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

From: [email-masked] (Dmitry Torokhov)
Date: Tue, 18 Nov 2014 13:44:33 -0800

On Tue, Nov 18, 2014 at 10:58:17PM +0100, Rafael J. Wysocki wrote:
> On Tuesday, November 18, 2014 01:02:29 PM Dmitry Torokhov wrote:
> > On Tue, Nov 18, 2014 at 10:17:46PM +0100, Rafael J. Wysocki wrote:
> > > On Tuesday, November 18, 2014 10:03:18 PM Rafael J. Wysocki wrote:
> > > > 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.
> > > 
> > > And by the way, things worked just fine for the ACPI PM domain before commit
> > > 46420dd73b80 (PM / Domains: Add APIs to attach/detach a PM domain for a device)
> > > which put the ACPI PM domain and genpd into one bag, which was a mistake,
> > > because they are different things.
> > >
> > 
> > Can we maybe settle on the naming then so that we do not mix them up in
> > the future? For me PM domain is group of devices that share certain
> > power constraints so that they have to be powered up and down together.
> > Is this definition is not correct (for genpd at least)?
> 
> It is correct for genpd, it isn't correct for the ACPI PM domain.
> 
> > And what is the proper definition for ACPI PM domain?
> 
> I agree that the terminology is (somewhat?) confusing.
> 
> From the code perspective, using a PM domain object is a way to provide PM
> callbacks that will be executed for a subset of devices instead of or in
> addition to the bus type (or class or device type) callbacks.  Of course,
> that applies to proper power domains in particular, but it can also apply
> to broader sets of devices.  In the ACPI PM domain case this covers devices
> with ACPI power management support (or more precisely, devices with ACPI
> companion objects that can provide PM support).  In this context the word
> "domain" means as much as "area of control" (which is a proper dictionary
> definition of it AFAICS).
> 
> genpd is all about proper power domains, like you said.

OK, thank you for explaining this. Up until now I was blissfully
oblivious to the majority of PM intricacies :)

-- 
Dmitry