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269 lines
12 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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Booting Linux on x86 with FIT
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=============================
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Background
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----------
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Generally Linux x86 uses its own very complex booting method. There is a setup
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binary which contains all sorts of parameters and a compressed self-extracting
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binary for the kernel itself, often with a small built-in serial driver to
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display decompression progress.
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The x86 CPU has various processor modes. I am no expert on these, but my
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understanding is that an x86 CPU (even a really new one) starts up in a 16-bit
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'real' mode where only 1MB of memory is visible, moves to 32-bit 'protected'
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mode where 4GB is visible (or more with special memory access techniques) and
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then to 64-bit 'long' mode if 64-bit execution is required.
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Partly the self-extracting nature of Linux was introduced to cope with boot
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loaders that were barely capable of loading anything. Even changing to 32-bit
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mode was something of a challenge, so putting this logic in the kernel seemed
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to make sense.
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Bit by bit more and more logic has been added to this post-boot pre-Linux
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wrapper:
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- Changing to 32-bit mode
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- Decompression
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- Serial output (with drivers for various chips)
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- Load address randomisation
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- Elf loader complete with relocation (for the above)
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- Random number generator via 3 methods (again for the above)
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- Some sort of EFI mini-loader (1000+ glorious lines of code)
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- Locating and tacking on a device tree and ramdisk
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To my mind, if you sit back and look at things from first principles, this
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doesn't make a huge amount of sense. Any boot loader worth its salts already
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has most of the above features and more besides. The boot loader already knows
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the layout of memory, has a serial driver, can decompress things, includes an
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ELF loader and supports device tree and ramdisks. The decision to duplicate
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all these features in a Linux wrapper caters for the lowest common
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denominator: a boot loader which consists of a BIOS call to load something off
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disk, followed by a jmp instruction.
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(Aside: On ARM systems, we worry that the boot loader won't know where to load
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the kernel. It might be easier to just provide that information in the image,
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or in the boot loader rather than adding a self-relocator to put it in the
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right place. Or just use ELF?
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As a result, the x86 kernel boot process is needlessly complex. The file
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format is also complex, and obfuscates the contents to a degree that it is
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quite a challenge to extract anything from it. This bzImage format has become
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so prevalent that is actually isn't possible to produce the 'raw' kernel build
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outputs with the standard Makefile (as it is on ARM for example, at least at
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the time of writing).
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This document describes an alternative boot process which uses simple raw
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images which are loaded into the right place by the boot loader and then
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executed.
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Build the kernel
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----------------
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Note: these instructions assume a 32-bit kernel. U-Boot also supports directly
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booting a 64-bit kernel by jumping into 64-bit mode first (see below).
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You can build the kernel as normal with 'make'. This will create a file called
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'vmlinux'. This is a standard ELF file and you can look at it if you like::
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$ objdump -h vmlinux
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vmlinux: file format elf32-i386
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Sections:
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Idx Name Size VMA LMA File off Algn
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0 .text 00416850 81000000 01000000 00001000 2**5
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
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1 .notes 00000024 81416850 01416850 00417850 2**2
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CONTENTS, ALLOC, LOAD, READONLY, CODE
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2 __ex_table 00000c50 81416880 01416880 00417880 2**3
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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3 .rodata 00154b9e 81418000 01418000 00419000 2**5
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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4 __bug_table 0000597c 8156cba0 0156cba0 0056dba0 2**0
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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5 .pci_fixup 00001b80 8157251c 0157251c 0057351c 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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6 .tracedata 00000024 8157409c 0157409c 0057509c 2**0
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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7 __ksymtab 00007ec0 815740c0 015740c0 005750c0 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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8 __ksymtab_gpl 00004a28 8157bf80 0157bf80 0057cf80 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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9 __ksymtab_strings 0001d6fc 815809a8 015809a8 005819a8 2**0
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CONTENTS, ALLOC, LOAD, READONLY, DATA
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10 __init_rodata 00001c3c 8159e0a4 0159e0a4 0059f0a4 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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11 __param 00000ff0 8159fce0 0159fce0 005a0ce0 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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12 __modver 00000330 815a0cd0 015a0cd0 005a1cd0 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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13 .data 00063000 815a1000 015a1000 005a2000 2**12
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CONTENTS, ALLOC, LOAD, RELOC, DATA
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14 .init.text 0002f104 81604000 01604000 00605000 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
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15 .init.data 00040cdc 81634000 01634000 00635000 2**12
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CONTENTS, ALLOC, LOAD, RELOC, DATA
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16 .x86_cpu_dev.init 0000001c 81674cdc 01674cdc 00675cdc 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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17 .altinstructions 0000267c 81674cf8 01674cf8 00675cf8 2**0
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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18 .altinstr_replacement 00000942 81677374 01677374 00678374 2**0
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CONTENTS, ALLOC, LOAD, READONLY, CODE
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19 .iommu_table 00000014 81677cb8 01677cb8 00678cb8 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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20 .apicdrivers 00000004 81677cd0 01677cd0 00678cd0 2**2
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CONTENTS, ALLOC, LOAD, RELOC, DATA
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21 .exit.text 00001a80 81677cd8 01677cd8 00678cd8 2**0
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
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22 .data..percpu 00007880 8167a000 0167a000 0067b000 2**12
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CONTENTS, ALLOC, LOAD, RELOC, DATA
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23 .smp_locks 00003000 81682000 01682000 00683000 2**2
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CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
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24 .bss 000a1000 81685000 01685000 00686000 2**12
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ALLOC
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25 .brk 00424000 81726000 01726000 00686000 2**0
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ALLOC
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26 .comment 00000049 00000000 00000000 00686000 2**0
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CONTENTS, READONLY
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27 .GCC.command.line 0003e055 00000000 00000000 00686049 2**0
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CONTENTS, READONLY
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28 .debug_aranges 0000f4c8 00000000 00000000 006c40a0 2**3
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CONTENTS, RELOC, READONLY, DEBUGGING
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29 .debug_info 0440b0df 00000000 00000000 006d3568 2**0
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CONTENTS, RELOC, READONLY, DEBUGGING
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30 .debug_abbrev 0022a83b 00000000 00000000 04ade647 2**0
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CONTENTS, READONLY, DEBUGGING
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31 .debug_line 004ead0d 00000000 00000000 04d08e82 2**0
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CONTENTS, RELOC, READONLY, DEBUGGING
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32 .debug_frame 0010a960 00000000 00000000 051f3b90 2**2
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CONTENTS, RELOC, READONLY, DEBUGGING
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33 .debug_str 001b442d 00000000 00000000 052fe4f0 2**0
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CONTENTS, READONLY, DEBUGGING
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34 .debug_loc 007c7fa9 00000000 00000000 054b291d 2**0
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CONTENTS, RELOC, READONLY, DEBUGGING
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35 .debug_ranges 00098828 00000000 00000000 05c7a8c8 2**3
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CONTENTS, RELOC, READONLY, DEBUGGING
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There is also the setup binary mentioned earlier. This is at
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arch/x86/boot/setup.bin and is about 12KB in size. It includes the command
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line and various settings need by the kernel. Arguably the boot loader should
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provide all of this also, but setting it up is some complex that the kernel
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helps by providing a head start.
