Re: [PATCH v5 7/7] iommu/exynos: Use device dependency links to control runtime pm

[Lukas Wunner]

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