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

To build for sandbox:

   UB=/tmp/b/chromeos_sandbox    # U-Boot build directory
   CROS=/home/sglass/cosarm      # Chromium OS directory
   make O=$UB/chromeos_sandbox_defconfig
   make O=$UB -j20 -s VBOOT_SOURCE=$CROS/src/platform/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