Message-ID: <509D8E52.1010505@wwwdotorg.org>
Date: Fri, 09 Nov 2012 16:14:26 -0700
From: Stephen Warren <swarren@wwwdotorg.org>
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To: David Gibson <david@gibson.dropbear.id.au>
CC: Grant Likely <grant.likely@secretlab.ca>, Kevin Hilman <khilman@ti.com>,
        Matt Porter <mporter@ti.com>, Koen Kooi <koen@dominion.thruhere.net>,
        Pantelis Antoniou <panto@antoniou-consulting.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Felipe Balbi <balbi@ti.com>, Deepak Saxena <dsaxena@linaro.org>,
        Scott Wood <scottwood@freescale.com>, Russ Dill <Russ.Dill@ti.com>,
        linux-omap@vger.kernel.org, devicetree-discuss@lists.ozlabs.org
Subject: Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices
 to mach-omap2)
References: <CACxGe6vu8ek7-K-yDDMXyg9x-oKiFt_cSz+Pz-yZf5U5vwe=0g@mail.gmail.com> <20121109022624.GI23553@truffula.fritz.box>
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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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