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SubjectRe: [PATCH 6/6] OF: Introduce DT overlay support.
From
Date
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@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.

Sorry, I can't parse this. You mean apply the overlay without converting
to live tree format?

>
> --
> 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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