linux-kernel - Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap

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Message-ID: <509D8E52.1010505@wwwdotorg.org>
Date:	Fri, 09 Nov 2012 16:14:26 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	David Gibson <david@...son.dropbear.id.au>
CC:	Grant Likely <grant.likely@...retlab.ca>,
	Kevin Hilman <khilman@...com>, Matt Porter <mporter@...com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Pantelis Antoniou <panto@...oniou-consulting.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Felipe Balbi <balbi@...com>,
	Deepak Saxena <dsaxena@...aro.org>,
	Scott Wood <scottwood@...escale.com>,
	Russ Dill <Russ.Dill@...com>, linux-omap@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org
Subject: Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices
 to mach-omap2)

On 11/08/2012 07:26 PM, David Gibson wrote:
...
> So, let me take a stab at this from a more bottom-up approach, and see
> if we meet in the middle somewhere.  As I discussed in the other
> thread with Daniel Mack, I can see two different operationso on the
> fdt that might be useful in this context.  I think of them as "graft"
> - which takes one fdt and adds it as a new subtree to an existing fdt
> - and "overlay" where a new fdt adds or overrides arbitrary properties
> in an existing tree.  Overlay is more or less what we do at the source
> level in dtc already.

One more thought on the differences between overlay and grafting:

With overlay, it's possible to make your overlay a complete DT tree
rooted at /. In some cases, you might find a lower-level node which all
overlaid elements share, and root the overlay process there.

With grafting, you're basically guaranteed to want the child/graft file
to have different parts of itself grafted into different points in the
parent/underlying tree, e.g. to add nodes to an I2C bus, an SPI bus, and
perhaps some bus-less devices at the root (e.g. a new gpio-leds device).
This will require that a child/graft file to consist of multiple chunks
of DT all to be grafted as separate operations, whereas with overlay you
may be able to get away with a single chunk to be overlaid (although
with some of the use-cases discussed, even the overlay approach might
require multiple chunks to be applied).
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