On Wed, Jan 23, 2013 at 01:01:58PM +0200, Pantelis Antoniou wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxx>
> >>>> ---
> >>>> 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.
It's really not that bad. In particular it's only tricky if you want
to make alterations to the flat tree, which you shouldn't need to
here.
> 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?
It would be, if you actually needed to do that. But what you can
actually do is:
1. parse the information you need out of the base tree
2. use the flat tree functions to retrieve the
__local_fixups__ and __fixups__ information from the overlay, parsing
just those into a temporary data structure
3. Perform a single pass over the flat tree overlay, inserting
each node and and property into the live tree, applying fixups as you
go.
--
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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