Kernel Configuration

Overview

Kernel configuration is a little complicated in Chromium OS. There are a number of different kernels based on family (chromeos, termina), architecture (x86_64, arm64), variants (generic, rockchip) and platforms (Seaboard, Mario). We want to split the config into a sort of hierarchy so that the common options are shared as shown in this figure (click to enlarge):

Shown here is one family (chromeos) with 6 flavours, three for ARM and three for Intel (it is not clear why flavours is spelt the British way).

The sharing of common config options makes it easier to maintain the variants.

Conversions

We need to be able to do the following:

create a .config file for a particular flavour - this is done simply by concatenating the three levels for that flavour. For example, we create the tegra2 flavour by concatenating base.config, armel/common.config and armel/chromeos-tegra2.flavour.config. This .config file is suitable for configuring the kernel.
split a .config into its constituent parts. This is a little more complicated and has to be done for all flavours at once. Given a .config file for each flavour we can look for items which are common to all flavours - these can be put into the family config. We also look for everything common within each architecture - these are put into the architecture files like config.common.arm. The splitconfig script does this.

Scripts

Scripts to deal with the kernel configuration are in the chromeos/scripts directory. A partial listing is shown below:

$ ls chromeos/scripts/
kernelconfig  prepareconfig

kernelconfig

This script operates on all flavours at once. It builds the config for each flavour, performs the requested operations, and splits the configs back into the proper hierarchy. It supports four operations:

oldconfig - runs a 'make oldconfig' on each config
olddefconfig - like 'oldconfig', but it accepts the default selections automatically
editconfig - runs a 'make menuconfig' on each config
genconfig - like oldconfig, but it leaves the generated full config files lying around afterwards in the CONFIGS directory

There is a bit of a problem with kernelconfig. If you want to change a kernel option which is common to all flavours, for example, then you must change it once for each flavour using 'kernelconfig editconfig'. At the moment this is ~10 times! If you miss one, then the result may be no change, which can be very confusing. See below for another approach.

prepareconfig

This is used to prepare the config for a particular flavour. It assembles the default config for the flavour. This is useful when starting development on a board and you want to build in-tree:

CHROMEOS_KERNEL_FAMILY=chromeos ./chromeos/scripts/prepareconfig chromeos-tegra2
export ARCH=arm
export CROSS_COMPILE=armv7a-cros-linux-gnueabi-
make olddefconfig    #(to pick up new options, if any)
make
...

prepareconfig requires an environment variable CHROMEOS_KERNEL_FAMILY to correctly determine the split config. The above example uses chromeos as the kernel family. Each supported kernel family's split config is maintained in chromeos/config/<family>/ directory.

This script is used in the emerge. An example resulting string is 'Ubuntu 2.6.32-0.1-chromeos-tegra2'.

Using the kernelconfig script

The kernel config is split into several config files - generic, chrome-specific, architecture-specific and board specific. Automatic tools split and combine these configs.

If you want to edit the configuration for a particular board, then you need to do something like:

$ cd .../src/third_party/kernel/v$VER/
$ ./chromeos/scripts/kernelconfig editconfig

The value for $VER depends on the board. You can find what kernel version a board uses by running `emerge-$BOARD -pv virtual/linux-sources`.

The editconfig command runs make menuconfig for all existing kernel configurations. You need to figure out which part your config options appear in, make the changes, save the file, and exit.

Alternative Method to Edit Config

This is faster, if you know what you're doing:

Search the split configs for the kernel config option you want to edit.

$ grep -rs CONFIG_LEGACY_PTYS  chromeos/config/
chromeos/config/chromeos/base.config:# CONFIG_LEGACY_PTYS is not set
chromeos/config/termina/base.config:# CONFIG_LEGACY_PTYS is not set

Now assuming that we are working with chromeos kernel family, edit the file chromeos/config/chromeos/base.config to change the config like this:
```
CONFIG_LEDS_CLASS=y
<s># CONFIG_LEGACY_PTYS is not set</s>
CONFIG_LEGACY_PTYS=y
CONFIG_LKDTM=y
```
Now run this to recreate all the configs based on your changes:
```
$ chromeos/scripts/kernelconfig olddefconfig
```
This will accept the default choices for any new or otherwise unspecified options.
Use git diff to ensure that the result is as expected. You may repeat editing the files and executing kernelconfig as needed.

If you find that during that last step, your configs are disappearing, then you've likely enabled a config without enabling all of its dependencies. You'll need to either 1) track down those dependencies and enable them as well, or 2) Use the editconfig method mentioned above.

Comparing in-tree config to split config

This might not be useful, but:

# get full flavour config sorted, without leading #
$ FLAVOUR=chromeos-tegra2
$ cat chromeos/config/*/base.config \
   chromeos/config/*/armel/common.config \
   chromeos/config/*/armel/$FLAVOUR.flavour.config | \
   grep CONFIG_ | sed "s/.*CONFIG/CONFIG/" | sort -u >1.emerge
# purify .config in the same way

make ARCH=${kernarch} savedefconfig

# compare
diff 1.emerge defconfig

Manual Building

Here is how to build the kernel yourself manually, from within the chroot.

Make sure you have done something like:

export B="tegra2_seaboard"
export ARCH=arm
export CROSS_COMPILE=armv7a-cros-linux-gnueabi-
# indicate that you are making kernel changes

cros_workon --board=$B start sys-kernel/chromeos-kernel-$VER

The exact value for $VER depends on your board as each one is different. You can find that out by seeing what version by running `emerge-$BOARD -pv virtual/linux-sources`.

As you probably know, the cros_workon start tells the build system to use your local source install of the official source. Then set up the config:

# set up the .config file
CHROMEOS_KERNEL_FAMILY=<kernel-family>  ./chromeos/scripts/prepareconfig chromeos-tegra2
make olddefconfig
# edit config
make menuconfig
# (make your changes)

Now, after you make a kernel change, you can simply type make as normal:

# build the kernel and modules
make
# or maybe: build the uImage on an 8 core machine
make -j10 uImage
# or both!
make && make uImage

Generate a new splitconfig after adding new Kconfig options

Run olddefconfig to regenerate the splitconfigs. The olddefconfig script will run "make olddefconfig" on each flavor config.

cd ~/trunk/src/third_party/kernel/v$VER
chromeos/scripts/kernelconfig olddefconfig # regenerate splitconfig

Adding a new config

Copy the new config to the appropriate place in the chromeos/config and then run oldconfig

cd ~/trunk/src/third_party/kernel/v$VER
cp config.flavour.chromeos-new .config
make ARCH=${ARCH} olddefconfig
cp .config chromeos/config/<kernel-family>/<arch>/<flavour>.flavour.config
chromeos/scripts/kernelconfig olddefconfig # regenerate splitconfig

How can I find the list of flavour configs

cd ~/trunk/src/third_party/kernel/v$VER
find chromeos/config -name \*.flavour.config

How to regenerate and validate splitconfigs from single config

cd ~/trunk/src/third_party/kernel/v$VER
cp <single config> chromeos/config/<kernel-family>/<arch>/<flavour>.flavour.config
chromeos/scripts/kernelconfig olddefconfig
CHROMEOS_KERNEL_FAMILY=<kernel-family>  chromeos/scripts/prepareconfig <flavour>
make ARCH=${ARCH} oldconfig
diff .config <single config>