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Using an upstream kernel on ChromeOS

Introduction

This page aims to document how to use an Upstream Kernel on Chrome OS. It's aimed at helping people with upstreaming for the Samsung ARM Series 3 Chromebook (AKA snow) and uses that in many examples. However, it's useful in other cases, too.

In general this documentation assumes that you've got the Chrome OS source tree synced down and have a build environment (and chroot) setup. It assumes you've built packages and an image for your board (like BOARD=daisy for the snow board). See the Developer Guide if you haven't done that.

Obtain source code and recent patches

The whole point here is to use the upstream kernel, so these instructions will direct you to linux-next. ...but linux-next is pretty darn bleeding edge. If you get stuck and things won't boot, you could always try a known-good state. In fact, many of the needed patches have now landed on even landed on the main linux tree now. See the section below "Patches relevant to the Samsung ARM Chromebook" for the last known good state.

The easiest way to get source code is to just add a remote git server to your normal kernel tree, fetch things, and then use patchwork to apply some patches.

Here's some steps to do that and start a new work branch on linux-next. WARNING: The fetch might be slow.

cd ~/trunk/src/third_party/kernel/files
git remote add linuxnext git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
git fetch linuxnext
git checkout -b my_branch linuxnext/master

Installing pwclient

Now that you've got the source code you should apply a few patches. In this case we'll use pwclient to get things out of patchwork.kernel.org.

First we need to install pwclient.

One way to install the patchwork client in the cros chroot is via its ebuild:

sudo emerge pwclient

Alternatively, you can install the latest pwclient directly from kernel.org:

sudo bash -c "curl 'https://patchwork.kernel.org/pwclient/' > /usr/local/bin/pwclient"
sudo chmod +x /usr/local/bin/pwclient

Next, setup the .pwmclientrc for the LKML project on patchwork.kernel.org:


# The following will clobber any existing ~/.pwclientrc you have (!)
curl 'https://patchwork.kernel.org/project/LKML/pwclientrc/' > ~/.pwclientrc

Patches relevant to the Samsung ARM Chromebook

This section should go away eventually once patches are upstream...

Relevant patch series that I've tested (as of 12/19/12)

Quick way to apply:

for id in 1966801 1966931 1966871 1966781 1966811; do
  pwclient git-am $id
done
pwclient git-am 1966911
pwclient git-am 1840211

Status on linux/master:

Status on linux-next/master:

Building and installing the kernel

Certainly there are lots of ways to build and install the kernel. I'm not going to copy them all here but I'll just point you at the Kernel FAQ. Specifically read the How to quickly test kernel modifications (the fast way) section carefully.

I would strongly suggest that you boot from a fast SD card rather than directly messing with what you have on eMMC. That way you can always get back to a running system. The quick set of steps for that is:

You might also want to look at Using nv-U-Boot on the Samsung ARM Chromebook.

The kernel config

One especially weird thing when working with the upstream kernel is that standard ChromeOS kernel splitconfig system isn't in place. The kernel ebuild used by ChromeOS will realize that and will pick up a backup config from chromiumos-overlay. On snow it will pick up the exynos5_defconfig in particular.

If you need to make changes to the kernel config locally the easiest thing to do is to just override that config. Here's a set of commands that might suit you:

DEFCONFIG=~/trunk/src/third_party/chromiumos-overlay/eclass/cros-kernel/exynos5_defconfig
cd ~/trunk/src/third_party/kernel/v3.8
cp ${DEFCONFIG} .config
ARCH=arm make menuconfig
# Make your changes
ARCH=arm make savedefconfig
cp defconfig ${DEFCONFIG}
make mrproper

If you have changes to the config that you think is relevant to others working on the upstream kernel then feel free to submit a patch!

Making sure other configs compile

It's hard to make 100% sure that your change won't break anyone else. ...but it's usually considered courtesy to make sure that some other boards compile. If you want to see if the exynos4_defconfig compiles you could use a recipe like this:

cd ~/trunk/src/third_party/kernel/v3.8
ARCH=arm CROSS_COMPILE=armv7a-cros-linux-gnueabi- make exynos4_defconfig
ARCH=arm CROSS_COMPILE=armv7a-cros-linux-gnueabi- make -j16
make mrproper

The exynos4_defconfig is particularly interesting when you're modifying exynos code because it is a non-device tree enabled board that shares a lot of code with the exynos config that we're using (so it's easy to break). It's actually broken at the git hash I listed above because I didn't run it before submitting my last patch. :(

Sending your change upstream

This is documented over at the Kernel FAQ, which will soon (hopefully) include the use of patman.

What is known to work (and known to not work)

As of 11/30/12:

Works:

Known not to work yet:

Musings

A few random thoughts:

Contributing

Patches are certainly appreciated as are reviews of patches posted upstream. In general most people on the Chrome OS team wouldn't object to people taking patches from the Chrome OS kernel tree, adjusting them, and posting them upstream (check with the author, though, since people may be midway through doing that themselves).