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Booting a Chromebook with Depthcharge and automating it with LAVA for kernelci.org

Some context

You might have heard of the Google Chromebook laptops. They come with Chrome OS, to run applications in the Chrome web browser such as Gmail, YouTube, Google Docs, Google Drive etc... Chromium OS is the open-source project behind Chrome OS, based on Linux (Gentoo). As part of the effort to keep the mainline Linux kernel working on these devices, they are being continuously tested on kernelci.org.

kernelci.org is a project dedicated to testing the mainline Linux kernel in order to find issues introduced during its development. It uses LAVA to run tests on a variety of platforms in many different test labs as explained on the kernelci.org wiki.

I've been enabling several Chromebooks to be tested on kernelci.org as part of my work at Collabora. There are quite a few steps to go through in order to be able to achieve this, so let's start from the beginning.

Prerequisites

In order to put the information below into practice, you'll need:

  • a Chromebook device with a compatible Servo debug board
  • a Linux PC to build the Chromebook bootloader from source
  • optionally a LAVA installation in order to automate running things on the device

It can be difficult to find a Servo debug board, but there are alternatives. Its PCB design is open source so you can in principle get one made. Some Servo boards will only work with Chromebook devices that have a special debug connector fitted on the motherboard. Newer Chromebooks can apparently be used with USB Type-C debug interfaces, although I haven't tried to do this myself.

The part about LAVA automation is useful for continuous testing, but for kernel development it can be easier to configure the device to load a kernel image from a fixed network location and have direct console interaction. The first part about Depthcharge with tftpboot is relevant in this case.

Depthcharge with tftpboot

The part of Chrome OS that loads the Linux kernel and starts the device is called Depthcharge (source code). It's a payload for the Coreboot bootloader. So the ideal way to boot a Chromebook is to use Depthcharge. For development and testing, it can download a Linux kernel image over the network (TFTP) by enabling a debug command line. This is how LAVA controls Chromebooks, an example of which can be seen in Collabora's LAVA lab.

The first step is to rebuild Depthcharge with the debug command line interface enabled, in order to be able to dynamically download a Linux kernel image using TFTP and boot it.

Here's a summary of the steps to follow to build this from source:

  • Get the Chromium OS source code and set up the build environment as per the quick start guide
  • Don't build all of Chromium OS! You can, but it's very large and we're only interested in the bootloader part here.
  • Find out the code name for the device you have as per the developer information page.
  • Enter the source code tree and build the firmware for your device.

Here are some sample commands for the Samsung Chromebook Plus, code named "gru-kevin":

cd chromeos
export PATH=$PWD/chromium/tools/depot_tools:$PATH
cros_sdk --enter --no-ns-pid
export ACCEPT_LICENSE=Google-TOS
export BOARD=gru
./setup_board --board=$BOARD
USE=fwconsole emerge-${BOARD} \
  libpayload \
  depthcharge \
  coreboot \
  vboot_reference \
  chromeos-bootimage

The "USE=fwconsole" option enables the console in Depthcharge.

I've done this for a few devices, so alternatively the build step can be skipped by downloading one of these binaries:

Once the build is complete (or a compatible binary has been downloaded) the firmware can be flashed onto the device. Each device type requires a slightly different flashing method, so here's an example for the same "gru-kevin" device: flash-kevin.sh. It will first read the current firmware and save it in a file, to be able to restore it later on if there was any problem with the new firmware.

When the device boots, the CPU serial console should show a prompt. Here's a typical command to boot over TFTP with the kernel command line stored in a file:

Starting depthcharge on gru...
gru: tftpboot 192.168.0.123 image.itb cmdline

The "image.dtb" file is a Linux kernel image in the FIT format. It can be created with a regular "mkimage" command:

mkimage \
  -D "-I dts -O dtb -p 2048" \
  -f auto \
  -A arm \
  -O linux \
  -T kernel \
  -C None \
  -d Image \
  -a 0 \
  -b rk3399-gru-kevin.dtb \
  -i ramdisk.cpio.gz \
  image.itb

The "cmdline" file simply contains the kernel command line, here's an example:

earlyprintk=ttyS2,115200n8
console=tty1
console=ttyS2,115200n8
root=/dev/ram0
ip=dhcp

Chromebooks in LAVA

This can now all be automated in LAVA. Some device types such as the "gru-kevin", "veyron-jaq" and "nyan-big" are part of the mainline LAVA releases, so relatively little configuration is required for them. Installing LAVA and managing devices in a lab is a whole topic which goes beyond what this blog post is about; a good place to start is the LAVA documentation.

In a nutshell, the power can be controlled with commands of this kind:

dut-control cold_reset:off
dut-control cold_reset:on

and the console is available over USB. For example, there are several Chromebook devices booting with Depthcharge in the Collabora LAVA lab (they use the firmware binaries listed above):

In order to be able to use dut-control without a Chrome OS build environment, and to automatically bring up the device connections, servod-tools can be used in conjunction with hdctools. Installing and using these still requires a fair amount of manual configuration, which could be a topic for a future blog post.

Chromebooks on kernelci.org

The main objective with doing all this was to be able to test the mainline Linux kernel on these Chromebook devices via kernelci.org. The same devices listed above have all been enabled, boot results can be found here:

Now that the basic infrastructure to run tests is available, we're working on adding many more functional tests to cover various areas of the Linux kernel via user-space APIs - but that's for another blog post.

Stay tuned!

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