linux/Documentation/powerpc/bootwrapper.txt
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   1The PowerPC boot wrapper
   2------------------------
   3Copyright (C) Secret Lab Technologies Ltd.
   4
   5PowerPC image targets compresses and wraps the kernel image (vmlinux) with
   6a boot wrapper to make it usable by the system firmware.  There is no
   7standard PowerPC firmware interface, so the boot wrapper is designed to
   8be adaptable for each kind of image that needs to be built.
   9
  10The boot wrapper can be found in the arch/powerpc/boot/ directory.  The
  11Makefile in that directory has targets for all the available image types.
  12The different image types are used to support all of the various firmware
  13interfaces found on PowerPC platforms.  OpenFirmware is the most commonly
  14used firmware type on general purpose PowerPC systems from Apple, IBM and
  15others.  U-Boot is typically found on embedded PowerPC hardware, but there
  16are a handful of other firmware implementations which are also popular.  Each
  17firmware interface requires a different image format.
  18
  19The boot wrapper is built from the makefile in arch/powerpc/boot/Makefile and
  20it uses the wrapper script (arch/powerpc/boot/wrapper) to generate target
  21image.  The details of the build system is discussed in the next section.
  22Currently, 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
 101Image types which embed a device tree blob (simpleImage, dtbImage, treeImage,
 102and cuImage) all generate the device tree blob from a file in the
 103arch/powerpc/boot/dts/ directory.  The Makefile selects the correct device
 104tree source based on the name of the target.  Therefore, if the kernel is
 105built with 'make treeImage.walnut simpleImage.virtex405-ml403', then the
 106build system will use arch/powerpc/boot/dts/walnut.dts to build
 107treeImage.walnut and arch/powerpc/boot/dts/virtex405-ml403.dts to build
 108the simpleImage.virtex405-ml403.
 109
 110Two special targets called 'zImage' and 'zImage.initrd' also exist.  These
 111targets build all the default images as selected by the kernel configuration.
 112Default images are selected by the boot wrapper Makefile
 113(arch/powerpc/boot/Makefile) by adding targets to the $image-y variable.  Look
 114at the Makefile to see which default image targets are available.
 115
 116How it is built
 117---------------
 118arch/powerpc is designed to support multiplatform kernels, which means
 119that a single vmlinux image can be booted on many different target boards.
 120It also means that the boot wrapper must be able to wrap for many kinds of
 121images on a single build.  The design decision was made to not use any
 122conditional compilation code (#ifdef, etc) in the boot wrapper source code.
 123All of the boot wrapper pieces are buildable at any time regardless of the
 124kernel configuration.  Building all the wrapper bits on every kernel build
 125also ensures that obscure parts of the wrapper are at the very least compile
 126tested in a large variety of environments.
 127
 128The wrapper is adapted for different image types at link time by linking in
 129just the wrapper bits that are appropriate for the image type.  The 'wrapper
 130script' (found in arch/powerpc/boot/wrapper) is called by the Makefile and
 131is responsible for selecting the correct wrapper bits for the image type.
 132The arguments are well documented in the script's comment block, so they
 133are not repeated here.  However, it is worth mentioning that the script
 134uses the -p (platform) argument as the main method of deciding which wrapper
 135bits to compile in.  Look for the large 'case "$platform" in' block in the
 136middle of the script.  This is also the place where platform specific fixups
 137can be selected by changing the link order.
 138
 139In particular, care should be taken when working with cuImages.  cuImage
 140wrapper bits are very board specific and care should be taken to make sure
 141the target you are trying to build is supported by the wrapper bits.
 142
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