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

[Rafael J. Wysocki]

On Tuesday, November 18, 2014 12:04:38 PM Dmitry Torokhov wrote:
> On Tue, Nov 18, 2014 at 09:14:56PM +0100, Rafael J. Wysocki wrote:
> > On Tuesday, November 18, 2014 09:55:15 AM Dmitry Torokhov wrote:
> > > On Tue, Nov 18, 2014 at 12:44:22PM -0500, Alan Stern wrote:
> > > > On Tue, 18 Nov 2014, Dmitry Torokhov wrote:
> > > > 
> > > > > OK. Another question then: pm_runtime_get_noresume() does literally this:
> > > > > 
> > > > > 	atomic_inc(&dev->power.usage_count);
> > > > > 
> > > > > So who is responsible for actually waking up parent device and/or power
> > > > > domain? Is it simply missing because we did not really have PM domains
> > > > > before?
> > > > 
> > > > Ths bus is responsible for making sure that all the standard resources
> > > > are available -- that is, all the resources that would be needed by a
> > > > normal device on that bus.  Anything beyond that (such as
> > > > special-purpose clocks) has to be set up by the driver.
> > > > 
> > > > Thus the bus would insure that the device was powered, its parent was
> > > > resumed, and the usual clock inputs were enabled.  And of course, one
> > > > mechanism for doing this is to runtime-resume the power domain.
> > > 
> > > This does not sound like anything bus-specific. Can we move powering on
> > > the domain before probing into the driver core, similarly to the default
> > > pin selection by pinctrl?
> > 
> > We could do that for genpd if devices were added to domains before registering
> > (those devices).  Otherwise, there's no guarantee that all has been set up yet.
> > 
> > Note that this would only be the case for genpd, not for the ACPI PM domain
> > in particular, for example.  The reason why is that the ACPI PM domain cannot
> > be used along with bus types that provide non-trivial PM callbacks, so pretty
> > much the bus type's ->probe needs to decide whether or not to use it.
> 
> In genpd code there is a notion of providers that match devices and
> domains. Can we do the same for ACPI and stuff all that knowledge into
> it's "provider"?

It is in ACPI like that too, but not in the form of the ACPI PM domain.

> IOW why ACPI is that special?

The ACPI PM domain is there specifically for bus types that don't provide
non-trivial PM callbacks to avoid duplication of code (if it didn't exist,
all of the bus types in question would need to provide callbacks with
optional ACPI handling in them).  That's all about it.

And there are bus types that provide non-trivial PM callbacks *and* use
ACPI in them, like PCI, and that is more interleaved with the native PM
in there.  For those bus types we can't add devices to the ACPI PM domain
just because they have ACPI companion objects.

I'm not really sure why it is important here, though.  We're talking about
genpd, aren't we?

I just wanted to indicate that the PM domains concept is not only about
handling power domains and not all of its use cases can be shoehorned into
the same scheme.

Rafael