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 The simpleImage is useful for booting systems with 74 an unknown firmware interface or for booting from 75 a debugger when no firmware is present (such as on 76 the Xilinx Virtex platform). The only assumption that 77 simpleImage makes is that RAM is correctly initialized 78 and that the MMU is either off or has RAM mapped to 79 base address 0. 80 81 simpleImage also supports inserting special platform 82 specific initialization code to the start of the bootup 83 sequence. The virtex405 platform uses this feature to 84 ensure that the cache is invalidated before caching 85 is enabled. Platform specific initialization code is 86 added as part of the wrapper script and is keyed on 87 the image target name. For example, all 88 simpleImage.virtex405-* targets will add the 89 virtex405-head.S initialization code (This also means 90 that the dts file for virtex405 targets should be 91 named (virtex405-<board>.dts). Search the wrapper 92 script for 'virtex405' and see the file 93 arch/powerpc/boot/virtex405-head.S for details. 94 95 treeImage.%; Image format for used with OpenBIOS firmware found 96 on some ppc4xx hardware. This image embeds a device 97 tree blob inside the image. 98 99 uImage: Native image format used by U-Boot. The uImage target 100 does not add any boot code. It just wraps a compressed 101 vmlinux in the uImage data structure. This image 102 requires a version of U-Boot that is able to pass 103 a device tree to the kernel at boot. If using an older 104 version of U-Boot, then you need to use a cuImage 105 instead. 106 107 zImage.%: Image format which does not embed a device tree. 108 Used by OpenFirmware and other firmware interfaces 109 which are able to supply a device tree. This image 110 expects firmware to provide the device tree at boot. 111 Typically, if you have general purpose PowerPC 112 hardware then you want this image format. 113 ==================== ======================================================== 114 115Image types which embed a device tree blob (simpleImage, dtbImage, treeImage, 116and cuImage) all generate the device tree blob from a file in the 117arch/powerpc/boot/dts/ directory. The Makefile selects the correct device 118tree source based on the name of the target. Therefore, if the kernel is 119built with 'make treeImage.walnut simpleImage.virtex405-ml403', then the 120build system will use arch/powerpc/boot/dts/walnut.dts to build 121treeImage.walnut and arch/powerpc/boot/dts/virtex405-ml403.dts to build 122the simpleImage.virtex405-ml403. 123 124Two special targets called 'zImage' and 'zImage.initrd' also exist. These 125targets build all the default images as selected by the kernel configuration. 126Default images are selected by the boot wrapper Makefile 127(arch/powerpc/boot/Makefile) by adding targets to the $image-y variable. Look 128at the Makefile to see which default image targets are available. 129 130How it is built 131--------------- 132arch/powerpc is designed to support multiplatform kernels, which means 133that a single vmlinux image can be booted on many different target boards. 134It also means that the boot wrapper must be able to wrap for many kinds of 135images on a single build. The design decision was made to not use any 136conditional compilation code (#ifdef, etc) in the boot wrapper source code. 137All of the boot wrapper pieces are buildable at any time regardless of the 138kernel configuration. Building all the wrapper bits on every kernel build 139also ensures that obscure parts of the wrapper are at the very least compile 140tested in a large variety of environments. 141 142The wrapper is adapted for different image types at link time by linking in 143just the wrapper bits that are appropriate for the image type. The 'wrapper 144script' (found in arch/powerpc/boot/wrapper) is called by the Makefile and 145is responsible for selecting the correct wrapper bits for the image type. 146The arguments are well documented in the script's comment block, so they 147are not repeated here. However, it is worth mentioning that the script 148uses the -p (platform) argument as the main method of deciding which wrapper 149bits to compile in. Look for the large 'case "$platform" in' block in the 150middle of the script. This is also the place where platform specific fixups 151can be selected by changing the link order. 152 153In particular, care should be taken when working with cuImages. cuImage 154wrapper bits are very board specific and care should be taken to make sure 155the target you are trying to build is supported by the wrapper bits. 156
