Hi David,
> On May 26, 2016, at 09:33 , David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, May 26, 2016 at 09:31:20AM +0300, Pantelis Antoniou wrote:
>> Hi David,
>>
>>> On May 26, 2016, at 09:28 , David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> On Thu, May 26, 2016 at 09:14:49AM +0300, Pantelis Antoniou wrote:
>>>> Hi Frank,
>>>>
>>>>> On May 25, 2016, at 22:13 , Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>>>>>
>>>>> On 5/24/2016 10:50 AM, Pantelis Antoniou wrote:
>>>>>> Provides the document explaining the internal mechanics of
>>>>>> plugins and options.
>>>>>>
>>>>>> Signed-off-by: Pantelis Antoniou <pantelis.antoniou@xxxxxxxxxxxx>
>>>>>> ---
>>>>>> Documentation/dt-object-internal.txt | 318 +++++++++++++++++++++++++++++++++++
>>>>>> 1 file changed, 318 insertions(+)
>>>>>> create mode 100644 Documentation/dt-object-internal.txt
>>>>>>
>>>>>> diff --git a/Documentation/dt-object-internal.txt b/Documentation/dt-object-internal.txt
>>>>>> new file mode 100644
>>>>>> index 0000000..d5b841e
>>>>>> --- /dev/null
>>>>>> +++ b/Documentation/dt-object-internal.txt
>>>>>> @@ -0,0 +1,318 @@
>>>>>> +Device Tree Dynamic Object format internals
>>>>>> +-------------------------------------------
>>>>>> +
>>>>>> +The Device Tree for most platforms is a static representation of
>>>>>> +the hardware capabilities. This is insufficient for many platforms
>>>>>> +that need to dynamically insert device tree fragments to the
>>>>>> +running kernel's live tree.
>>>>>> +
>>>>>> +This document explains the the device tree object format and the
>>>>>> +modifications made to the device tree compiler, which make it possible.
>>>>>> +
>>>>>> +1. Simplified Problem Definition
>>>>>> +--------------------------------
>>>>>> +
>>>>>> +Assume we have a platform which boots using following simplified device tree.
>>>>>> +
>>>>>> +---- foo.dts -----------------------------------------------------------------
>>>>>> + /* FOO platform */
>>>>>> + / {
>>>>>> + compatible = "corp,foo";
>>>>>> +
>>>>>> + /* shared resources */
>>>>>> + res: res {
>>>>>> + };
>>>>>> +
>>>>>> + /* On chip peripherals */
>>>>>> + ocp: ocp {
>>>>>> + /* peripherals that are always instantiated */
>>>>>> + peripheral1 { ... };
>>>>>> + };
>>>>>> + };
>>>>>> +---- foo.dts -----------------------------------------------------------------
>>>>>> +
>>>>>> +We have a number of peripherals that after probing (using some undefined method)
>>>>>> +should result in different device tree configuration.
>>>>>> +
>>>>>> +We cannot boot with this static tree because due to the configuration of the
>>>>>> +foo platform there exist multiple conficting peripherals DT fragments.
>>>>>> +
>>>>>> +So for the bar peripheral we would have this:
>>>>>> +
>>>>>> +---- 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 -------------------------------------------------------------
>>>>>> +
>>>>>> +While for the baz peripheral we would have this:
>>>>>> +
>>>>>> +---- foo+baz.dts -------------------------------------------------------------
>>>>>> + /* FOO platform + baz peripheral */
>>>>>> + / {
>>>>>> + compatible = "corp,foo";
>>>>>> +
>>>>>> + /* shared resources */
>>>>>> + res: res {
>>>>>> + /* baz resources */
>>>>>> + baz_res: res_baz { ... };
>>>>>> + };
>>>>>> +
>>>>>> + /* On chip peripherals */
>>>>>> + ocp: ocp {
>>>>>> + /* peripherals that are always instantiated */
>>>>>> + peripheral1 { ... };
>>>>>> +
>>>>>> + /* baz peripheral */
>>>>>> + baz {
>>>>>> + compatible = "corp,baz";
>>>>>> + /* reference to another point in the tree */
>>>>>> + ref-to-res = <&baz_res>;
>>>>>> + ... /* various properties and child nodes */
>>>>>> + };
>>>>>> + };
>>>>>> + };
>>>>>> +---- foo+baz.dts -------------------------------------------------------------
>>>>>> +
>>>>>> +We note that the baz case is more complicated, since the baz peripheral needs to
>>>>>> +reference another node in the DT tree.
>>>>>> +
>>>>>> +2. Device Tree Object Format Requirements
>>>>>> +-----------------------------------------
>>>>>> +
>>>>>> +Since the device tree is used for booting a number of very different hardware
>>>>>> +platforms it is imperative that we tread very carefully.
>>>>>> +
>>>>>> +2.a) No changes to the Device Tree binary format for the base tree. We cannot
>>>>>> +modify the tree format at all and all the information we require should be
>>>>>> +encoded using device tree itself. We can add nodes that can be safely ignored
>>>>>> +by both bootloaders and the kernel. The plugin dtb's are optionally tagged
>>>>>> +with a different magic number in the header but otherwise they too are simple
>>>>>> +blobs.
