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SubjectRe: [PATCH 6/6] OF: Introduce DT overlay support.
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On Jan 23, 2013, at 7:12 AM, David Gibson wrote:

> 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@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?
>
> Yes.
>

The gymnastics required when operating on the flat tree will make grown, tough as nails
s/w developers cry.

In essence you will have to replicate the unflattening functionality again, create a
similar tree structure as the live tree, do your work, and then discard it.

A bit excessive don't you think?


> --
> 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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