[PATCH] PM / Domains: Power on the PM domain right after attach completes

[Rafael J. Wysocki]

On Wednesday, November 19, 2014 09:54:00 AM Ulf Hansson wrote:
> [...]
> 
> >>
> >> Scenario 5), a platform driver with/without runtime PM callbacks.
> >> ->probe()
> >> - do some initialization
> >> - may fetch handles to runtime PM resources
> >> - pm_runtime_enable()
> >
> > Well, and now how the driver knows if the device is "on" before accessing it?
> 
> In this case the driver don't need to access the device during
> ->probe(). That's postponed until sometime later.

If this is a platform driver, it rather does need to access the device,
precisely because it doesn't know what power state the device is in otherwise.
See below.

> >> Note 1)
> >> Scenario 1) and 2), both relies on the approach to power on the PM
> >> domain by using pm_runtime_get_sync(). That approach didn't work when
> >> CONFIG_PM_RUNTIME was unset, but we recently decided to fixed that by
> >> the below patch, so that's good!
> >> "[PATCH] PM / domains: Kconfig: always enable PM_RUNTIME when genpd enabled"
> >>
> >> Note 2)
> >> Scenario 3) and 4) use the same principles for managing runtime PM.
> >> These scenarios needs a way to power on the generic PM domain prior
> >> probing the device. The call to pm_runtime_set_active(), prevents an
> >> already powered PM domain from power off until after probe, but that's
> >> not enough.
> >>
> >> Note 3)
> >> The $subject patch, tried to address the issues for scenario 3) and
> >> 4). It does so, but will affect scenario 5) which was working nicely
> >> before. In scenario 5), the $subject patch will cause the generic PM
> >> domain to potentially stay powered after ->probe() even if the device
> >> is runtime PM suspended.
> >
> > Why would it?  If the device is runtime-suspended, the domain will know
> > that, because its callbacks will be used for that.  At least, that's
> > what I'd expect to happen, so is there a problem here?
> 
> Genpd do knows about the device but it doesn’t get a "notification" to
> power off. There are no issues whatsoever for driver.

Except that the driver is arguably buggy.

> This is a somewhat special case. Let's go through an example.
> 
> 1. The PM domain is initially in powered off state.
> 2. The bus ->probe() invokes dev_pm_domain_attach() and then the PM
> domain gets attached to the device.
> 3. $subject patch causes the PM domain to power on.
> 4. A driver ->probe() sequence start, following the Scenario 5).
> 5. The device is initially in runtime PM suspended state and it will
> remain so during ->probe().

But is it physically suspended?

The runtime PM status of the device after ->probe is required to reflect its
real state if runtime PM is enabled.  If that's not the case, it is a bug.

Now, for platform drivers, the driver can't really assume anything in
particular about the current power state of the device at ->probe time,
because different platforms including devices handled by that driver may
behave differently.

A good example would be two platforms A and B where the same device X is in a
power domain such that A boots with the domain (physically) "on", while B boots
with the domain "off".  If the driver for X assumes anything about the initial
power state of the device, it may not work on either A or B.

> 6. The pm_request_idle() invoked after really_probe() in driver core,
> won't trigger a runtime PM suspend callback to be invoked. In other
> words, genpd don't get a "notification" that it may power off.
> 
> In this state, genpd will either power off from the late_initcall,
> genpd_poweroff_unused() (depending on when the driver was probed) or
> wait until some device's runtime PM suspend callback is invoked at any
> later point.

Which sounds OK to me, so why is it a problem?

> >> I see three options going forward.
> >>
> >> Option 1)
> >> Convert scenario 3) and 4) into using the pm_runtime_get_sync()
> >> approach. There are no theoretical obstacles to do this, but pure
> >> practical. There are a lot of drivers that needs to be converted and
> >> we also need to teach driver authors future wise to not use
> >> pm_runtime_set_active() in this manner.
> >
> > I'd say we need to do something like this anyway.  That is, standardize on
> > *one* approach.  I'm actually not sure what approach is the most useful,
> > but the pm_runtime_get_sync() one seems to be the most popular to me.
> >
> >> Option 2)
> >> Add some kind of get/put API for PM domains. The buses invokes it to
> >> control the power to the PM domain. From what I understand, that's
> >> also what Dmitry think is needed!?
> >> Anyway, that somehow means to proceed with the approach I took in the
> >> below patchset.
> >> [PATCH v3 0/9] PM / Domains: Fix race conditions during boot
> >> http://marc.info/?t=141320907000003&r=1&w=2
> >
> > I don't like that.  The API is already quite complicated in my view and
> > adding even more complexity to it is not going to help in the long run.
> 
> I absolutely agree that we shouldn't add unnecessary APIs and keep
> APIs as simple as possible.
> 
> In that context, I think the effect from proceeding with Option 2)
> also means there are no need for the below APIs any more.
> pm_genpd_poweron()
> pm_genpd_name_poweron() (requires some additional work though)
> pm_genpd_poweroff_unused()
> pm_genpd_dev_need_restore()
> 
> I guess you figured out that I am in favour of Option 2). :-)
> Especially since it cover all scenarios and we don't have to go a fix
> a vast amount of drivers.

Adding more callbacks to struct dev_pm_domain just in order to handle some
genpd-specific use case is out of the question.  If you find a way within
genpd to address this the way you like, I'm open for ideas.  Otherwise,
let's just do what I said.

> >
> >> Option 3)
> >> Live we the limitation this $subject patch introduces for scenario 5).
> >
> > I'd say, 3) for now and 1) going forward.
> 
> This could work!
> 
> The hardest part is to know when we should revert $subject patch, to
> fix the regression introduced for scenario 5).

Well, I don't think so.

First of all, there is a (very) limited number of platforms using genpd today
and it should be perfectly possible to go through the platform drivers used
by them and see which of those drivers are affected.  Fixing them should not
be too hard either.

Rafael