src/kernel/linux/v4.19/Documentation/powerpc/bootwrapper.txt

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.