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. -- 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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