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

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

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.

Thanks.

-- 
Dmitry