Re: [PATCH v10 2/4] dtc: Document the dynamic plugin internals

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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On Mon, Nov 28, 2016 at 10:29:05PM +0200, Pantelis Antoniou wrote:
> Hi Stephen,
> 
> > On Nov 28, 2016, at 22:03 , Stephen Boyd <stephen.boyd-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
> > 
> > Quoting Pantelis Antoniou (2016-11-25 04:32:09)
> >> 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
> > 
> > s/many//
> > 
> >> +that need to dynamically insert device tree fragments to the
> > 
> > that need to dynamically insert device tree fragments into the
> > 
> > Also, should device tree be capitalized here?
> > 
> >> +running kernel's live tree.
> > 
> > Drop "running kernel's" as it's implicit with "live tree"?
> > 
> >> +
> >> +This document explains the the device tree object format and the
> > 
> > s/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
> > 
> > s/dtb's/dtbs/
> > 
> >> +with a different magic number in the header but otherwise they too are simple
> >> +blobs.
> > 
> > but otherwise they're simple blobs.
> > 
> 
> OK on the spelling/grammar fixes above.
> 
> >> +
> >> +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.
> > 
> > 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?
> > 
> 
> Actually now we do. The last patchset does automatically generate those nodes
> if a /plugin/ tag is encountered. For base dtbs I would suggest that generating
> the symbols node automatically is what’s sane too, but unfortunately there are
> some platforms out there that are having trouble with larger dtbs than what they
> expect.
> 
> It is your call whether to enable it by default I guess.
> 
> >> +
> >> +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).
> > 
> > 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.
> > 
> 
> Hmm, yeah.
> 
> > 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.
> > 
> 
> No, we can’t do that; the end-game of this is for overlays to be portable
> for use in platforms having the same kind of connectors.

That's kind of true, but actually I think we want to redesign the
connector format.  Obviously we'll take some stuff from the current
overlay format, but we can't be fully compatible with them, so we
should take the opportunity to remove some of the sillier design flaws
in the overlay format.

That said, even with their flaws and limitations, overlays in the
current format can sometimes be portable to multiple base trees.

-- 
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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