From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pantelis Antoniou Subject: Re: [PATCH v7 3/5] dtc: Document the dynamic plugin internals Date: Thu, 26 May 2016 09:14:49 +0300 Message-ID: <1151E0EF-B811-4C0B-858A-00810BE9BA42@konsulko.com> References: <1464112239-29856-1-git-send-email-pantelis.antoniou@konsulko.com> <1464112239-29856-4-git-send-email-pantelis.antoniou@konsulko.com> <5745F95F.6000600@gmail.com> Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <5745F95F.6000600-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> Sender: devicetree-compiler-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org 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 Hi Frank, > On May 25, 2016, at 22:13 , Frank Rowand wro= te: >=20 > On 5/24/2016 10:50 AM, 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 | 318 +++++++++++++++++++++++++= ++++++++++ >> 1 file changed, 318 insertions(+) >> create mode 100644 Documentation/dt-object-internal.txt >>=20 >> 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 possi= ble. >> + >> +1. Simplified Problem Definition >> +-------------------------------- >> + >> +Assume we have a platform which boots using following simplified de= vice 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 unde= fined method) >> +should result in different device tree configuration. >> + >> +We cannot boot with this static tree because due to the configurati= on of the >> +foo platform there exist multiple conficting peripherals DT fragmen= ts. >> + >> +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 periph= eral 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 differen= t 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 sh= ould be >> +encoded using device tree itself. We can add nodes that can be safe= ly 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 bo= ot 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 sho= uld be >> +possible to express everything using the existing DT syntax. >> + >> +3. Implementation >> +----------------- >> + >> +The basic unit of addressing in Device Tree is the phandle. Turns o= ut it's >> +relatively simple to extend the way phandles are generated and refe= renced >> +so that it's possible to dynamically convert symbolic references (l= abels) >> +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 roo= t 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 =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__ n= ode will >> +be safely ignored both by the bootloader and the kernel (the only l= oss will >> +be a few bytes of memory and disk space). >> + >> +3.b) The Device Tree fragments must be compiled with the same optio= n 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 =3D <&ocp>; >> + __overlay__ { >> + /* bar peripheral */ >> + bar { >> + compatible =3D "corp,bar"; >> + ... /* various properties and child nodes */ >> + } >=20 > }; >=20 >> + }; >> + }; >> +}; >=20 > 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. >=20 > Without those added, the overlay dts becomes simpler (though for a > multi-node target path example this would be more complex unless a la= bel > was used for the target node): >=20 > +/dts-v1/ /plugin/; /* allow undefined references and record them */ > +/ { > + .... /* various properties for loader use; i.e. part id etc. */ > + ocp { > + /* bar peripheral */ > + bar { > + compatible =3D "corp,bar"; > + ... /* various properties and child nodes */ > + }; > + }; > +}; >=20 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 x= 86 platform. >> + >> +Note that there's a target property that specifies the location whe= re 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 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 handl= e >> +value is filled with the illegal value '0xffffffff', while a __fixu= ps__ >> +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 >> + >> +