dtc: Add some documentation for the dts formta

This patch adds a dts-format.txt in the Documentation directory, with
an introduction to the dtc source format.  Note that this
documentation is also going into the upcoming ePAPR specification.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Documentation/dts-format.txt

@@ -0,0 +1,110 @@
+Device Tree Source Format (version 1)
+=====================================
+
+The Device Tree Source (DTS) format is a textual representation of a
+device tree in a form that can be processed by dtc into a binary
+device tree in the form expected by the kernel.  The description below
+is not a formal syntax definition of DTS, but describes the basic
+constructs used to represent device trees.
+
+Node and property definitions
+-----------------------------
+
+Device tree nodes are defined with a node name and unit address with
+braces marking the start and end of the node definition.  They may be
+preceded by a label.
+
+	[label:] node-name[@unit-address] {
+		[properties definitions]
+		[child nodes]
+	}
+
+Nodes may contain property definitions and/or child node
+definitions. If both are present, properties must come before child
+nodes.
+
+Property definitions are name value pairs in the form:
+	[label:] property-name = value;
+except for properties with empty (zero length) value which have the
+form:
+	[label:] property-name;
+
+Property values may be defined as an array of 32-bit integer cells, as
+NUL-terminated strings, as bytestrings or a combination of these.
+
+* Arrays of cells are represented by angle brackets surrounding a
+  space separated list of C-style integers
+
+	e.g. interrupts = <17 0xc>;
+
+* A 64-bit value is represented with two 32-bit cells.
+
+	e.g. clock-frequency = <0x00000001 0x00000000>;
+
+* A NUL-terminated string value is represented using double quotes
+  (the property value is considered to include the terminating NUL
+  character).
+
+	e.g. compatible = "simple-bus";
+
+* A bytestring is enclosed in square brackets [] with each byte
+  represented by two hexadecimal digits.  Spaces between each byte are
+  optional.
+
+	e.g. local-mac-address = [00 00 12 34 56 78]; or equivalently
+	     local-mac-address = [000012345678];
+
+* Values may have several comma-separated components, which are
+  concatenated together.
+	e.g. compatible = "ns16550", "ns8250";
+	     example = <0xf00f0000 19>, "a strange property format";
+
+* In a cell array a reference to another node will be expanded to that
+  node's phandle.  References may by '&' followed by a node's label:
+	e.g. interrupt-parent = < &mpic >;
+  or they may be '&' followed by a node's full path in braces:
+	e.g. interrupt-parent = < &{/soc/interrupt-controller@40000} >;
+
+* Outside a cell array, a reference to another node will be expanded
+  to that node's full path.
+	e.g. ethernet0 = &EMAC0;
+
+* Labels may also appear before or after any component of a property
+  value, or between cells of a cell array, or between bytes of a
+  bytestring.
+	e.g. reg = reglabel: <0 sizelabel: 0x1000000>;
+	e.g. prop = [ab cd ef byte4: 00 ff fe];
+	e.g. str = start: "string value" end: ;
+
+
+File layout
+-----------
+
+Version 1 DTS files have the overall layout:
+	/dts-v1/;
+
+	[memory reservations]
+
+	/ {
+		[property definitions]
+		[child nodes]
+	};
+
+* The "/dts-v1/;" must be present to identify the file as a version 1
+  DTS (dts files without this tag will be treated by dtc as being in
+  the obsolete "version 0", which uses a different format for integers
+  amongst other small but incompatible changes).
+
+* Memory reservations define an entry for the device tree blob's
+  memory reservation table.  They have the form:
+	e.g. /memreserve/ <address> <length>;
+  Where <address> and <length> are 64-bit C-style integers.
+
+* The / { ... }; section defines the root node of the device tree.
+
+* C style (/* ... */) and C++ style (// ...) comments are supported.
+
+
+
+	-- David Gibson <david@gibson.dropbear.id.au>
+	-- Yoder Stuart <stuart.yoder@freescale.com>