels of runtime PM handling,
> > > driver, (possibly) bus type and (possibly) PM domain (and multiple levels
> > > of these are possible in principle).  All of them have to be initialized
> > > at different times.
> > >
> > > Quite arguably, the PM domain and/or bus type runtime PM handling should
> > > be initialized even before registerind the device or during device
> > > registration.  Doing that later may be too late.  When the device has been
> > > registered, runtime PM should work to an extent allowing the driver to access
> > > the device and configure it further after calling pm_runtime_resume().
> > >
> > > Of course, if ->probe() is to call pm_runtime_resume() for this purpose,
> > > it must take the fact that the driver's own ->runtime_resume() may be called
> > > as a result of this into account.  That's why I'm asking whether or not the
> > > core should call pm_runtime_resume() before calling really_probe() in a
> > > followup branch of this thread.
> > 
> > I am reading the other thread, let's see.
> > 
> > >
> > > The driver's own runtime PM handling must be initialized in the driver and
> > > the only place suitable for that is ->probe().  However, it needs to be done
> > > *before* the driver's own ->runtime_resume() or ->runtime_suspend() callback
> > > is executed.  If that is done properly, it should be possible to cover
> > > both the CONFIG_PM_RUNTIME set/unset cases in that code.
> > >
> > > And I wouldn't recommend anyone to do the runtime PM initialization in
> > > ->runtime_resume() (when it is called for the first time), as that would be
> > > error prone and fragile.
> > 
> > Great! That's means we are at least aligned on this topic. :-)
> > 
> > >
> > >> The AMBA bus and some of its drivers a good example of how this has
> > >> been implemented:
> > >> driver/amba/bus.c
> > >> drivers/mmc/host/mmci.c
> > >> drivers/spi/spi-pl022.c
> > >>
> > >> This conclusion I have made from this is:
> > >> - Using pm_runtime_get_sync() during the ->probe() path to explicitly
> > >> power up a PM domain, is not suitable as the _common_ solution to
> > >> solve the race condition. It certainly may work for some scenarios,
> > >> but not for those I am looking at.
> > >
> > > I think, however, that it might work if the core calls pm_runtime_get_sync()
> > > from driver_probe_device().
> > 
> > Currently this won't work.
> > 
> > That's because the buses' ->probe() are invoked in this path and they
> > are doing the attachment of the device to its PM domain.
> > 
> > In other words, we can't power up the PM domain using
> > pm_runtime_get_sync(), until the device has been attached to its PM
> > domain. Right?
> 
> 
> I think this is one of the issues that we have there. Why do we conflate
> probing and placing the device into a power domain? The latter should
> happen when we register the device. The fact that a device was probed
> and has a driver bound or not bound to it should have no bearing on
> whether the device is member of power domain or not.

Agreed.

Rafael