The PowerPC boot wrapper
  ------------------------
  Copyright (C) Secret Lab Technologies Ltd.
  
  PowerPC image targets compresses and wraps the kernel image (vmlinux) with
  a boot wrapper to make it usable by the system firmware.  There is no
  standard PowerPC firmware interface, so the boot wrapper is designed to
  be adaptable for each kind of image that needs to be built.
  
  The boot wrapper can be found in the arch/powerpc/boot/ directory.  The
  Makefile in that directory has targets for all the available image types.
  The different image types are used to support all of the various firmware
  interfaces found on PowerPC platforms.  OpenFirmware is the most commonly
  used firmware type on general purpose PowerPC systems from Apple, IBM and
  others.  U-Boot is typically found on embedded PowerPC hardware, but there
  are a handful of other firmware implementations which are also popular.  Each
  firmware interface requires a different image format.
  
  The boot wrapper is built from the makefile in arch/powerpc/boot/Makefile and
  it uses the wrapper script (arch/powerpc/boot/wrapper) to generate target
  image.  The details of the build system is discussed in the next section.
  Currently, the following image format targets exist:
  
     cuImage.%:		Backwards compatible uImage for older version of
  			U-Boot (for versions that don't understand the device
  			tree).  This image embeds a device tree blob inside
  			the image.  The boot wrapper, kernel and device tree
  			are all embedded inside the U-Boot uImage file format
  			with boot wrapper code that extracts data from the old
  			bd_info structure and loads the data into the device
  			tree before jumping into the kernel.
  			  Because of the series of #ifdefs found in the
  			bd_info structure used in the old U-Boot interfaces,
  			cuImages are platform specific.  Each specific
  			U-Boot platform has a different platform init file
  			which populates the embedded device tree with data
  			from the platform specific bd_info file.  The platform
  			specific cuImage platform init code can be found in
  			arch/powerpc/boot/cuboot.*.c.  Selection of the correct
  			cuImage init code for a specific board can be found in
  			the wrapper structure.
     dtbImage.%:		Similar to zImage, except device tree blob is embedded
  			inside the image instead of provided by firmware.  The
  			output image file can be either an elf file or a flat
  			binary depending on the platform.
  			  dtbImages are used on systems which do not have an
  			interface for passing a device tree directly.
  			dtbImages are similar to simpleImages except that
  			dtbImages have platform specific code for extracting
  			data from the board firmware, but simpleImages do not
  			talk to the firmware at all.
  			  PlayStation 3 support uses dtbImage.  So do Embedded
  			Planet boards using the PlanetCore firmware.  Board
  			specific initialization code is typically found in a
  			file named arch/powerpc/boot/<platform>.c; but this
  			can be overridden by the wrapper script.
     simpleImage.%:	Firmware independent compressed image that does not
  			depend on any particular firmware interface and embeds
  			a device tree blob.  This image is a flat binary that
  			can be loaded to any location in RAM and jumped to.
  			Firmware cannot pass any configuration data to the
  			kernel with this image type and it depends entirely on
  			the embedded device tree for all information.
  			  The simpleImage is useful for booting systems with
  			an unknown firmware interface or for booting from
  			a debugger when no firmware is present (such as on
  			the Xilinx Virtex platform).  The only assumption that
  			simpleImage makes is that RAM is correctly initialized
  			and that the MMU is either off or has RAM mapped to
  			base address 0.
  			  simpleImage also supports inserting special platform
  			specific initialization code to the start of the bootup
  			sequence.  The virtex405 platform uses this feature to
  			ensure that the cache is invalidated before caching
  			is enabled.  Platform specific initialization code is
  			added as part of the wrapper script and is keyed on
  			the image target name.  For example, all
  			simpleImage.virtex405-* targets will add the
  			virtex405-head.S initialization code (This also means
  			that the dts file for virtex405 targets should be
  			named (virtex405-<board>.dts).  Search the wrapper
  			script for 'virtex405' and see the file
  			arch/powerpc/boot/virtex405-head.S for details.
     treeImage.%;		Image format for used with OpenBIOS firmware found
  			on some ppc4xx hardware.  This image embeds a device
  			tree blob inside the image.
     uImage:		Native image format used by U-Boot.  The uImage target
  			does not add any boot code.  It just wraps a compressed
  			vmlinux in the uImage data structure.  This image
  			requires a version of U-Boot that is able to pass
  			a device tree to the kernel at boot.  If using an older
  			version of U-Boot, then you need to use a cuImage
  			instead.
     zImage.%:		Image format which does not embed a device tree.
  			Used by OpenFirmware and other firmware interfaces
  			which are able to supply a device tree.  This image
  			expects firmware to provide the device tree at boot.
  			Typically, if you have general purpose PowerPC
  			hardware then you want this image format.
  
  Image types which embed a device tree blob (simpleImage, dtbImage, treeImage,
  and cuImage) all generate the device tree blob from a file in the
  arch/powerpc/boot/dts/ directory.  The Makefile selects the correct device
  tree source based on the name of the target.  Therefore, if the kernel is
  built with 'make treeImage.walnut simpleImage.virtex405-ml403', then the
  build system will use arch/powerpc/boot/dts/walnut.dts to build
  treeImage.walnut and arch/powerpc/boot/dts/virtex405-ml403.dts to build
  the simpleImage.virtex405-ml403.
  
  Two special targets called 'zImage' and 'zImage.initrd' also exist.  These
  targets build all the default images as selected by the kernel configuration.
  Default images are selected by the boot wrapper Makefile
  (arch/powerpc/boot/Makefile) by adding targets to the $image-y variable.  Look
  at the Makefile to see which default image targets are available.
  
  How it is built
  ---------------
  arch/powerpc is designed to support multiplatform kernels, which means
  that a single vmlinux image can be booted on many different target boards.
  It also means that the boot wrapper must be able to wrap for many kinds of
  images on a single build.  The design decision was made to not use any
  conditional compilation code (#ifdef, etc) in the boot wrapper source code.
  All of the boot wrapper pieces are buildable at any time regardless of the
  kernel configuration.  Building all the wrapper bits on every kernel build
  also ensures that obscure parts of the wrapper are at the very least compile
  tested in a large variety of environments.
  
  The wrapper is adapted for different image types at link time by linking in
  just the wrapper bits that are appropriate for the image type.  The 'wrapper
  script' (found in arch/powerpc/boot/wrapper) is called by the Makefile and
  is responsible for selecting the correct wrapper bits for the image type.
  The arguments are well documented in the script's comment block, so they
  are not repeated here.  However, it is worth mentioning that the script
  uses the -p (platform) argument as the main method of deciding which wrapper
  bits to compile in.  Look for the large 'case "$platform" in' block in the
  middle of the script.  This is also the place where platform specific fixups
  can be selected by changing the link order.
  
  In particular, care should be taken when working with cuImages.  cuImage
  wrapper bits are very board specific and care should be taken to make sure
  the target you are trying to build is supported by the wrapper bits.