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