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