Quoting Pantelis Antoniou (2016-11-25 04:32:09)
> 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
s/many//
> +that need to dynamically insert device tree fragments to the
that need to dynamically insert device tree fragments into the
Also, should device tree be capitalized here?
> +running kernel's live tree.
Drop "running kernel's" as it's implicit with "live tree"?
> +
> +This document explains the the device tree object format and the
s/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
s/dtb's/dtbs/
> +with a different magic number in the header but otherwise they too are simple
> +blobs.
but otherwise they're 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.
Why? We can't figure that out based on the /plugin/ label within the dts
file? And shouldn't we always generate a __symbols__ node in the base
dtb?
> +
> +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).
This never really mentions why we need to generate a symbols node.
Perhaps we should say something like "we generate a __symbols__ node to
record nodes that had labels in the base tree so they can be matched up
with the fragments which reference the same labels"? Or something like
that.
I also wonder why it's even necessary. Couldn't we require overlays to
be compiled with the original dts files? Then we could encode the full
path of nodes referenced in the overlay into the overlay dtb itself.
> +
> +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 */
> + }
> + };
> + };
> +};
> +
> +Note that there's a target property that specifies the location where the
> +contents of the overlay node will be placed, and it references the node
> +in the foo.dts file.
> +
> +$ dtc -@ -O dtb -o bar.dtbo -b 0 bar.dts
> +$ fdtdump bar.dtbo
> +...
> +/ {
> + ... /* properties */
> + fragment@0 {
> + target = <0xffffffff>;
> + __overlay__ {
> + bar {
> + compatible = "corp,bar";
> + ... /* various properties and child nodes */
> + }
> + };
> + };
> + __fixups__ {
> + ocp = "/fragment@0:target:0";
> + };
> +};
> +
> +No __symbols__ has been generated (no label in bar.dts).
Add "node" after __symbols__ here?
> +Note that the target's ocp label is undefined, so the phandle handle
Drop handle after phandle?
> +value is filled with the illegal value '0xffffffff', while a __fixups__
> +node has been generated, which marks the location in the tree where
> +the label lookup should store the runtime phandle value of the ocp node.
> +
> +The format of the __fixups__ node entry is
> +
> + <label> = "<local-full-path>:<property-name>:<offset>";
> +
> +<label> Is the label we're referring
> +<local-full-path> Is the full path of the node the reference is
> +<property-name> Is the name of the property containing the
Weird alignment here.
> + reference
> +<offset> The offset (in bytes) of where the property's
> + phandle value is located.
located within the property? Or "offset relative to the start of the
property in bytes where the phandle value is located"?
Is this a list? So multiple properties can be fixed up with the same
label? If so that isn't clear from this description.
> +
> +Doing the same with the baz peripheral's DTS format is a little bit more
> +involved, since baz contains references to local labels which require
> +local fixups.
> +
> +/dts-v1/ /plugin/; /* allow undefined label references and record them */
> +/ {
> + .... /* various properties for loader use; i.e. part id etc. */
> + fragment@0 {
> + target = <&res>;
> + __overlay__ {
> + /* baz resources */
> + baz_res: res_baz { ... };
> + };
> + };
> + fragment@1 {
> + target = <&ocp>;
> + __overlay__ {
> + /* baz peripheral */
> + baz {
> + compatible = "corp,baz";
> + /* reference to another point in the tree */
> + ref-to-res = <&baz_res>;
> + ... /* various properties and child nodes */
> + }
> + };
> + };
> +};
> +
> +Note that &bar_res reference.
> +
> +$ dtc -@ -O dtb -o baz.dtbo -b 0 baz.dts
> +$ fdtdump baz.dtbo
> +...
> +/ {
> + ... /* properties */
> + fragment@0 {
> + target = <0xffffffff>;
> + __overlay__ {
> + res_baz {
> + ....
> + phandle = <0x00000001>;
> + };
> + };
> + };
> + fragment@1 {
> + target = <0xffffffff>;
> + __overlay__ {
> + baz {
> + compatible = "corp,baz";
> + ... /* various properties and child nodes */
> + ref-to-res = <0x00000001>;
> + }
> + };
> + };
> + __fixups__ {
> + res = "/fragment@0:target:0";
> + ocp = "/fragment@1:target:0";
> + };
> + __local_fixups__ {
> + fragment@1 {
> + __overlay__ {
> + baz {
> + ref-to-res = <0>;
> + };
> + };
> + };
> + };
> +};
> +
> +This is similar to the bar case, but the reference of a local label by the
> +baz node generates a __local_fixups__ entry that records the place that the
> +local reference is being made. No matter how phandles are allocated from dtc
> +the run time loader must apply an offset to each phandle in every dynamic
> +DT object loaded. The __local_fixups__ node records the place of every
records the offset relative to the start of the property of every local
reference within that property so that the loader...
> +local reference so that the loader can apply the offset.
> +
> +There is an alternative syntax to the expanded form for overlays with phandle
> +targets which makes the format similar to the one using in .dtsi include files.
> +
> +So for the &ocp target example above one can simply write:
> +
> +/dts-v1/ /plugin/;
> +&ocp {
> + /* bar peripheral */
> + bar {
> + compatible = "corp,bar";
> + ... /* various properties and child nodes */
> + }
> +};
> +
> +The resulting dtb object is identical.
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