Documentation/powerpc/bootwrapper.rst - linux/kernel/git/paulmck/linux-rcu - Git at Google

 ========================
 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.

    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', then the build system will use
 arch/powerpc/boot/dts/walnut.dts to build treeImage.walnut.

 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.
	========================
	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.

	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', then the build system will use
	arch/powerpc/boot/dts/walnut.dts to build treeImage.walnut.

	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.