From mboxrd@z Thu Jan 1 00:00:00 1970 From: Frank Rowand Subject: Re: [PATCH v7 3/5] dtc: Document the dynamic plugin internals Date: Wed, 25 May 2016 12:13:35 -0700 Message-ID: <5745F95F.6000600@gmail.com> References: <1464112239-29856-1-git-send-email-pantelis.antoniou@konsulko.com> <1464112239-29856-4-git-send-email-pantelis.antoniou@konsulko.com> Reply-To: frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1464112239-29856-4-git-send-email-pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org> Sender: devicetree-compiler-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Pantelis Antoniou Cc: David Gibson , Jon Loeliger , Grant Likely , Rob Herring , Mark Rutland , Jan Luebbe , Sascha Hauer , Matt Porter , devicetree-compiler-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: devicetree@vger.kernel.org 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 > --- > 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 */ + }; + }; +}; > + > +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). > +Note that the target's ocp label is undefined, so the phandle handle > +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 > + > +