bootwrapper.rst

  1========================
  2The PowerPC boot wrapper
  3========================
  4
  5Copyright (C) Secret Lab Technologies Ltd.
  6
  7PowerPC image targets compresses and wraps the kernel image (vmlinux) with
  8a boot wrapper to make it usable by the system firmware.  There is no
  9standard PowerPC firmware interface, so the boot wrapper is designed to
 10be adaptable for each kind of image that needs to be built.
 11
 12The boot wrapper can be found in the arch/powerpc/boot/ directory.  The
 13Makefile in that directory has targets for all the available image types.
 14The different image types are used to support all of the various firmware
 15interfaces found on PowerPC platforms.  OpenFirmware is the most commonly
 16used firmware type on general purpose PowerPC systems from Apple, IBM and
 17others.  U-Boot is typically found on embedded PowerPC hardware, but there
 18are a handful of other firmware implementations which are also popular.  Each
 19firmware interface requires a different image format.
 20
 21The boot wrapper is built from the makefile in arch/powerpc/boot/Makefile and
 22it uses the wrapper script (arch/powerpc/boot/wrapper) to generate target
 23image.  The details of the build system is discussed in the next section.
 24Currently, the following image format targets exist:
 25
 26   ==================== ========================================================
 27   cuImage.%:		Backwards compatible uImage for older version of
 28			U-Boot (for versions that don't understand the device
 29			tree).  This image embeds a device tree blob inside
 30			the image.  The boot wrapper, kernel and device tree
 31			are all embedded inside the U-Boot uImage file format
 32			with boot wrapper code that extracts data from the old
 33			bd_info structure and loads the data into the device
 34			tree before jumping into the kernel.
 35
 36			Because of the series of #ifdefs found in the
 37			bd_info structure used in the old U-Boot interfaces,
 38			cuImages are platform specific.  Each specific
 39			U-Boot platform has a different platform init file
 40			which populates the embedded device tree with data
 41			from the platform specific bd_info file.  The platform
 42			specific cuImage platform init code can be found in
 43			`arch/powerpc/boot/cuboot.*.c`. Selection of the correct
 44			cuImage init code for a specific board can be found in
 45			the wrapper structure.
 46
 47   dtbImage.%:		Similar to zImage, except device tree blob is embedded
 48			inside the image instead of provided by firmware.  The
 49			output image file can be either an elf file or a flat
 50			binary depending on the platform.
 51
 52			dtbImages are used on systems which do not have an
 53			interface for passing a device tree directly.
 54			dtbImages are similar to simpleImages except that
 55			dtbImages have platform specific code for extracting
 56			data from the board firmware, but simpleImages do not
 57			talk to the firmware at all.
 58
 59			PlayStation 3 support uses dtbImage.  So do Embedded
 60			Planet boards using the PlanetCore firmware.  Board
 61			specific initialization code is typically found in a
 62			file named arch/powerpc/boot/<platform>.c; but this
 63			can be overridden by the wrapper script.
 64
 65   simpleImage.%:	Firmware independent compressed image that does not
 66			depend on any particular firmware interface and embeds
 67			a device tree blob.  This image is a flat binary that
 68			can be loaded to any location in RAM and jumped to.
 69			Firmware cannot pass any configuration data to the
 70			kernel with this image type and it depends entirely on
 71			the embedded device tree for all information.
 72
 73   treeImage.%;		Image format for used with OpenBIOS firmware found
 74			on some ppc4xx hardware.  This image embeds a device
 75			tree blob inside the image.
 76
 77   uImage:		Native image format used by U-Boot.  The uImage target
 78			does not add any boot code.  It just wraps a compressed
 79			vmlinux in the uImage data structure.  This image
 80			requires a version of U-Boot that is able to pass
 81			a device tree to the kernel at boot.  If using an older
 82			version of U-Boot, then you need to use a cuImage
 83			instead.
 84
 85   zImage.%:		Image format which does not embed a device tree.
 86			Used by OpenFirmware and other firmware interfaces
 87			which are able to supply a device tree.  This image
 88			expects firmware to provide the device tree at boot.
 89			Typically, if you have general purpose PowerPC
 90			hardware then you want this image format.
 91   ==================== ========================================================
 92
 93Image types which embed a device tree blob (simpleImage, dtbImage, treeImage,
 94and cuImage) all generate the device tree blob from a file in the
 95arch/powerpc/boot/dts/ directory.  The Makefile selects the correct device
 96tree source based on the name of the target.  Therefore, if the kernel is
 97built with 'make treeImage.walnut', then the build system will use
 98arch/powerpc/boot/dts/walnut.dts to build treeImage.walnut.
 99
100Two special targets called 'zImage' and 'zImage.initrd' also exist.  These
101targets build all the default images as selected by the kernel configuration.
102Default images are selected by the boot wrapper Makefile
103(arch/powerpc/boot/Makefile) by adding targets to the $image-y variable.  Look
104at the Makefile to see which default image targets are available.
105
106How it is built
107---------------
108arch/powerpc is designed to support multiplatform kernels, which means
109that a single vmlinux image can be booted on many different target boards.
110It also means that the boot wrapper must be able to wrap for many kinds of
111images on a single build.  The design decision was made to not use any
112conditional compilation code (#ifdef, etc) in the boot wrapper source code.
113All of the boot wrapper pieces are buildable at any time regardless of the
114kernel configuration.  Building all the wrapper bits on every kernel build
115also ensures that obscure parts of the wrapper are at the very least compile
116tested in a large variety of environments.
117
118The wrapper is adapted for different image types at link time by linking in
119just the wrapper bits that are appropriate for the image type.  The 'wrapper
120script' (found in arch/powerpc/boot/wrapper) is called by the Makefile and
121is responsible for selecting the correct wrapper bits for the image type.
122The arguments are well documented in the script's comment block, so they
123are not repeated here.  However, it is worth mentioning that the script
124uses the -p (platform) argument as the main method of deciding which wrapper
125bits to compile in.  Look for the large 'case "$platform" in' block in the
126middle of the script.  This is also the place where platform specific fixups
127can be selected by changing the link order.
128
129In particular, care should be taken when working with cuImages.  cuImage
130wrapper bits are very board specific and care should be taken to make sure
131the target you are trying to build is supported by the wrapper bits.