>>>>>> +
>>>>>> +2.b) Changes to the DTS source format should be absolutely minimal, and should
>>>>>> +only be needed for the DT fragment definitions, and not the base boot DT.
>>>>>> +
>>>>>> +2.c) An explicit option should be used to instruct DTC to generate the required
>>>>>> +information needed for object resolution. Platforms that don't use the
>>>>>> +dynamic object format can safely ignore it.
>>>>>> +
>>>>>> +2.d) Finally, DT syntax changes should be kept to a minimum. It should be
>>>>>> +possible to express everything using the existing DT syntax.
>>>>>> +
>>>>>> +3. Implementation
>>>>>> +-----------------
>>>>>> +
>>>>>> +The basic unit of addressing in Device Tree is the phandle. Turns out it's
>>>>>> +relatively simple to extend the way phandles are generated and referenced
>>>>>> +so that it's possible to dynamically convert symbolic references (labels)
>>>>>> +to phandle values. This is a valid assumption as long as the author uses
>>>>>> +reference syntax and does not assign phandle values manually (which might
>>>>>> +be a problem with decompiled source files).
>>>>>> +
>>>>>> +We can roughly divide the operation into two steps.
>>>>>> +
>>>>>> +3.a) Compilation of the base board DTS file using the '-@' option
>>>>>> +generates a valid DT blob with an added __symbols__ node at the root node,
>>>>>> +containing a list of all nodes that are marked with a label.
>>>>>> +
>>>>>> +Using the foo.dts file above the following node will be generated;
>>>>>> +
>>>>>> +$ dtc -@ -O dtb -o foo.dtb -b 0 foo.dts
>>>>>> +$ fdtdump foo.dtb
>>>>>> +...
>>>>>> +/ {
>>>>>> + ...
>>>>>> + res {
>>>>>> + ...
>>>>>> + phandle = <0x00000001>;
>>>>>> + ...
>>>>>> + };
>>>>>> + ocp {
>>>>>> + ...
>>>>>> + phandle = <0x00000002>;
>>>>>> + ...
>>>>>> + };
>>>>>> + __symbols__ {
>>>>>> + res="/res";
>>>>>> + ocp="/ocp";
>>>>>> + };
>>>>>> +};
>>>>>> +
>>>>>> +Notice that all the nodes that had a label have been recorded, and that
>>>>>> +phandles have been generated for them.
>>>>>> +
>>>>>> +This blob can be used to boot the board normally, the __symbols__ node will
>>>>>> +be safely ignored both by the bootloader and the kernel (the only loss will
>>>>>> +be a few bytes of memory and disk space).
>>>>>> +
>>>>>> +3.b) The Device Tree fragments must be compiled with the same option but they
>>>>>> +must also have a tag (/plugin/) that allows undefined references to nodes
>>>>>> +that are not present at compilation time to be recorded so that the runtime
>>>>>> +loader can fix them.
>>>>>> +
>>>>>> +So the bar peripheral's DTS format would be of the form:
>>>>>> +
>>>>>> +/dts-v1/ /plugin/; /* allow undefined 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 */
>>>>>> + }
>>>>>
>>>>> };
>>>>>
>>>>>> + };
>>>>>> + };
>>>>>> +};
>>>>>
>>>>> Other than the fact that the above syntax is already in the Linux
>>>>> kernel overlay implementation, is there a need for the target
>>>>> property and the __overlay__ node? I haven't figured out what
>>>>> extra value they provide.
>>>>>
>>>>> Without those added, the overlay dts becomes simpler (though for a
>>>>> multi-node target path example this would be more complex unless a label
>>>>> was used for the target node):
>>>>>
>>>>> +/dts-v1/ /plugin/; /* allow undefined references and record them */
>>>>> +/ {
>>>>> + .... /* various properties for loader use; i.e. part id etc. */
>>>>> + ocp {
>>>>> + /* bar peripheral */
>>>>> + bar {
>>>>> + compatible = "corp,bar";
>>>>> + ... /* various properties and child nodes */
>>>>> + };
>>>>> + };
>>>>> +};
>>>>>
>>>>
>>>> No.
>>>>
>>>> That only works if the overlay is applied in a single platform.
>>>>
>>>> I have working cases where the same overlay is applied on a ppc and a x86
>>>> platform.
>>>
>>> Huh? How so..
>>>
>>
>> Yes, it does work. Yes it’s being used right now. It is a very valid use case.
>>
>> Think carrier boards on enterprise routers, plugging to a main board
>> that’s either ppc or x86 (or anything else for that matter).
>
> Sorry, I wasn't clear. I have no problem believing overlays can be
> applied on multiple platforms.
>
> What I can't see is how Frank's format breaks that. AFAICT it
> contains exactly the same information in a simpler encoding.
>
It breaks it because it’s missing the target property.
The layout of the base tree is not going to be the same in different
platforms, so in the above example ‘ocp’ would not exist in x86 for
instance.
> --
> 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
Regards
— Pantelis
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