Re: [PATCH v7 3/5] dtc: Document the dynamic plugin internals

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

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On Thu, May 26, 2016 at 09:14:49AM +0300, Pantelis Antoniou wrote:
> Hi Frank,
> 
> > On May 25, 2016, at 22:13 , Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > 
> > 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 <pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org>
> >> ---
> >> 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 */
> > +			};
> > +	};
> > +};
> > 
> 
> 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 x86
> platform.

Huh?  How so..

-- 
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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