From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Gibson Subject: Re: [PATCH v4 3/3] dtc: Document the dynamic plugin internals Date: Wed, 4 Mar 2015 22:29:20 +1100 Message-ID: <20150304112920.GE18072@voom.fritz.box> References: <1425063346-14554-1-git-send-email-pantelis.antoniou@konsulko.com> <1425063346-14554-4-git-send-email-pantelis.antoniou@konsulko.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="PPYy/fEw/8QCHSq3" Return-path: Content-Disposition: inline In-Reply-To: <1425063346-14554-4-git-send-email-pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org> Sender: devicetree-compiler-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: To: Pantelis Antoniou Cc: Jon Loeliger , Grant Likely , Rob Herring , devicetree-compiler-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org --PPYy/fEw/8QCHSq3 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Feb 27, 2015 at 08:55:46PM +0200, Pantelis Antoniou wrote: > Provides the document explaining the internal mechanics of > plugins and options. >=20 > Signed-off-by: Pantelis Antoniou > --- > Documentation/dt-object-internal.txt | 301 +++++++++++++++++++++++++++++= ++++++ > 1 file changed, 301 insertions(+) > create mode 100644 Documentation/dt-object-internal.txt >=20 > diff --git a/Documentation/dt-object-internal.txt b/Documentation/dt-obje= ct-internal.txt > new file mode 100644 > index 0000000..b5ce9b4 > --- /dev/null > +++ b/Documentation/dt-object-internal.txt > @@ -0,0 +1,301 @@ > +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 =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 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 =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 nodes */ > + } > + } > + }; > +---- 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 tree */ > + ref-to-res =3D <&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 har= dware > +platforms it is imperative that we tread very carefully. > + > +2.a) No changes to the Device Tree binary format. We cannot modify the t= ree > +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 bootloa= ders and > +the kernel. I really don't understand this requirement. I mean, obviously it makes sense for the "base" dt, since that can be used as-is, ignoring the overlay stuff. But the overlay fragments themselves are useless if not properly processed, in which case what's the point of maintaining the same dtb format. It really seems like you're trying to force out-of-band information into band. The dt is flexible enough that you *can* do that, but that doesn't mean it's a good idea. > +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 r= equired > +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. > + > +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 nod= e, > +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 { > + ... > + linux,phandle =3D <0x00000001>; > + phandle =3D <0x00000001>; > + ... > + }; > + ocp { > + ... > + linux,phandle =3D <0x00000002>; > + phandle =3D <0x00000002>; > + ... > + }; > + __symbols__ { > + res=3D"/res"; > + ocp=3D"/ocp"; So, the __symbols__ node looks an awful lot like the OF-standard aliases node. Is there any reason you can't use /aliases for this instead? (Automatically generating aliases for nodes with a label is a dtc feature I've had vaguely in mind since forever). > + }; > +}; > + > +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 w= ill > +be safely ignored both by the bootloader and the kernel (the only loss w= ill > +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 labe= ls > +that are not present at compilation time to be recorded so that the runt= ime > +loader can fix them. > + > +So the bar peripheral's DTS format would be of the form: > + > +/plugin/; /* allow undefined label 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 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 label > +in the foo.dts file. > + > +$ dtc -@ -O dtb -o bar.dtbo -b 0 bar.dts > +$ fdtdump bar.dtbo > +... > +/ { > + ... /* properties */ > + fragment@0 { > + target =3D <0xdeadbeef>; The target property is specific to the overlay format. So specifying it as a nonsense phandle, which must then be fixed up is silly. It should be encoded directly as a symbolic name. > + __overlay__ { > + bar { > + compatible =3D "corp,bar"; > + ... /* various properties and child nodes */ > + } > + }; > + }; > + __fixups__ { > + ocp =3D "/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 '0xdeadbeef', 0xdeadbeef is not an illegal phandle value. Use either 0 or -1 if you need an illegal phandle value. > 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 > + > +