On 08/27/2013 11:01 AM, Josh Cartwright wrote:
...

If we want to ensure for the generic bindings that we are fulling
characterizing/describing the SPMI bus, then we'll additionally need to
tackle an additional identified assumption:

  4. One master per SPMI bus.  (The SPMI spec allows for up to 4
     masters)

On the Snapdragon 800 series, there exists only one software-controlled
master, but it is conceivably possible to have a setup with two
software-controlled masters on the same SPMI bus.

This necessarily means that the description of the slaves and the
masters will need to be decoupled; I'm imagining a generic binding
supporting multiple masters would look something like this:

Is there a need to represent the other masters in the DT? Sure they're
there in HW, but if there's no specific way for the
CPU-to-which-the-DT-applies to actually interact with those other
masters (except perhaps by experiencing some arbitration delays) then
presumably there's no need to represent the other masters in DT?

	master0: master at 0 {
		compatible = "...";
		#spmi-master-cells = <0>;
		spmi-mid = <0>;

		...
	};

	master2: master at 2 {
		compatible = "...";
		#spmi-master-cells = <0>;
		spmi-mid = <2>;

		...
	};

	spmi_bus {
		compatible = "...";

		spmi-masters = <&master0 &master2>;

		foo at 0 {
			compatible = "...";
			reg = <0 ...>;
		};

		foo at 8 {
			compatible = "...";
			reg = <8 ...>;
		};
	};

(This will also necessitate a change in the underlying SPMI driver
model, in the current implementation, a SPMI master 'owns' a particular
device.  This is not a valid assumption to make.)

Would this property-containing-phandle-vector be considered the
canonical way of representing nodes with multiple parents in the device
tree?

I don't think I've seen anything like this before, although that
in-and-of-itself doesn't make it wrong.

Another approach might be to encode master-vs-slave into a cell in the
reg property? Something like:

cell 0 - address type (0: master, 1: unique ID, 2: group ID, ...)
cell 1 - address value

I haven't thought much about that; perhaps there are disadvantages doing
that.