[RFC PATCH 3/7] doc: dt: mtd: partition: add on-flash format binding

[Brian Norris <computersforpeace-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

The platform description (such as the type of partition formats used on
a given flash) should be done independently of the flash driver in use.
However, we can't reasonably have *all* partition parsers run on all
flash (until they find a match), so let's overload the 'partitions'
subnode to support specifying which format(s) to try in the device tree.

Start by supporting Google's (Chrome OS) FMAP structure.

There have been others interested in extending devicetree support to
other parsers, like the bcm47xxpart parser:

  http://patchwork.ozlabs.org/patch/475986/

and the AFS (ARM Flash Structure?) parser:

  http://patchwork.ozlabs.org/patch/537827/

Signed-off-by: Brian Norris <computersforpeace-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
---
 .../devicetree/bindings/mtd/partition.txt          | 75 ++++++++++++++++++++--
 1 file changed, 69 insertions(+), 6 deletions(-)

diff --git a/Documentation/devicetree/bindings/mtd/partition.txt b/Documentation/devicetree/bindings/mtd/partition.txt
index 28ae56f5c972..1bf9a7243993 100644
--- a/Documentation/devicetree/bindings/mtd/partition.txt
+++ b/Documentation/devicetree/bindings/mtd/partition.txt
@@ -1,29 +1,56 @@
-Representing flash partitions in devicetree
+Flash partitions in device tree
+===============================
 
-Partitions can be represented by sub-nodes of an mtd device. This can be used
+Flash devices can be partitioned into one or more functional ranges (e.g.,
+"boot code", "nvram", and "kernel") in at least two distinct ways:
+
+ (A) a fixed flash layout at production time or
+ (B) with an on-flash partition table, such as RedBoot, that describes the
+     geometry and naming/purpose of each functional region
+
+The former typically requires an operating system to learn about the
+partitioning from some kind of metadata provided by the bootloader/firmware.
+Such partitions can be described using the method in "Section A: Fixed Partitions".
+
+The latter is somewhat analogous to partition tables used on block devices
+(e.g., MBR or GPT), except that there is less standardization for flash
+devices, and it is not always safe or efficient to attempt to search for all of
+them on every flash device in the system, particularly since many of them allow
+their data structures to be placed anywhere on the flash, and so require
+scanning the entire flash device to find them.
+
+To assist system software in locating these partition tables, we provide a
+binding to describe which partition format(s) may be used on a given flash,
+found below in "Section B: On-Flash Partition Tables".
+
+
+Section A: Fixed Partitions
+---------------------------
+
+Partitions can be represented by sub-nodes of a flash device. This can be used
 on platforms which have strong conventions about which portions of a flash are
 used for what purposes, but which don't use an on-flash partition table such
 as RedBoot.
 
-The partition table should be a subnode of the mtd node and should be named
+The partition table should be a subnode of the flash node and should be named
 'partitions'. This node should have the following property:
 - compatible : (required) must be "partitions"
 Partitions are then defined in subnodes of the partitions node.
 
-For backwards compatibility partitions as direct subnodes of the mtd device are
+For backwards compatibility partitions as direct subnodes of the flash device are
 supported. This use is discouraged.
 NOTE: also for backwards compatibility, direct subnodes that have a compatible
 string are not considered partitions, as they may be used for other bindings.
 
 #address-cells & #size-cells must both be present in the partitions subnode of the
-mtd device. There are two valid values for both:
+flash device. There are two valid values for both:
 <1>: for partitions that require a single 32-bit cell to represent their
      size/address (aka the value is below 4 GiB)
 <2>: for partitions that require two 32-bit cells to represent their
      size/address (aka the value is 4 GiB or greater).
 
 Required properties:
-- reg : The partition's offset and size within the mtd bank.
+- reg : The partition's offset and size within the flash
 
 Optional properties:
 - label : The label / name for this partition.  If omitted, the label is taken
@@ -89,3 +116,39 @@ flash@2 {
 		};
 	};
 };
+
+
+Section B: On-Flash Partition Tables
+------------------------------------
+
+System designers use a variety of on-flash data structures to describe the
+layout of the flash. Because it's not always optimal for system software to
+scan for every sort of data structure that might be used, one can specify which
+structure(s) might be used on a given flash using the 'partitions' subnode of
+the flash node.
+
+Node name: partitions
+Properties:
+ - compatible: (required) used to define which partition format(s) may be in
+   use on this flash may contain one or more of the strings defined in the
+   following subsections
+
+
+Google's FMAP (Flash MAP)
+#########################
+
+Often used to describe the boot flash on Chrome OS devices. Specified here:
+
+  https://github.com/dhendrix/flashmap/blob/wiki/FmapSpec.md
+
+Properties:
+- compatible: (required) must include "google,fmap"
+
+
+Examples:
+
+flash@0 {
+	partitions {
+		compatible = "google,fmap";
+	};
+};
-- 
2.6.0.rc2.230.g3dd15c0

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