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

Ulf Hansson ulf.hansson at linaro.org
Tue Nov 4 08:42:13 PST 2014

On 4 November 2014 14:51, Rafael J. Wysocki <rjw at rjwysocki.net> wrote:
> On Tuesday, November 04, 2014 09:54:19 AM 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?
> Yes, but my point was that those bus types might need to be changed.
> We can't make everyone happy at the same time if their ideas about what to do
> are different.

Urgh. I fail to understand this comment.

Why do we prefer the pm_runtime_get_sync() solution in favour of this
pathset's approach?

What are the benefit do we get with pm_runtime_get_sync()?

>> [...]
>> >> For PM domains that are initialized in powered off state, we can't
>> >> rely on CONFIG_PM_RUNTIME and thus not on pm_runtime_get_sync() to
>> >> power on these PM domains. We need a different mechanism, which is
>> >> suggested in this v3 patchset.
>> >
>> > That is quite simple to address, though.  You can register a bus type
>> > notifier that will power up the domain on BUS_NOTIFY_ADD_DEVICE events
>> > (where the target device belongs to the domain), and do that only for
>> > CONFIG_PM_RUNTIME unset (otherwise runtime PM should take care of this).
>> I guess we could use notifiers, but I am not sure I see any benefit.
>> The code will be more complex and we need error handling as well.
> Like "oh, I can't power up this thing, so I should fail ->probe()"?
> Then your driver would need to depend on the specific knowledge about the
> given PM domain, I'm afraid.
> If you want error handling like that, it needs to be handled by the core,
> so as to avoid calling the bus type's ->probe() as well in that case.

Yes, I want this error handling - but I fail to understand why the bus
can't handle the errors.

If works perfectly in this patchset's approach.

> So to summarize:
> - Devices need to be added to power domains before really_probe() is called
>   for them.  Otherwise we'll have ordering problems all over.

What ordering problems?

> - Runtime PM (if compiled in) needs to be enabled for all devices in power
>   domains by default.  Otherwise devices may lose power as a result for
>   power management of the other devices in the same domain.
> - The core should try to power up domains before calling really_probe() both
>   for CONFIG_PM_RUNTIME set and unset, so ->probe() can always make the
>   "device is accessible" assumption.

And how exactly will you then power up the PM domain when

> - Bus types may need to do more on top of that in their ->probe(), so the
>   driver's ->probe() can make that assumption too in all cases.
> Does that make sense to you?

I working on fully understanding your idea. :-)

Kind regards

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