Re: How-to: Uniquely identify a DT node in the driver?

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

Hi,

Sorry for the corss-talk on my previous reply.

> I am trying to instantiate generic PM domains for different CPU clusters
> commonly. Individual platform code may have different actions for
> powering down/up the CPU domain. It could be different set of bucks that
> they need to talk to or bunch of devices. The common code would like to
> provide the opportunity for the platform code to perform their
> activities.
> 
> CPUs may be organized into 'n' clusters, the common code would create
> genpd for each of these clusters. In a multi machine image, to identify
> the right platform driver for the cluster, is a challenge. I am trying
> to solve it the same way as CPUidle - using __init section tables.  To
> uniquely identify a cluster in a SoC, I need a way to match the domain
> provider's DT node, with a callback in the driver. Like the 'method'
> attribute of the CPUIdle macros. The CPU compatibles are too generic and
> could duplicate across SoCs to be used for comparison. For e.g, you
> could have two clusters of A53 cores that could use the same compatible
> string. Distinguishing the domains for each of these clusters is a pain
> (but doable using phandles to the domain referenced by the CPU).
> 
> To make it easy for the driver, I could only think of adding an unique
> compatible string to the domain node and the platform driver would then
> be able to same compatible string to distinguish between domains for the
> different clusters.
> 
> Alternately, I was exploring a way to use phandles for the device nodes
> as unique comparison attributes, but that is more complex and doesnt
> provide any better benefit than the compatible.

I don't believe using compatible strings is the right thing to do. The
thing which varies per-domain is the relationships of various
components, which should be described with phandles. At the level of the
domain, the interface is identical, and thus they should have the same
compatible string.

Using different compatible strings implies that we have to add new
compatible strings for each new variation that appears, leaving us with
a completely arbitrary set of compatible strings with (likely)
ill-defined semantics. That makes it really difficult to reuse code, and
necessitates adding far more.

The inter-device relationships (and the attibutes of those devices)
should be explicit in the DT.

Thanks,
Mark.
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