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1 The PowerPC boot wrapper
2 ------------------------
3 Copyright (C) Secret Lab Technologies Ltd.
5 PowerPC image targets compresses and wraps the kernel image (vmlinux) with
6 a boot wrapper to make it usable by the system firmware. There is no
7 standard PowerPC firmware interface, so the boot wrapper is designed to
8 be adaptable for each kind of image that needs to be built.
10 The boot wrapper can be found in the arch/powerpc/boot/ directory. The
11 Makefile in that directory has targets for all the available image types.
12 The different image types are used to support all of the various firmware
13 interfaces found on PowerPC platforms. OpenFirmware is the most commonly
14 used firmware type on general purpose PowerPC systems from Apple, IBM and
15 others. U-Boot is typically found on embedded PowerPC hardware, but there
16 are a handful of other firmware implementations which are also popular. Each
17 firmware interface requires a different image format.
19 The boot wrapper is built from the makefile in arch/powerpc/boot/Makefile and
20 it uses the wrapper script (arch/powerpc/boot/wrapper) to generate target
21 image. The details of the build system is discussed in the next section.
22 Currently, the following image format targets exist:
24 cuImage.%: Backwards compatible uImage for older version of
25 U-Boot (for versions that don't understand the device
26 tree). This image embeds a device tree blob inside
27 the image. The boot wrapper, kernel and device tree
28 are all embedded inside the U-Boot uImage file format
29 with boot wrapper code that extracts data from the old
30 bd_info structure and loads the data into the device
31 tree before jumping into the kernel.
32 Because of the series of #ifdefs found in the
33 bd_info structure used in the old U-Boot interfaces,
34 cuImages are platform specific. Each specific
35 U-Boot platform has a different platform init file
36 which populates the embedded device tree with data
37 from the platform specific bd_info file. The platform
38 specific cuImage platform init code can be found in
39 arch/powerpc/boot/cuboot.*.c. Selection of the correct
40 cuImage init code for a specific board can be found in
41 the wrapper structure.
42 dtbImage.%: Similar to zImage, except device tree blob is embedded
43 inside the image instead of provided by firmware. The
44 output image file can be either an elf file or a flat
45 binary depending on the platform.
46 dtbImages are used on systems which do not have an
47 interface for passing a device tree directly.
48 dtbImages are similar to simpleImages except that
49 dtbImages have platform specific code for extracting
50 data from the board firmware, but simpleImages do not
51 talk to the firmware at all.
52 PlayStation 3 support uses dtbImage. So do Embedded
53 Planet boards using the PlanetCore firmware. Board
54 specific initialization code is typically found in a
55 file named arch/powerpc/boot/<platform>.c; but this
56 can be overridden by the wrapper script.
57 simpleImage.%: Firmware independent compressed image that does not
58 depend on any particular firmware interface and embeds
59 a device tree blob. This image is a flat binary that
60 can be loaded to any location in RAM and jumped to.
61 Firmware cannot pass any configuration data to the
62 kernel with this image type and it depends entirely on
63 the embedded device tree for all information.
64 The simpleImage is useful for booting systems with
65 an unknown firmware interface or for booting from
66 a debugger when no firmware is present (such as on
67 the Xilinx Virtex platform). The only assumption that
68 simpleImage makes is that RAM is correctly initialized
69 and that the MMU is either off or has RAM mapped to
70 base address 0.
71 simpleImage also supports inserting special platform
72 specific initialization code to the start of the bootup
73 sequence. The virtex405 platform uses this feature to
74 ensure that the cache is invalidated before caching
75 is enabled. Platform specific initialization code is
76 added as part of the wrapper script and is keyed on
77 the image target name. For example, all
78 simpleImage.virtex405-* targets will add the
79 virtex405-head.S initialization code (This also means
80 that the dts file for virtex405 targets should be
81 named (virtex405-<board>.dts). Search the wrapper
82 script for 'virtex405' and see the file
83 arch/powerpc/boot/virtex405-head.S for details.
84 treeImage.%; Image format for used with OpenBIOS firmware found
85 on some ppc4xx hardware. This image embeds a device
86 tree blob inside the image.
87 uImage: Native image format used by U-Boot. The uImage target
88 does not add any boot code. It just wraps a compressed
89 vmlinux in the uImage data structure. This image
90 requires a version of U-Boot that is able to pass
91 a device tree to the kernel at boot. If using an older
92 version of U-Boot, then you need to use a cuImage
93 instead.
94 zImage.%: Image format which does not embed a device tree.
95 Used by OpenFirmware and other firmware interfaces
96 which are able to supply a device tree. This image
97 expects firmware to provide the device tree at boot.
98 Typically, if you have general purpose PowerPC
99 hardware then you want this image format.
101 Image types which embed a device tree blob (simpleImage, dtbImage, treeImage,
102 and cuImage) all generate the device tree blob from a file in the
103 arch/powerpc/boot/dts/ directory. The Makefile selects the correct device
104 tree source based on the name of the target. Therefore, if the kernel is
105 built with 'make treeImage.walnut simpleImage.virtex405-ml403', then the
106 build system will use arch/powerpc/boot/dts/walnut.dts to build
107 treeImage.walnut and arch/powerpc/boot/dts/virtex405-ml403.dts to build
108 the simpleImage.virtex405-ml403.
110 Two special targets called 'zImage' and 'zImage.initrd' also exist. These
111 targets build all the default images as selected by the kernel configuration.
112 Default images are selected by the boot wrapper Makefile
113 (arch/powerpc/boot/Makefile) by adding targets to the $image-y variable. Look
114 at the Makefile to see which default image targets are available.
116 How it is built
117 ---------------
118 arch/powerpc is designed to support multiplatform kernels, which means
119 that a single vmlinux image can be booted on many different target boards.
120 It also means that the boot wrapper must be able to wrap for many kinds of
121 images on a single build. The design decision was made to not use any
122 conditional compilation code (#ifdef, etc) in the boot wrapper source code.
123 All of the boot wrapper pieces are buildable at any time regardless of the
124 kernel configuration. Building all the wrapper bits on every kernel build
125 also ensures that obscure parts of the wrapper are at the very least compile
126 tested in a large variety of environments.
128 The wrapper is adapted for different image types at link time by linking in
129 just the wrapper bits that are appropriate for the image type. The 'wrapper
130 script' (found in arch/powerpc/boot/wrapper) is called by the Makefile and
131 is responsible for selecting the correct wrapper bits for the image type.
132 The arguments are well documented in the script's comment block, so they
133 are not repeated here. However, it is worth mentioning that the script
134 uses the -p (platform) argument as the main method of deciding which wrapper
135 bits to compile in. Look for the large 'case "$platform" in' block in the
136 middle of the script. This is also the place where platform specific fixups
137 can be selected by changing the link order.
139 In particular, care should be taken when working with cuImages. cuImage
140 wrapper bits are very board specific and care should be taken to make sure
141 the target you are trying to build is supported by the wrapper bits.