Documentation/arch/powerpc/bootwrapper.rst

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========================
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

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