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- Chromium OS Support in U-Boot
- =============================
- Introduction
- ------------
- This describes how to use U-Boot with Chromium OS. Several options are
- available:
- - Running U-Boot from the 'altfw' feature, which is available on selected
- Chromebooks from 2019 onwards (initially Grunt). Press '1' from the
- developer-mode screen to get into U-Boot. See here for details:
- https://sites.google.com/a/chromium.org/dev/chromium-os/poking-around-your-chrome-os-device?pli=1
- - Running U-Boot from the disk partition. This involves signing U-Boot and
- placing it on the disk, for booting as a 'kernel'. See
- README.chromium-chainload for information on this. This is the only
- option on non-U-Boot Chromebooks from 2013 to 2018 and is somewhat
- more involved.
- - Running U-Boot with Chromium OS verified boot. This allows U-Boot to be
- used instead of either or both of depthcharge (a bootloader which forked
- from U-Boot in 2013) and coreboot. See below for more information on
- this.
- U-Boot with Chromium OS verified boot
- -------------------------------------
- To obtain:
- git clone https://github.com/sglass68/u-boot.git
- cd u-boot
- git checkout cros-master
- cd ..
- git clone https://chromium.googlesource.com/chromiumos/platform/vboot_reference
- cd vboot_reference
- git checkout 45964294
- # futility: updater: Correct output version for Snow
- To build for sandbox:
- UB=/tmp/b/chromeos_sandbox # U-Boot build directory
- cd u-boot
- make O=$UB chromeos_sandbox_defconfig
- make O=$UB -j20 -s VBOOT_SOURCE=/path/to/vboot_reference \
- MAKEFLAGS_VBOOT=DEBUG=1 QUIET=1
- Replace sandbox with another supported target.
- This produces $UB/image.bin which contains the firmware binaries in a SPI
- flash image.
- To run on sandbox:
- $UB/tpl/u-boot-tpl -d $UB/u-boot.dtb.out \
- -L6 -c "host bind 0 $CROS/src/build/images/cheza/latest/chromiumos_image.bin; vboot go auto" \
- -l -w -s state.dtb -r
- To run on other boards:
- Install image.bin in the SPI flash of your device
- Boot your system
- Sandbox
- -------
- Most Chromium OS development with U-Boot is undertaken using sandbox. There is
- a sandbox target available (chromeos_sandbox) which allows running U-Boot on
- a Linux machine completion with emulations of the display, TPM, disk, etc.
- Running sandbox starts TPL, which contains the first phase of vboot, providing
- a device tree and binding a Chromium OS disk image for use to find kernels
- (any Chromium OS image will do). It also saves driver state between U-Boot
- phases into state.dtb and will automatically ensure that memory is shared
- between all phases. TPL will jump to SPL and then on to U-Boot proper.
- It is possible to run with debugging on, e.g.
- gdb --args $UB/tpl/u-boot-tpl -d ....
- Breakpoints can be set in any U-Boot phase. Overall this is a good debugging
- environment for new verified-boot features.
- Samus
- -----
- Basic support is available for samus, using the chromeos_samus target. If you
- have an em100, use:
- sudo em100 -s -c W25Q128FW -d $UB/image.bin -t -r
- to write the image and then boot samus (Power-Refresh).
- Boot flow
- ---------
- Verified boot starts in TPL, which selects the A or B SPL, which in turn selects
- the A or B U-Boot. Then this jumps to the selected kernel. If anything goes
- wrong, the device reboots and the recovery SPL and U-Boot are used instead.
- More details are available here:
- https://www.chromium.org/chromium-os/chromiumos-design-docs/firmware-boot-and-recovery
- New uclasses
- ------------
- Several uclasses are provided in cros/:
- UCLASS_CROS_AUX_FW Chrome OS auxiliary firmware
- UCLASS_CROS_FWSTORE Chrome OS firmware storage
- UCLASS_CROS_NVDATA Chrome OS non-volatile data device
- UCLASS_CROS_VBOOT_EC Chrome OS vboot EC operations
- UCLASS_CROS_VBOOT_FLAG Chrome OS verified boot flag
- The existing UCLASS_CROS_EC is also used.
- Commands
- --------
- A new 'vboot' command is provided to run particular vboot stages. The most
- useful command is 'vboot go auto', which continues where the last stage left
- off.
- Note that TPL and SPL do not supports commands as yet, so the vboot code is
- called directly from the SPL boot devices (BOOT_DEVICE_CROS_VBOOT). See
- cros_load_image_tpl() and cros_load_image_spl() which both call
- vboot_run_auto().
- Config options
- --------------
- The main option is CONFIG_CHROMEOS, which enables a wide array of other options
- so that the required features are present.
- Device-tree config
- ------------------
- Various options are available which control the operation of verified boot.
- See cros/dts/bindings/config.txt for details. Most config is handled at run-
- time, although build-time config (with Kconfig) could also be added fairly
- easily.
- Porting to other hardware
- -------------------------
- A basic port to samus (Chromebook Pixel 2015) is in a basic working state,
- using the chromeos_samus target. Patches will likely be forthcoming in early
- 2019. Ports to an ARM board and coreboot (for x86 Chromebooks) are in the
- dreaming state.
- Tests
- -----
- Chromium OS firmware has a very limited set of tests. The tests that originally
- existed in U-Boot were not brought over to coreboot or depthcharge.
- The U-Boot tests ('make check') do operate, but at present there are no
- Chromium OS tests available. These will hopefully come together over time. Of
- course the above sandbox feature provides a sort of functional test and can
- detecte problems that affect the flow or particular vboot features.
- TO DO
- -----
- - Support for booting from coreboot (patches expected March 2019)
- - Support for booting from an ARM board, e.g. bob
- Simon Glass
- sjg@chromium.org
- 7 October 2018
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