Running U-Boot from coreboot on Chromebooks
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===========================================
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U-Boot can be used as a secondary boot loader in a few situations such as from
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UEFI and coreboot (see README.x86). Recent Chromebooks use coreboot even on
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ARM platforms to start up the machine.
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This document aims to provide a guide to booting U-Boot on a Chromebook. It
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is only a starting point, and there are many guides on the interwebs. But
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placing this information in the U-Boot tree should make it easier to find for
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those who use U-Boot habitually.
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Most of these platforms are supported by U-Boot natively, but it is risky to
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replace the ROM unless you have a servo board and cable to restore it with.
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For all of these the standard U-Boot build instructions apply. For example on
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ARM:
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sudo apt install gcc-arm-linux-gnueabi
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mkdir b
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make O=b/nyan_big CROSS_COMPILE=arm-linux-gnueabi- nyan-big_defconfig all
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You can obtain the vbutil_kernel utility here:
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https://drive.google.com/open?id=0B7WYZbZ9zd-3dHlVVXo4VXE2T0U
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Snow (Samsung ARM Chromebook)
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-----------------------------
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See here:
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https://www.chromium.org/chromium-os/firmware-porting-guide/using-nv-u-boot-on-the-samsung-arm-chromebook
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Nyan-big
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--------
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Compiled based on information here:
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https://lists.denx.de/pipermail/u-boot/2015-March/209530.html
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https://git.collabora.com/cgit/user/tomeu/u-boot.git/commit/?h=nyan-big
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https://lists.denx.de/pipermail/u-boot/2017-May/289491.html
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https://github.com/chromeos-nvidia-androidtv/gnu-linux-on-acer-chromebook-13#copy-data-to-the-sd-card
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1. Patch U-Boot
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Open include/configs/tegra124-common.h
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Change:
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#define CONFIG_SYS_TEXT_BASE 0x80110000
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to:
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#define CONFIG_SYS_TEXT_BASE 0x81000100
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2. Build U-Boot
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mkdir b
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make -j8 O=b/nyan-big CROSS_COMPILE=arm-linux-gnueabi- nyan-big_defconfig all
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3. Select a .its file
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Select something from doc/chromium which matches your board, or create your
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own.
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Note that the device tree node is required, even though it is not actually
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used by U-Boot. This is because the Chromebook expects to pass it to the
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kernel, and crashes if it is not present.
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4. Build and sign an image
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./b/nyan-big/tools/mkimage -f doc/chromium/nyan-big.its u-boot-chromium.fit
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echo test >dummy.txt
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vbutil_kernel --arch arm --keyblock doc/chromium/devkeys/kernel.keyblock \
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--signprivate doc/chromium/devkeys/kernel_data_key.vbprivk \
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--version 1 --config dummy.txt --vmlinuz u-boot-chromium.fit \
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--bootloader dummy.txt --pack u-boot.kpart
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5. Prepare an SD card
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DISK=/dev/sdc # Replace with your actual SD card device
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sudo cgpt create $DISK
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sudo cgpt add -b 34 -s 32768 -P 1 -S 1 -t kernel $DISK
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sudo cgpt add -b 32802 -s 2000000 -t rootfs $DISK
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sudo gdisk $DISK # Enter command 'w' to write a protective MBR to the disk
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6. Write U-Boot to the SD card
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sudo dd if=u-boot.kpart of=/dev/sdc1; sync
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7. Start it up
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Reboot the device in dev mode. Make sure that you have USB booting enabled. To
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do this, login as root (via Ctrl-Alt-forward_arrow) and type
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'enable_dev_usb_boot'. You only need to do this once.
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Reboot the device with the SD card inserted. Press Clrl-U at the developer
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mode screen. It should show something like the following on the display:
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U-Boot 2017.07-00637-g242eb42-dirty (May 22 2017 - 06:14:21 -0600)
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Model: Acer Chromebook 13 CB5-311
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Board: Google/NVIDIA Nyan-big, ID: 1
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Net: No ethernet found.
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Hit any key to stop autoboot: 0
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Tegra124 (Nyan-big) #
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8. Known problems
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On the serial console the word MMC is chopped at the start of the line:
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C: sdhci@700b0000: 2, sdhci@700b0400: 1, sdhci@700b0600: 0
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This is likely due to some problem with change-over of the serial driver
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during relocation (or perhaps updating the clock setup in board_init()).
