|The PowerPC boot wrapper
|Copyright (C) Secret Lab Technologies Ltd.
|PowerPC image targets compresses and wraps the kernel image (vmlinux) with
|a boot wrapper to make it usable by the system firmware. There is no
|standard PowerPC firmware interface, so the boot wrapper is designed to
|be adaptable for each kind of image that needs to be built.
|The boot wrapper can be found in the arch/powerpc/boot/ directory. The
|Makefile in that directory has targets for all the available image types.
|The different image types are used to support all of the various firmware
|interfaces found on PowerPC platforms. OpenFirmware is the most commonly
|used firmware type on general purpose PowerPC systems from Apple, IBM and
|others. U-Boot is typically found on embedded PowerPC hardware, but there
|are a handful of other firmware implementations which are also popular. Each
|firmware interface requires a different image format.
|The boot wrapper is built from the makefile in arch/powerpc/boot/Makefile and
|it uses the wrapper script (arch/powerpc/boot/wrapper) to generate target
|image. The details of the build system is discussed in the next section.
|Currently, the following image format targets exist:
| cuImage.%: Backwards compatible uImage for older version of
| U-Boot (for versions that don't understand the device
| tree). This image embeds a device tree blob inside
| the image. The boot wrapper, kernel and device tree
| are all embedded inside the U-Boot uImage file format
| with boot wrapper code that extracts data from the old
| bd_info structure and loads the data into the device
| tree before jumping into the kernel.
| Because of the series of #ifdefs found in the
| bd_info structure used in the old U-Boot interfaces,
| cuImages are platform specific. Each specific
| U-Boot platform has a different platform init file
| which populates the embedded device tree with data
| from the platform specific bd_info file. The platform
| specific cuImage platform init code can be found in
| arch/powerpc/boot/cuboot.*.c. Selection of the correct
| cuImage init code for a specific board can be found in
| the wrapper structure.
| dtbImage.%: Similar to zImage, except device tree blob is embedded
| inside the image instead of provided by firmware. The
| output image file can be either an elf file or a flat
| binary depending on the platform.
| dtbImages are used on systems which do not have an
| interface for passing a device tree directly.
| dtbImages are similar to simpleImages except that
| dtbImages have platform specific code for extracting
| data from the board firmware, but simpleImages do not
| talk to the firmware at all.
| PlayStation 3 support uses dtbImage. So do Embedded
| Planet boards using the PlanetCore firmware. Board
| specific initialization code is typically found in a
| file named arch/powerpc/boot/<platform>.c; but this
| can be overridden by the wrapper script.
| simpleImage.%: Firmware independent compressed image that does not
| depend on any particular firmware interface and embeds
| a device tree blob. This image is a flat binary that
| can be loaded to any location in RAM and jumped to.
| Firmware cannot pass any configuration data to the
| kernel with this image type and it depends entirely on
| the embedded device tree for all information.
| The simpleImage is useful for booting systems with
| an unknown firmware interface or for booting from
| a debugger when no firmware is present (such as on
| the Xilinx Virtex platform). The only assumption that
| simpleImage makes is that RAM is correctly initialized
| and that the MMU is either off or has RAM mapped to
| base address 0.
| simpleImage also supports inserting special platform
| specific initialization code to the start of the bootup
| sequence. The virtex405 platform uses this feature to
| ensure that the cache is invalidated before caching
| is enabled. Platform specific initialization code is
| added as part of the wrapper script and is keyed on
| the image target name. For example, all
| simpleImage.virtex405-* targets will add the
| virtex405-head.S initialization code (This also means
| that the dts file for virtex405 targets should be
| named (virtex405-<board>.dts). Search the wrapper
| script for 'virtex405' and see the file
| arch/powerpc/boot/virtex405-head.S for details.
| treeImage.%; Image format for used with OpenBIOS firmware found
| on some ppc4xx hardware. This image embeds a device
| tree blob inside the image.
| uImage: Native image format used by U-Boot. The uImage target
| does not add any boot code. It just wraps a compressed
| vmlinux in the uImage data structure. This image
| requires a version of U-Boot that is able to pass
| a device tree to the kernel at boot. If using an older
| version of U-Boot, then you need to use a cuImage
| zImage.%: Image format which does not embed a device tree.
| Used by OpenFirmware and other firmware interfaces
| which are able to supply a device tree. This image
| expects firmware to provide the device tree at boot.
| Typically, if you have general purpose PowerPC
| hardware then you want this image format.
|Image types which embed a device tree blob (simpleImage, dtbImage, treeImage,
|and cuImage) all generate the device tree blob from a file in the
|arch/powerpc/boot/dts/ directory. The Makefile selects the correct device
|tree source based on the name of the target. Therefore, if the kernel is
|built with 'make treeImage.walnut simpleImage.virtex405-ml403', then the
|build system will use arch/powerpc/boot/dts/walnut.dts to build
|treeImage.walnut and arch/powerpc/boot/dts/virtex405-ml403.dts to build
|Two special targets called 'zImage' and 'zImage.initrd' also exist. These
|targets build all the default images as selected by the kernel configuration.
|Default images are selected by the boot wrapper Makefile
|(arch/powerpc/boot/Makefile) by adding targets to the $image-y variable. Look
|at the Makefile to see which default image targets are available.
|How it is built
|arch/powerpc is designed to support multiplatform kernels, which means
|that a single vmlinux image can be booted on many different target boards.
|It also means that the boot wrapper must be able to wrap for many kinds of
|images on a single build. The design decision was made to not use any
|conditional compilation code (#ifdef, etc) in the boot wrapper source code.
|All of the boot wrapper pieces are buildable at any time regardless of the
|kernel configuration. Building all the wrapper bits on every kernel build
|also ensures that obscure parts of the wrapper are at the very least compile
|tested in a large variety of environments.
|The wrapper is adapted for different image types at link time by linking in
|just the wrapper bits that are appropriate for the image type. The 'wrapper
|script' (found in arch/powerpc/boot/wrapper) is called by the Makefile and
|is responsible for selecting the correct wrapper bits for the image type.
|The arguments are well documented in the script's comment block, so they
|are not repeated here. However, it is worth mentioning that the script
|uses the -p (platform) argument as the main method of deciding which wrapper
|bits to compile in. Look for the large 'case "$platform" in' block in the
|middle of the script. This is also the place where platform specific fixups
|can be selected by changing the link order.
|In particular, care should be taken when working with cuImages. cuImage
|wrapper bits are very board specific and care should be taken to make sure
|the target you are trying to build is supported by the wrapper bits.