U-Boot new uImage source file format (bindings definition)

  ==========================================================
  
  Author: Marian Balakowicz <m8@semihalf.com>

  External data additions, 25/1/16 Simon Glass <sjg@chromium.org>

  
  1) Introduction
  ---------------
  
  Evolution of the 2.6 Linux kernel for embedded PowerPC systems introduced new
  booting method which requires that hardware description is available to the
  kernel in the form of Flattened Device Tree.
  
  Booting with a Flattened Device Tree is much more flexible and is intended to
  replace direct passing of 'struct bd_info' which was used to boot pre-FDT
  kernels.

  However, U-Boot needs to support both techniques to provide backward

  compatibility for platforms which are not FDT ready. Number of elements
  playing role in the booting process has increased and now includes the FDT
  blob. Kernel image, FDT blob and possibly ramdisk image - all must be placed
  in the system memory and passed to bootm as a arguments. Some of them may be
  missing: FDT is not present for legacy platforms, ramdisk is always optional.
  Additionally, old uImage format has been extended to support multi sub-images
  but the support is limited by simple format of the legacy uImage structure.
  Single binary header 'struct image_header' is not flexible enough to cover all
  possible scenarios.
  
  All those factors combined clearly show that there is a need for new, more
  flexible, multi component uImage format.
  
  
  2) New uImage format assumptions
  --------------------------------
  
  a) Implementation
  
  Libfdt has been selected for the new uImage format implementation as (1) it
  provides needed functionality, (2) is actively maintained and developed and

  (3) increases code reuse as it is already part of the U-Boot source tree.

  
  b) Terminology
  
  This document defines new uImage structure by providing FDT bindings for new

  uImage internals. Bindings are defined from U-Boot perspective, i.e. describe
  final form of the uImage at the moment when it reaches U-Boot. User

  perspective may be simpler, as some of the properties (like timestamps and

  hashes) will need to be filled in automatically by the U-Boot mkimage tool.

  
  To avoid confusion with the kernel FDT the following naming convention is
  proposed for the new uImage format related terms:
  
  FIT	- Flattened uImage Tree
  
  FIT is formally a flattened device tree (in the libfdt meaning), which
  conforms to bindings defined in this document.
  
  .its	- image tree source

  .itb	- flattened image tree blob

  
  c) Image building procedure
  
  The following picture shows how the new uImage is prepared. Input consists of
  image source file (.its) and a set of data files. Image is created with the

  help of standard U-Boot mkimage tool which in turn uses dtc (device tree

  compiler) to produce image tree blob (.itb).  Resulting .itb file is the

  actual binary of a new uImage.
  
  
  tqm5200.its
  +

  vmlinux.bin.gz	   mkimage + dtc	       xfer to target

  eldk-4.2-ramdisk  --------------> tqm5200.itb --------------> bootm

  tqm5200.dtb			     /|\
  ...				      |
  				 'new uImage'

  
  	- create .its file, automatically filled-in properties are omitted
  	- call mkimage tool on a .its file
  	- mkimage calls dtc to create .itb image and assures that
  	  missing properties are added
  	- .itb (new uImage) is uploaded onto the target and used therein
  
  
  d) Unique identifiers
  
  To identify FIT sub-nodes representing images, hashes, configurations (which
  are defined in the following sections), the "unit name" of the given sub-node
  is used as it's identifier as it assures uniqueness without additional
  checking required.
  
  
  3) Root node properties
  -----------------------
  
  Root node of the uImage Tree should have the following layout:
  
  / o image-tree
      |- description = "image description"
      |- timestamp = <12399321>
      |- #address-cells = <1>
      |
      o images
      | |

      | o image-1 {...}
      | o image-2 {...}

      | ...
      |
      o configurations

        |- default = "conf-1"

        |

        o conf-1 {...}
        o conf-2 {...}

        ...
  
  
    Optional property:
    - description : Textual description of the uImage
  
    Mandatory property:
    - timestamp : Last image modification time being counted in seconds since
      1970-01-01 00:00:00 - to be automatically calculated by mkimage tool.
  
    Conditionally mandatory property:
    - #address-cells : Number of 32bit cells required to represent entry and
      load addresses supplied within sub-image nodes. May be omitted when no
      entry or load addresses are used.
  
    Mandatory node:
    - images : This node contains a set of sub-nodes, each of them representing
      single component sub-image (like kernel, ramdisk, etc.). At least one
      sub-image is required.
  
    Optional node:
    - configurations : Contains a set of available configuration nodes and
      defines a default configuration.
  
  
  4) '/images' node
  -----------------
  
  This node is a container node for component sub-image nodes. Each sub-node of
  the '/images' node should have the following layout:

   o image-1

     |- description = "component sub-image description"
     |- data = /incbin/("path/to/data/file.bin")
     |- type = "sub-image type name"
     |- arch = "ARCH name"
     |- os = "OS name"
     |- compression = "compression name"
     |- load = <00000000>
     |- entry = <00000000>
     |

     o hash-1 {...}
     o hash-2 {...}

     ...
  
    Mandatory properties:
    - description : Textual description of the component sub-image
    - type : Name of component sub-image type, supported types are:

      "standalone", "kernel", "kernel_noload", "ramdisk", "firmware", "script",
      "filesystem", "flat_dt" and others (see uimage_type in common/image.c).

