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diff --git a/Documentation/dt-object-internal.txt b/Documentation/dt-object-internal.txt deleted file mode 100644 index 51d68ab93ac9..000000000000 --- a/Documentation/dt-object-internal.txt +++ /dev/null @@ -1,310 +0,0 @@ -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. |