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As you can see the code loads to address 0x01000000 and everything else
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follows after that. We could load this image using the 'bootelf' command but
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we would still need to provide the setup binary. This is not supported by
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U-Boot although I suppose you could mostly script it. This would permit the
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use of a relocatable kernel.
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All we need to boot is the vmlinux file and the setup.bin file.
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Create a FIT
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------------
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To create a FIT you will need a source file describing what should go in the
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FIT. See kernel.its for an example for x86 and also instructions on setting
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the 'arch' value for booting 64-bit kernels if desired. Put this into a file
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called image.its.
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Note that setup is loaded to the special address of 0x90000 (a special address
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you just have to know) and the kernel is loaded to 0x01000000 (the address you
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saw above). This means that you will need to load your FIT to a different
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address so that U-Boot doesn't overwrite it when decompressing. Something like
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0x02000000 will do so you can set CONFIG_SYS_LOAD_ADDR to that.
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In that example the kernel is compressed with lzo. Also we need to provide a
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flat binary, not an ELF. So the steps needed to set things are are::
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# Create a flat binary
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objcopy -O binary vmlinux vmlinux.bin
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# Compress it into LZO format
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lzop vmlinux.bin
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# Build a FIT image
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mkimage -f image.its image.fit
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(be careful to run the mkimage from your U-Boot tools directory since it
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will have x86_setup support.)
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You can take a look at the resulting fit file if you like::
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$ dumpimage -l image.fit
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FIT description: Simple image with single Linux kernel on x86
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Created: Tue Oct 7 10:57:24 2014
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Image 0 (kernel)
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Description: Vanilla Linux kernel
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Created: Tue Oct 7 10:57:24 2014
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Type: Kernel Image
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Compression: lzo compressed
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Data Size: 4591767 Bytes = 4484.15 kB = 4.38 MB
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Architecture: Intel x86
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OS: Linux
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Load Address: 0x01000000
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Entry Point: 0x00000000
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Hash algo: sha1
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Hash value: 446b5163ebfe0fb6ee20cbb7a8501b263cd92392
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Image 1 (setup)
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Description: Linux setup.bin
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Created: Tue Oct 7 10:57:24 2014
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Type: x86 setup.bin
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Compression: uncompressed
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Data Size: 12912 Bytes = 12.61 kB = 0.01 MB
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Hash algo: sha1
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Hash value: a1f2099cf47ff9816236cd534c77af86e713faad
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Default Configuration: 'config-1'
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Configuration 0 (config-1)
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Description: Boot Linux kernel
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Kernel: kernel
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Booting the FIT
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---------------
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To make it boot you need to load it and then use 'bootm' to boot it. A
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suitable script to do this from a network server is::
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bootp
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tftp image.fit
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bootm
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This will load the image from the network and boot it. The command line (from
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the 'bootargs' environment variable) will be passed to the kernel.
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If you want a ramdisk you can add it as normal with FIT. If you want a device
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tree then x86 doesn't normally use those - it has ACPI instead.
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Why Bother?
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-----------
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#. It demystifies the process of booting an x86 kernel
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#. It allows use of the standard U-Boot boot file format
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#. It allows U-Boot to perform decompression - problems will provide an error
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message and you are still in the boot loader. It is possible to investigate.
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#. It avoids all the pre-loader code in the kernel which is quite complex to
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follow
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#. You can use verified/secure boot and other features which haven't yet been
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added to the pre-Linux
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#. It makes x86 more like other architectures in the way it boots a kernel.
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You can potentially use the same file format for the kernel, and the same
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procedure for building and packaging it.
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References
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----------
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In the Linux kernel, Documentation/x86/boot.txt defines the boot protocol for
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the kernel including the setup.bin format. This is handled in U-Boot in
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arch/x86/lib/zimage.c and arch/x86/lib/bootm.c.
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Various files in the same directory as this file describe the FIT format.
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.. sectionauthor:: Simon Glass <sjg@chromium.org> 7-Oct-2014
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