    - data : Path to the external file which contains this node's binary data.
    - compression : Compression used by included data. Supported compressions
      are "gzip" and "bzip2". If no compression is used compression property

      should be set to "none". If the data is compressed but it should not be
      uncompressed by U-Boot (e.g. compressed ramdisk), this should also be set
      to "none".

  
    Conditionally mandatory property:

    - os : OS name, mandatory for types "kernel" and "ramdisk". Valid OS names
      are: "openbsd", "netbsd", "freebsd", "4_4bsd", "linux", "svr4", "esix",
      "solaris", "irix", "sco", "dell", "ncr", "lynxos", "vxworks", "psos", "qnx",
      "u_boot", "rtems", "unity", "integrity".

    - arch : Architecture name, mandatory for types: "standalone", "kernel",
      "firmware", "ramdisk" and "fdt". Valid architecture names are: "alpha",
      "arm", "i386", "ia64", "mips", "mips64", "ppc", "s390", "sh", "sparc",

      "sparc64", "m68k", "microblaze", "nios2", "blackfin", "avr32", "st200",
      "sandbox".

    - entry : entry point address, address size is determined by
      '#address-cells' property of the root node. Mandatory for for types:
      "standalone" and "kernel".
    - load : load address, address size is determined by '#address-cells'
      property of the root node. Mandatory for types: "standalone" and "kernel".
  
    Optional nodes:

    - hash-1 : Each hash sub-node represents separate hash or checksum

      calculated for node's data according to specified algorithm.
  
  
  5) Hash nodes
  -------------

  o hash-1

    |- algo = "hash or checksum algorithm name"
    |- value = [hash or checksum value]
  
    Mandatory properties:
    - algo : Algorithm name, supported are "crc32", "md5" and "sha1".
    - value : Actual checksum or hash value, correspondingly 4, 16 or 20 bytes
      long.
  
  
  6) '/configurations' node
  -------------------------
  
  The 'configurations' node is optional. If present, it allows to create a
  convenient, labeled boot configurations, which combine together kernel images
  with their ramdisks and fdt blobs.
  
  The 'configurations' node has has the following structure:
  
  o configurations
    |- default = "default configuration sub-node unit name"
    |

    o config-1 {...}
    o config-2 {...}

    ...
  
  
    Optional property:
    - default : Selects one of the configuration sub-nodes as a default
      configuration.
  
    Mandatory nodes:
    - configuration-sub-node-unit-name : At least one of the configuration
      sub-nodes is required.
  
  
  7) Configuration nodes
  ----------------------
  
  Each configuration has the following structure:

  o config-1

    |- description = "configuration description"
    |- kernel = "kernel sub-node unit name"
    |- ramdisk = "ramdisk sub-node unit name"

    |- fdt = "fdt sub-node unit-name" [, "fdt overlay sub-node unit-name", ...]

    |- fpga = "fpga sub-node unit-name"

    |- loadables = "loadables sub-node unit-name"

  
  
    Mandatory properties:
    - description : Textual configuration description.
    - kernel : Unit name of the corresponding kernel image (image sub-node of a
      "kernel" type).
  
    Optional properties:
    - ramdisk : Unit name of the corresponding ramdisk image (component image
      node of a "ramdisk" type).
    - fdt : Unit name of the corresponding fdt blob (component image node of a

      "fdt type"). Additional fdt overlay nodes can be supplied which signify
      that the resulting device tree blob is generated by the first base fdt
      blob with all subsequent overlays applied.

    - setup : Unit name of the corresponding setup binary (used for booting
      an x86 kernel). This contains the setup.bin file built by the kernel.

    - fpga : Unit name of the corresponding fpga bitstream blob
      (component image node of a "fpga type").

    - loadables : Unit name containing a list of additional binaries to be
      loaded at their given locations.  "loadables" is a comma-separated list

      of strings. U-Boot will load each binary at its given start-address and
      may optionaly invoke additional post-processing steps on this binary based
      on its component image node type.

  
  The FDT blob is required to properly boot FDT based kernel, so the minimal
  configuration for 2.6 FDT kernel is (kernel, fdt) pair.
  
  Older, 2.4 kernel and 2.6 non-FDT kernel do not use FDT blob, in such cases
  'struct bd_info' must be passed instead of FDT blob, thus fdt property *must
  not* be specified in a configuration node.

  8) External data
  ----------------
  
  The above format shows a 'data' property which holds the data for each image.
  It is also possible for this data to reside outside the FIT itself. This
  allows the FIT to be quite small, so that it can be loaded and scanned
  without loading a large amount of data. Then when an image is needed it can
  be loaded from an external source.
  
  In this case the 'data' property is omitted. Instead you can use:
  
    - data-offset : offset of the data in a separate image store. The image
      store is placed immediately after the last byte of the device tree binary,
      aligned to a 4-byte boundary.
    - data-size : size of the data in bytes

  The 'data-offset' property can be substituted with 'data-position', which
  defines an absolute position or address as the offset. This is helpful when

  booting U-Boot proper before performing relocation. Pass '-p [offset]' to
  mkimage to enable 'data-position'.

  Normal kernel FIT image has data embedded within FIT structure. U-Boot image
  for SPL boot has external data. Existence of 'data-offset' can be used to
  identify which format is used.

  9) Examples

  -----------

  Please see doc/uImage.FIT/*.its for actual image source files.