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.
132