dtc: Document the dynamic plugin internals

Provides the document explaining the internal mechanics of
plugins and options.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Documentation/dt-object-internal.txt

@@ -0,0 +1,310 @@
+Device Tree Dynamic Object format internals
+-------------------------------------------
+
+The Device Tree for most platforms is a static representation of
+the hardware capabilities. This is insufficient for platforms
+that need to dynamically insert Device Tree fragments into the
+live tree.
+
+This document explains the the Device Tree object format and
+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 dtbs are optionally tagged
+with a different magic number in the header but otherwise they're 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).
+
+We generate a __symbols__ node to record nodes that had labels in the base
+tree (or subsequent loaded overlays) so that they can be matched up with
+references made to them in Device Tree objects.
+
+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 */
+			}
+		};
+	};
+};
+
+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 = <0xffffffff>;
+		__overlay__ {
+			bar {
+				compatible = "corp,bar";
+				... /* various properties and child nodes */
+			}
+		};
+	};
+	__fixups__ {
+	    ocp = "/fragment@0:target:0";
+	};
+};
+
+No __symbols__ node has been generated (no label in bar.dts).
+Note that the target's ocp label is undefined, so the phandle
+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 node.
+
+The format of the __fixups__ node entry is
+
+  <label> = "<local-full-path>:<property-name>:<offset>" 
+	    [, "<local-full-path>:<property-name>:<offset>"...];
+
+  <label> 		Is the label we're referring
+  <local-full-path>	Is the full path of the node the reference is
+  <property-name>	Is the name of the property containing the
+			reference
+  <offset>		The offset (in bytes) of where the property's
+			phandle value is located.
+
+Doing the same with the baz peripheral's DTS format is a little bit more
+involved, since baz contains references to local labels which require
+local fixups.
+
+/dts-v1/;
+/plugin/;	/* allow undefined label references and record them */
+/ {
+	....	/* various properties for loader use; i.e. part id etc. */
+	fragment@0 {
+		target = <&res>;
+		__overlay__ {
+			/* baz resources */
+			baz_res: res_baz { ... };
+		};
+	};
+	fragment@1 {
+		target = <&ocp>;
+		__overlay__ {
+			/* baz peripheral */
+			baz {
+				compatible = "corp,baz";
+				/* reference to another point in the tree */
+				ref-to-res = <&baz_res>;
+				... /* various properties and child nodes */
+			}
+		};
+	};
+};
+
+Note that &bar_res reference.
+
+$ dtc -@ -O dtb -o baz.dtbo -b 0 baz.dts
+$ fdtdump baz.dtbo
+...
+/ {
+	... /* properties */
+	fragment@0 {
+		target = <0xffffffff>;
+		__overlay__ {
+			res_baz {
+				....
+				phandle = <0x00000001>;
+			};
+		};
+	};
+	fragment@1 {
+		target = <0xffffffff>;
+		__overlay__ {
+			baz {
+				compatible = "corp,baz";
+				... /* various properties and child nodes */
+				ref-to-res = <0x00000001>;
+			}
+		};
+	};
+	__fixups__ {
+		res = "/fragment@0:target:0";
+		ocp = "/fragment@1:target:0";
+	};
+	__local_fixups__ {
+		fragment@1 {
+			__overlay__ {
+				baz {
+					ref-to-res = <0>;
+				};
+			};
+		};
+	};
+};
+
+This is similar to the bar case, but the reference of a local label by the
+baz node generates a __local_fixups__ entry that records the place that the
+local reference is being made. No matter how phandles are allocated from dtc
+the run time loader must apply an offset to each phandle in every dynamic
+DT object loaded. The __local_fixups__ node records the offset relative to the
+start of every local reference within that property so that the loader can apply
+the offset.