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