Documentation/powerpc/bootwrapper.txt

   1 The PowerPC boot wrapper
   2 ------------------------
   3 Copyright (C) Secret Lab Technologies Ltd.
   4
   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.
   9
  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.
  18
  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:
  23
  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.
 100
 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.
 109
 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 $image-y variable.  Look
 114 at the Makefile to see which default image targets are available.
 115
 116 How it is built
 117 ---------------
 118 arch/powerpc is designed to support multiplatform kernels, which means
 119 that a single vmlinux image can be booted on many different target boards.
 120 It also means that the boot wrapper must be able to wrap for many kinds of
 121 images on a single build.  The design decision was made to not use any
 122 conditional compilation code (#ifdef, etc) in the boot wrapper source code.
 123 All of the boot wrapper pieces are buildable at any time regardless of the
 124 kernel configuration.  Building all the wrapper bits on every kernel build
 125 also ensures that obscure parts of the wrapper are at the very least compile
 126 tested in a large variety of environments.
 127
 128 The wrapper is adapted for different image types at link time by linking in
 129 just the wrapper bits that are appropriate for the image type.  The 'wrapper
 130 script' (found in arch/powerpc/boot/wrapper) is called by the Makefile and
 131 is responsible for selecting the correct wrapper bits for the image type.
 132 The arguments are well documented in the script's comment block, so they
 133 are not repeated here.  However, it is worth mentioning that the script
 134 uses the -p (platform) argument as the main method of deciding which wrapper
 135 bits to compile in.  Look for the large 'case "$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.
 138
 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.