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.