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9. Notes
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To check that you copied the u-boot.its file correctly, use these commands.
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You should see that the data at 0x100 in u-boot-chromium.fit is the first few
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bytes of U-Boot:
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hd u-boot-chromium.fit |head -20
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...
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00000100 b8 00 00 ea 14 f0 9f e5 14 f0 9f e5 14 f0 9f e5 |................|
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hd b/nyan-big/u-boot.bin |head
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00000000 b8 00 00 ea 14 f0 9f e5 14 f0 9f e5 14 f0 9f e5 |................|
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The 'data' property of the FIT is set up to start at offset 0x100 bytes into
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the file. The change to CONFIG_SYS_TEXT_BASE is also an offset of 0x100 bytes
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from the load address. If this changes, you either need to modify U-Boot to be
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fully relocatable, or expect it to hang.
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chromebook_jerry
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----------------
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The instruction are similar to those for Nyan with changes as noted below:
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1. Patch U-Boot
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Open include/configs/rk3288_common.h
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Change:
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#define CONFIG_SYS_TEXT_BASE 0x00100000
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to:
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#define CONFIG_SYS_TEXT_BASE 0x02000100
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2. Build U-Boot
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mkdir b
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make -j8 O=b/chromebook_jerry CROSS_COMPILE=arm-linux-gnueabi- \
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chromebook_jerry_defconfig all
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3. See above
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4. Build and sign an image
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./b/chromebook_jerry/tools/mkimage -f doc/chromium/chromebook_jerry.its \
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u-boot-chromium.fit
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echo test >dummy.txt
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vbutil_kernel --arch arm --keyblock doc/chromium/devkeys/kernel.keyblock \
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--signprivate doc/chromium/devkeys/kernel_data_key.vbprivk \
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--version 1 --config dummy.txt --vmlinuz u-boot-chromium.fit \
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--bootloader dummy.txt --pack u-boot.kpart
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5. See above
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6. See above
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7. Start it up
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Reboot the device in dev mode. Make sure that you have USB booting enabled. To
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do this, login as root (via Ctrl-Alt-forward_arrow) and type
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'enable_dev_usb_boot'. You only need to do this once.
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Reboot the device with the SD card inserted. Press Clrl-U at the developer
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mode screen. It should show something like the following on the display:
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U-Boot 2017.05-00649-g72acdbf-dirty (May 29 2017 - 14:57:05 -0600)
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Model: Google Jerry
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Net: Net Initialization Skipped
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No ethernet found.
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Hit any key to stop autoboot: 0
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8. Known problems
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None as yet.
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9. Notes
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None as yet.
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Other notes
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===========
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flashrom
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--------
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Used to make a backup of your firmware, or to replace it.
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See: https://www.chromium.org/chromium-os/packages/cros-flashrom
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coreboot
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--------
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Coreboot itself is not designed to actually boot an OS. Instead, a program
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called Depthcharge is used. This originally came out of U-Boot and was then
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heavily hacked and modified such that is is almost unrecognisable. It does
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include a very small part of the U-Boot command-line interface but is not
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usable as a general-purpose boot loader.
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In addition, it has a very unusual design in that it does not do device init
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itself, but instead relies on coreboot. This is similar to (in U-Boot) having
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a SPI driver with an empty probe() method, relying on whatever was set up
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beforehand. It can be quite hard to figure out between these two code bases
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what settings are actually used. When chain-loading into U-Boot we must be
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careful to reinit anything that U-Boot expects. If not, some peripherals (or
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the whole machine) may not work. This makes the process of chainloading more
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complicated than it could be on some platforms.
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Finally, it supports only a subset of the U-Boot's FIT format. In particular
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it uses a fixed address to load the FIT and does not support load/exec
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addresses. This means that U-Boot must be able to boot from whatever
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address Depthcharge happens to use (it is the CONFIG_KERNEL_START setting
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in Depthcharge). In practice this means that the data in the kernel@1 FIT node
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(see above) must start at the same address as U-Boot's CONFIG_SYS_TEXT_BASE.
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