Tools for cross-compiling linux kernels for the Next Thing Co. Pocket CHIP using docker. Follow the steps in order below, at the end of this all the loot/ folder in this repository will contain your new kernel image and modules. If you've had success with this build process please let me know, so far this process has been reported to work well with:

• 4.4.13-ntc-mlc

This is where you get to decide what kernel revision you want to build and the name of your new kernel:

• FROM_KERNEL will be used to download the correct kernel sources.
• NEW_KERNEL should match the version of FROM_KERNEL but with a new postfix and "+".
• CHIP_IP is used to copy the correct kernel config from your CHIP via scp.
• MAKE_J should be equal to N+1, where N is the number of CPU cores on your computer.
• BASE_DIR can just be left as is.

$ export FROM_KERNEL=4.4.13-ntc-mlc
$ export NEW_KERNEL=4.4.13-rhodey+
$ export CHIP_IP=192.168.1.10
$ export MAKE_J=9
$ export BASE_DIR=/root/chip-linux

Here we're going to build a new docker image that contains all the dependencies needed to compile the linux kernel. This image will also contain the sources for CHIP-linux along with the RTL8723BS wifi/bluetooth driver.

$ docker build \
    -t chip-base \
    --build-arg "FROM_KERNEL=$FROM_KERNEL" \
    --build-arg "NEW_KERNEL=$NEW_KERNEL" \
    --build-arg "BASE_DIR=$BASE_DIR" \
    -f Dockerfile.base .

Next we need to compile the make menuconfig build target, this is an essential step in any kernel compilation. The output of this process will be a file named .config that instructs the rest of the build process as to what modules should be included in the kernel. It's best to start this process from an existing config file and so that's what the scp line below is about, it is an attempt to copy an existing (reliable) config file from your CHIP.

If this is your first try at kernel compilation you should avoid customizing too much from the defaults, the only setting you must change is the "Local version" string, it must match the postfix you added in the NEW_KERNEL environment variable above. After running the first two commands below do it like this:

1. select "Load".
2. enter .config-in and then "OK".
3. select "General setup".
4. select "Local version - Append to kernel release".
5. enter in your kernel postfix (except the "+", in my example -rhodey) and then "OK".
6. select "Save".
7. enter .config and then "OK".
8. exit by pressing escape four times.

Don't forget to run the last command below, this $ docker commit ... command is a fancy little trick to get around the bother that docker doesn't support interactive build processes. Basically what we're doing is creating a docker image from a stopped docker container, specifically our make menuconfig process.

$ scp chip@$CHIP_IP:/boot/config-$FROM_KERNEL .config-in
$ docker run \
    --name chip-config \
    -v $(pwd)/.config-in:$BASE_DIR/kernel/.config-in:ro \
    -it chip-base \
    bash -c 'make ARCH=arm CROSS_COMPILE=${CC_TOOL} menuconfig'
$ docker commit chip-config chip-config.img && \
    docker rm chip-config

Yayyy! It is time to build the kernel, pretty easy so far huh? This process should take anywhere from 5 to 30 minutes depending on your system specs.

$ docker build \
    -t chip-kernel \
    --build-arg "BASE_DIR=$BASE_DIR" \
    --build-arg "MAKE_J=$MAKE_J" .

And that's it! Now all that's left to do is copy the kernel image, modules, and firmware out of the docker image we just built in the step above. All these files will be copied to the loot/ folder in this repository.

$ ./tools/loot.sh

The final step is to install the new kernel image, modules, and firmware onto your CHIP. All of the commands below are perfectly safe to run except the last, the last command replaces the default kernel image with the one we just built. If you want to be careful you can follow this procedure to test out the new kernel before replacing the default.

Worst case your CHIP won't boot and you'll need to find a USB-to-Serial converter to fix it. However, if anything does go wrong it's most likely that you're CHIP will still boot fine but just not load a module or two. You can restore the default kernel at anytime by running # cp /boot/zImage.bak /boot/zImage.

$ scp loot/vmlinuz-* loot/config-* loot/System.map-* root@$CHIP_IP:/boot
$ cd loot/modules && tar cf - $NEW_KERNEL | ssh root@$CHIP_IP 'cd /lib/modules; tar xf -'
$ cd ../firmware && tar cf - $NEW_KERNEL | ssh root@$CHIP_IP 'cd /lib/firmware; tar xf -' && cd ../..
$ ssh root@$CHIP_IP cp /boot/vmlinuz-$NEW_KERNEL /boot/zImage

Thanks to Next Thing Co for bringing such a cute, smol linux device to life. Also a big thanks to @renzo for their how-to which gave me a good starting place.

Copyright 2017 Rhodey Orbits, GPLv3.