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 <[email protected]> > --- > 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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