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diff --git a/dtc/Documentation/dt-object-internal.txt b/dtc/Documentation/dt-object-internal.txt new file mode 100644 index 000000000..51d68ab93 --- /dev/null +++ b/dtc/Documentation/dt-object-internal.txt @@ -0,0 +1,310 @@ +Device Tree Dynamic Object format internals +------------------------------------------- + +The Device Tree for most platforms is a static representation of +the hardware capabilities. This is insufficient for platforms +that need to dynamically insert Device Tree fragments into the +live tree. + +This document explains the the Device Tree object format and +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 dtbs are optionally tagged +with a different magic number in the header 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. + +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). + +We generate a __symbols__ node to record nodes that had labels in the base +tree (or subsequent loaded overlays) so that they can be matched up with +references made to them in Device Tree objects. + +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__ node has been generated (no label in bar.dts). +Note that the target's ocp label is undefined, so the 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>" + [, "<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 + reference + <offset> The offset (in bytes) of where the property's + phandle value is located. + +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 offset relative to the +start of every local reference within that property so that the loader can apply +the offset. |