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

[Ulf Hansson]

On 4 November 2014 10:05, Dmitry Torokhov <dmitry.torokhov at gmail.com> 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.

Devices that are created while "discoverable buses" are being probed
can't be attached to a PM domain before the probing is done, because
those simply doesn't exist.

Now, I haven't yet seen a demand for such a cases, but it seems wrong
to not consider them. The current solution cover these.

Kind regards
Uffe