Date:	Wed, 23 Jan 2013 16:12:07 +1100
From:	David Gibson <david@...son.dropbear.id.au>
To:	Pantelis Antoniou <panto@...oniou-consulting.com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>, Jon Loeliger <jdl@....com>,
	Tony Lindgren <tony@...mide.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Benoit Cousson <b-cousson@...com>,
	Mitch Bradley <wmb@...mworks.com>,
	Alan Tull <atull@...era.com>, linux-omap@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Matt Porter <mporter@...com>,
	Russ Dill <Russ.Dill@...com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Joel A Fernandes <agnel.joel@...il.com>,
	Rob Clark <robdclark@...il.com>,
	Jason Kridner <jkridner@...gleboard.org>,
	Matt Ranostay <mranostay@...il.com>
Subject: Re: [PATCH 6/6] OF: Introduce DT overlay support.

On Tue, Jan 22, 2013 at 01:08:04PM +0200, Pantelis Antoniou wrote:
> Hi
> 
> On Jan 22, 2013, at 5:50 AM, David Gibson wrote:
> 
> > On Fri, Jan 04, 2013 at 09:31:10PM +0200, Pantelis Antoniou wrote:
> >> Introduce DT overlay support.
> >> Using this functionality it is possible to dynamically overlay a part of
> >> the kernel's tree with another tree that's been dynamically loaded.
> >> It is also possible to remove node and properties.
> >> 
> >> Signed-off-by: Pantelis Antoniou <panto@...oniou-consulting.com>
> >> ---
> >> Documentation/devicetree/overlay-notes.txt | 179 +++++++
> >> drivers/of/Kconfig                         |  10 +
> >> drivers/of/Makefile                        |   1 +
> >> drivers/of/overlay.c                       | 831 +++++++++++++++++++++++++++++
> >> include/linux/of.h                         | 107 ++++
> >> 5 files changed, 1128 insertions(+)
> >> create mode 100644 Documentation/devicetree/overlay-notes.txt
> >> create mode 100644 drivers/of/overlay.c
> >> 
> >> diff --git a/Documentation/devicetree/overlay-notes.txt b/Documentation/devicetree/overlay-notes.txt
> >> new file mode 100644
> >> index 0000000..5289cbb
> >> --- /dev/null
> >> +++ b/Documentation/devicetree/overlay-notes.txt
> >> @@ -0,0 +1,179 @@
> >> +Device Tree Overlay Notes
> >> +-------------------------
> >> +
> >> +This document describes the implementation of the in-kernel
> >> +device tree overlay functionality residing in drivers/of/overlay.c and is a
> >> +companion document to Documentation/devicetree/dt-object-internal.txt[1] &
> >> +Documentation/devicetree/dynamic-resolution-notes.txt[2]
> >> +
> >> +How overlays work
> >> +-----------------
> >> +
> >> +A Device Tree's overlay purpose is to modify the kernel's live tree, and
> >> +have the modification affecting the state of the the kernel in a way that
> >> +is reflecting the changes.
> > 
> > Um.. I'm having a great deal of trouble parsing that sentence.
> > 
> >> +Since the kernel mainly deals with devices, any new device node that result
> >> +in an active device should have it created while if the device node is either
> >> +disabled or removed all together, the affected device should be deregistered.
> >> +
> >> +Lets take an example where we have a foo board with the following base tree
> >> +which is taken from [1].
> >> +
> >> +---- foo.dts -----------------------------------------------------------------
> >> +	/* FOO platform */
> >> +	/ {
> >> +		compatible = "corp,foo";
> >> +
> >> +		/* shared resources */
> >> +		res: res {
> >> +		};
> >> +
> >> +		/* On chip peripherals */
> >> +		ocp: ocp {
> >> +			/* peripherals that are always instantiated */
> >> +			peripheral1 { ... };
> >> +		}
> >> +	};
> >> +---- foo.dts -----------------------------------------------------------------
> >> +
> >> +The overlay bar.dts, when loaded (and resolved as described in [2]) should
> >> +
> >> +---- bar.dts -----------------------------------------------------------------
> >> +/plugin/;	/* allow undefined label references and record them */
> >> +/ {
> >> +	....	/* various properties for loader use; i.e. part id etc. */
> >> +	fragment@0 {
> >> +		target = <&ocp>;
> >> +		__overlay__ {
> >> +			/* bar peripheral */
> >> +			bar {
> >> +				compatible = "corp,bar";
> >> +				... /* various properties and child nodes */
> >> +			}
> >> +		};
> >> +	};
> >> +};
> >> +---- bar.dts -----------------------------------------------------------------
> >> +
> >> +result in foo+bar.dts
> >> +
> >> +---- foo+bar.dts -------------------------------------------------------------
> >> +	/* FOO platform + bar peripheral */
> >> +	/ {
> >> +		compatible = "corp,foo";
> >> +
> >> +		/* shared resources */
> >> +		res: res {
> >> +		};
> >> +
> >> +		/* On chip peripherals */
> >> +		ocp: ocp {
> >> +			/* peripherals that are always instantiated */
> >> +			peripheral1 { ... };
> >> +
> >> +			/* bar peripheral */
> >> +			bar {
> >> +				compatible = "corp,bar";
> >> +				... /* various properties and child nodes */
> >> +			}
> >> +		}
> >> +	};
> >> +---- foo+bar.dts -------------------------------------------------------------
> >> +
> >> +As a result of the the overlay, a new device node (bar) has been created
> >> +so a bar platform device will be registered and if a matching device driver
> >> +is loaded the device will be created as expected.
> > 
> > Hrm.  This all seems rather complicated.  Maybe it needs to be, but
> > I'm not entirely convinced yet.
> > 
> > One other point - both of these patches are assuming that the overlay
> > is in the "live tree" format, but it still needs a bunch of extra
> > mangling.  Would it simplify things to just go straight from the
> > overlay in flat tree form to modifications to the system-wide live
> > tree.
> 
> Sorry, I can't parse this. You mean apply the overlay without converting
> to live tree format?

Yes.

-- 
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

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