On Thu, Nov 10, 2016 at 12:56:14AM +0100, Rafael J. Wysocki wrote:

The idea, roughly, is that if there is a single on/off switch acting
on multiple devices, you can (a) set up a PM domain tracking all of
those device's runtime PM invocations and (b) maintaining a reference
counter of devices still not suspended.  This way it would only turn
the switch off when all of the devices in question had been suspended.
Analogously, it would turn the switch on before resuming the first
device in the domain.  Of course, that code isn't available as a
library, you would need to implement it (or use genpd, but chances are
it is too heavy weight for the job).

My understanding is that the hierarchy of struct generic_pm_domain
is created by the platform on boot.  For an embedded platform, this
is encoded in the device tree, but what about ACPI which doesn't
know anything about struct generic_pm_domain?  I would have to lump
devices into generic_pm_domains after the fact, after the platform
has scanned the buses, but this seems to be forbidden according to
this slide deck, which calls that a "layering violation":

https://events.linuxfoundation.org/images/stories/pdf/lcjp2012_wysocki.pdf

(Quote: "Adding and Removing Devices [...] Supposed to be called by
the platform (calling one of them from a device driver is a layering
violation).")

So it seems that using struct generic_pm_domain is never an option
on ACPI, is that correct?

Thanks,

Lukas