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