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SubjectRe: [PATCH 6/6] OF: Introduce DT overlay support.
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@antoniou-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.

--
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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