linux-kernel - Re: [PATCH RFC v2 3/5] spmi: add generic SPMI controller binding documentation

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Date:	Tue, 27 Aug 2013 15:55:19 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Josh Cartwright <joshc@...eaurora.org>
CC:	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Kumar Gala <galak@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org,
	Sagar Dharia <sdharia@...eaurora.org>,
	Gilad Avidov <gavidov@...eaurora.org>,
	Michael Bohan <mbohan@...eaurora.org>,
	devicetree@...r.kernel.org
Subject: Re: [PATCH RFC v2 3/5] spmi: add generic SPMI controller binding
 documentation

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@0 {
> 		compatible = "...";
> 		#spmi-master-cells = <0>;
> 		spmi-mid = <0>;
> 
> 		...
> 	};
> 
> 	master2: master@2 {
> 		compatible = "...";
> 		#spmi-master-cells = <0>;
> 		spmi-mid = <2>;
> 
> 		...
> 	};
> 
> 	spmi_bus {
> 		compatible = "...";
> 
> 		spmi-masters = <&master0 &master2>;
> 
> 		foo@0 {
> 			compatible = "...";
> 			reg = <0 ...>;
> 		};
> 
> 		foo@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.
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