From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Gibson Subject: Re: [PATCH v10 2/4] dtc: Document the dynamic plugin internals Date: Tue, 29 Nov 2016 13:01:45 +1100 Message-ID: <20161129020145.GA13307@umbus.fritz.box> References: <1480077131-14526-1-git-send-email-pantelis.antoniou@konsulko.com> <1480077131-14526-3-git-send-email-pantelis.antoniou@konsulko.com> <148036343076.23275.14028691096221007535@sboyd-linaro> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="zhXaljGHf11kAtnf" Return-path: Content-Disposition: inline In-Reply-To: <148036343076.23275.14028691096221007535@sboyd-linaro> Sender: devicetree-compiler-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Stephen Boyd Cc: Pantelis Antoniou , Jon Loeliger , Grant Likely , Frank Rowand , Rob Herring , Jan Luebbe , Sascha Hauer , Phil Elwell , Simon Glass , Maxime Ripard , Thomas Petazzoni , Boris Brezillon , Antoine Tenart , Devicetree Compiler , devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: devicetree@vger.kernel.org --zhXaljGHf11kAtnf Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Nov 28, 2016 at 12:03:50PM -0800, Stephen Boyd wrote: > Quoting Pantelis Antoniou (2016-11-25 04:32:09) > > diff --git a/Documentation/dt-object-internal.txt b/Documentation/dt-ob= ject-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 >=20 > s/many// >=20 > > +that need to dynamically insert device tree fragments to the >=20 > that need to dynamically insert device tree fragments into the >=20 > Also, should device tree be capitalized here? >=20 > > +running kernel's live tree. >=20 > Drop "running kernel's" as it's implicit with "live tree"? >=20 > > + > > +This document explains the the device tree object format and the >=20 > s/the// >=20 > > +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 devic= e tree. > > + > > +---- foo.dts ---------------------------------------------------------= -------- > > + /* FOO platform */ > > + / { > > + compatible =3D "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 undefin= ed 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 =3D "corp,foo"; > > + > > + /* shared resources */ > > + res: res { > > + }; > > + > > + /* On chip peripherals */ > > + ocp: ocp { > > + /* peripherals that are always instantiated */ > > + peripheral1 { ... }; > > + > > + /* bar peripheral */ > > + bar { > > + compatible =3D "corp,bar"; > > + ... /* various properties and child nod= es */ > > + }; > > + }; > > + }; > > +---- foo+bar.dts -----------------------------------------------------= -------- > > + > > +While for the baz peripheral we would have this: > > + > > +---- foo+baz.dts -----------------------------------------------------= -------- > > + /* FOO platform + baz peripheral */ > > + / { > > + compatible =3D "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 =3D "corp,baz"; > > + /* reference to another point in the tr= ee */ > > + ref-to-res =3D <&baz_res>; > > + ... /* various properties and child nod= es */ > > + }; > > + }; > > + }; > > +---- foo+baz.dts -----------------------------------------------------= -------- > > + > > +We note that the baz case is more complicated, since the baz periphera= l 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 h= ardware > > +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 shoul= d 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 ta= gged >=20 > s/dtb's/dtbs/ >=20 > > +with a different magic number in the header but otherwise they too are= simple > > +blobs. >=20 > but otherwise they're simple blobs. >=20 > > + > > +2.b) Changes to the DTS source format should be absolutely minimal, an= d 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. >=20 > 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? No, given it's a nonstandard extension on the basic device tree contents, I don't think we should generate the symbol information by default. /plugin/ can let you determine whether to generate fixups, but you need the symbols for the base tree. > > + > > +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 referen= ced > > +so that it's possible to dynamically convert symbolic references (labe= ls) > > +to phandle values. This is a valid assumption as long as the author us= es > > +reference syntax and does not assign phandle values manually (which mi= ght > > +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 n= ode, > > +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 =3D <0x00000001>; > > + ... > > + }; > > + ocp { > > + ... > > + phandle =3D <0x00000002>; > > + ... > > + }; > > + __symbols__ { > > + res=3D"/res"; > > + ocp=3D"/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). >=20 > 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. >=20 > 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. That's one of many different design decisions that could have been made, and might have been a better idea. But the current design is out in the wild now, flaws and all, so we do need to implement it. > > + > > +3.b) The Device Tree fragments must be compiled with the same option b= ut they > > +must also have a tag (/plugin/) that allows undefined references to no= des > > +that are not present at compilation time to be recorded so that the ru= ntime > > +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 =3D <&ocp>; > > + __overlay__ { > > + /* bar peripheral */ > > + bar { > > + compatible =3D "corp,bar"; > > + ... /* various properties and child nod= es */ > > + } > > + }; > > + }; > > +}; > > + > > +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 =3D <0xffffffff>; > > + __overlay__ { > > + bar { > > + compatible =3D "corp,bar"; > > + ... /* various properties and child nod= es */ > > + } > > + }; > > + }; > > + __fixups__ { > > + ocp =3D "/fragment@0:target:0"; > > + }; > > +}; > > + > > +No __symbols__ has been generated (no label in bar.dts). >=20 > Add "node" after __symbols__ here? >=20 > > +Note that the target's ocp label is undefined, so the phandle handle >=20 > Drop handle after phandle? >=20 > > +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 nod= e. > > + > > +The format of the __fixups__ node entry is > > + > > +