[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[David Gibson]

On Mon, Nov 12, 2012 at 10:22:07PM -0700, Stephen Warren wrote:
> On 11/12/2012 06:05 PM, David Gibson wrote:
> > On Fri, Nov 09, 2012 at 09:42:37PM +0000, Grant Likely wrote:
> ...
> > 2) graft bundle
> > 
> > The base tree has something like this:
> > 
> > 	...
> > 	i2c at XXX {
> > 		...
> > 		cape-socket {
> > 			compatible = "vendor,cape-socket";
> > 			id = "Socket-A";
> > 			piece-id = "i2c";
> > 			ranges = < ... >;
> > 		};
> > 	};
> > 	...
> > 	spi at YYY {
> > 		...
> > 		cape-socket {
> > 			compatible = "vendor,cape-socket";
> > 			id = "Socket-A";
> > 			piece-id = "spi";
> > 			ranges = < ... >;
> > 		};
> > 	};
> > 	...
> > 	cape-socket {
> > 		compatible = "vendor,cape-socket";
> > 		id = "Socket-A";
> > 		piece-id = "misc";
> > 		interrupt-map = < ... >;
> > 		interrupt-map-mask = < ... >;
> > 		gpio-map = < ... >;
> > 		gpio-map-mask = < ... >;
> > 	};
> > 
> > Then instead of grafting a single subtree for the socket, we install a
> > "bundle" of subtrees, one each for each of the pieces within the
> > socket.  That bundle could either be an actual set of multiple fdts,
> > or they could be placed into one fdt with a dummy root node, something like:
> >
> > 	/ {
> > 		plugin-bundle;
> > 		compatible = "vendor,cape-plugin";
> > 		version = ...;
> > 		i2c-piece = {
> > 			piece-id = "i2c";
> > 			...
> > 		};
> > 		misc-piece = {
> > 			piece-id = "misc";
> > 			...
> > 		};
> > 	};
> 
> I do like this approach; it's the kind of thing I proposed at:
> 
> > http://www.mail-archive.com/devicetree-discuss@lists.ozlabs.org/msg20414.html

Roughly, yes, though a little streamlined from the syntax suggested
there.

> One question though: Perhaps the base board has two instances of the
> same type of connector vendor,cape-socket, allowing 2 independent capes
> to be plugged in. When overlaying/grafting the child board's .dts, we'd
> need some way to specify the socket ID that was being plugged into. Is
> that the intent of the "id" property in your base board example above?

Yes, that's exactly what I had in mind for the "id" property.
Property names and other details entirely negotiable at this stage,
of course.

By the by, I think having multiple interchangable sockets could break
the convention based approach for avoiding collisions between phandles
I suggested, but another mail with some better thoughts on that
shortly to be posted.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson