kern: add an extres-style power domain framework

[kevans]

Summary

Aptly named 'powerdom', the upcoming Apple Silicon power controller
driver among other drivers in the family will need this.

[mmel]

LGTM. Just note that the newly added OF_device_unregister_xref() function does not work as expected without some sort of reference couting. A single device can (and do it in many cases) provide multiple functions (for example - pinmux, gpio, and interrupt controller), so is registered using respective frameworks 3x...

[kevans]

In D49655#1132012, @mmel wrote:

LGTM. Just note that the newly added OF_device_unregister_xref() function does not work as expected without some sort of reference couting. A single device can (and do it in many cases) provide multiple functions (for example - pinmux, gpio, and interrupt controller), so is registered using respective frameworks 3x...

I'll work on a patch for that; I'm hesitant to revert it since it was sitting in Phab for years and it works for the trivial cases I'm doing, but I'm happy to iterate and fix it. I do wonder, looking at the final implementation that landed and looking at the version I actually had locally, if we should've checked xi->dev and avoided doing anything if they don't match for some reason.

[andrew]

How will multiple devices inside a single power domain be handled?

[mmel]

I really didn't want to force you to revert it. And it's just my problem that I didn't notice it at the right time. I just wanted to pass on this (perhaps obvious) information to someone else who is less sclerotic than me, that's all. Mostly because I've been super lazy lately, so no one can expect anything from me that just a few words.

[mmel]

In D49655#1132066, @andrew wrote:

How will multiple devices inside a single power domain be handled?

imho, the provider device for a domain must be only one device. Whether it is a virtual "coordination" device or a real but simple driver is probably not important now. It does not prevent further implementation and is sufficient for many use cases.

[andrew]

In sys/contrib/device-tree/src/arm64/arm/juno-scmi.dtsi I see 29 instances of power-domains = <&scmi_devpd 8>;. Will the scmi_devpd (and all other power domain controllers) device need to reference count these, or could we move this to the powerdom code?

[kevans]

In D49655#1132084, @andrew wrote:

In sys/contrib/device-tree/src/arm64/arm/juno-scmi.dtsi I see 29 instances of power-domains = <&scmi_devpd 8>;. Will the scmi_devpd (and all other power domain controllers) device need to reference count these, or could we move this to the powerdom code?

That's a fair point... I think you're right that it makes more sense for the framework to deal with it as ubiquitous as it'll be. I'll pull the provider_dev out into a refcounted struct and adjust the rest accordingly.