devicetree: bindings: Document qcom board compatible format

Some qcom based bootloaders identify the dtb blob based on a set
of device properties like SoC, platform, PMIC, and revisions of
those components. In downstream kernels, these values are added
to the different component dtsi files (i.e. pmic dtsi file, SoC
dtsi file, board dtsi file, etc.) via qcom specific DT
properties. The dtb files are parsed by a program called dtbTool
that picks out these properties and creates a table of contents
binary blob with the property information and some offsets into
the concatenation of all the dtbs (termed a QCDT image).

The suggestion is to do this via the board compatible string
instead, because these qcom specific properties are never used by
the kernel. Add a document describing the format of the
compatible string that encodes all this information that's
currently encoded in the qcom,{msm-id,board-id,pmic-id}
properties in downstream devicetrees. Future bootloaders may be
updated to look at the compatible field instead of looking for
the table of contents image. For non-updateable bootloaders, a
new dtbTool program will parse the compatible string and generate
a QCDT image from it.
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>

Documentation/devicetree/bindings/arm/qcom.txt

+QCOM device tree bindings
+-------------------------
+
+Some qcom based bootloaders identify the dtb blob based on a set of
+device properties like SoC and platform and revisions of those components.
+To support this scheme, we encode this information into the board compatible
+string.
+
+Each board must specify a top-level board compatible string with the following
+format:
+
+	compatible = "qcom,<SoC>[-<soc_version>][-<foundry_id>]-<board>[/<subtype>][-<board_version>]"
+
+The 'SoC' and 'board' elements are required. All other elements are optional.
+
+The 'SoC' element must be one of the following strings:
+
+	apq8016
+	apq8074
+	apq8084
+	apq8096
+	msm8916
+	msm8974
+	msm8996
+
+The 'board' element must be one of the following strings:
+
+	cdp
+	liquid
+	dragonboard
+	mtp
+	sbc
+
+The 'soc_version' and 'board_version' elements take the form of v<Major>.<Minor>
+where the minor number may be omitted when it's zero, i.e.  v1.0 is the same
+as v1. If all versions of the 'board_version' elements match, then a
+wildcard '*' should be used, e.g. 'v*'.
+
+The 'foundry_id' and 'subtype' elements are one or more digits from 0 to 9.
+
+Examples:
+
+	"qcom,msm8916-v1-cdp-pm8916-v2.1"
+
+A CDP board with an msm8916 SoC, version 1 paired with a pm8916 PMIC of version
+2.1.
+
+	"qcom,apq8074-v2.0-2-dragonboard/1-v0.1"
+
+A dragonboard board v0.1 of subtype 1 with an apq8074 SoC version 2, made in
+foundry 2.