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Kernel Building

There are two main methods for building the kernel. You can build locally on a Raspberry Pi which will take a long time; or you can cross-compile, which is much quicker, but requires more setup.

Local building

On a Raspberry Pi first install the latest version of Raspbian from the downloads page. Then boot your Pi, plug in Ethernet to give you access to the sources, and log in.

First get the sources, which will take some time:

git clone --depth=1 https://github.com/raspberrypi/linux

Add missing dependencies:

sudo apt-get install bc

Configure the kernel - as well as the default configuration you may wish to configure your kernel in more detail or apply patches from another source to add or remove required functionality:

Run the following commands depending on your Raspberry Pi version.

Raspberry Pi 1 (or Compute Module) Default Build Configuration

cd linux
KERNEL=kernel
make bcmrpi_defconfig

Raspberry Pi 2/3 Default Build Configuration

cd linux
KERNEL=kernel7
make bcm2709_defconfig

Build and install the kernel, modules and Device Tree blobs; this step takes a long time...

make zImage modules dtbs
sudo make modules_install
sudo cp arch/arm/boot/dts/*.dtb /boot/
sudo cp arch/arm/boot/dts/overlays/*.dtb* /boot/overlays/
sudo cp arch/arm/boot/dts/overlays/README /boot/overlays/
sudo scripts/mkknlimg arch/arm/boot/zImage /boot/$KERNEL.img

Note: On a Raspberry Pi 2/3, adding -j4 (make -j4 zImage modules dtbs) splits the work between all four cores, speeding up compilation significantly.

Cross-compiling

First you are going to require a suitable Linux cross-compilation host. We tend to use Ubuntu; since Raspbian is also a Debian distribution it means using similar command lines and so on.

You can either do this using VirtualBox (or VMWare) on Windows, or install it directly onto your computer. For reference you can follow instructions online at Wikihow.

Install toolchain

Use the following command:

git clone https://github.com/raspberrypi/tools

You can then copy the toolchain to a common location such as /tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian, and add /tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin to your $PATH in the .bashrc in your home directory. For 64-bit host systems, use /tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin. While this step is not strictly necessary, it does make it easier for later command lines!

Get sources

To get the sources, refer to the original GitHub repository for the various branches.

$ git clone --depth=1 https://github.com/raspberrypi/linux

Build sources

To build the sources for cross-compilation there may be extra dependencies beyond those you've installed by default with Ubuntu. If you find you need other things please submit a pull request to change the documentation.

Enter the following commands to build the sources and Device Tree files.

For Pi 1 or Compute Module:

cd linux
KERNEL=kernel
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- bcmrpi_defconfig

For Pi 2/3:

cd linux
KERNEL=kernel7
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- bcm2709_defconfig

Then for both:

make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- zImage modules dtbs

Note: To speed up compilation on multiprocessor systems, and get some improvement on single processor ones, use -j n where n is number of processors * 1.5. Alternatively, feel free to experiment and see what works!

Install directly onto the SD card

Having built the kernel you need to copy it onto your Raspberry Pi and install the modules; this is best done directly using an SD card reader.

First use lsblk before and after plugging in your SD card to identify which one it is; you should end up with something like this:

sdb
   sdb1
   sdb2

If it is a NOOBS card you should see something like this:

sdb
  sdb1
  sdb2
  sdb3
  sdb5
  sdb6

In the first case sdb1/sdb5 is the FAT partition, and sdb2/sdb6 is the ext4 filesystem image (NOOBS).

Mount these first:

mkdir mnt/fat32
mkdir mnt/ext4
sudo mount /dev/sdb1 mnt/fat32
sudo mount /dev/sdb2 mnt/ext4

Adjust the partition numbers for the NOOBS images.

Next, install the modules:

sudo make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- INSTALL_MOD_PATH=mnt/ext4 modules_install

Finally, copy the kernel and Device Tree blobs onto the SD card, making sure to back up your old kernel:

sudo cp mnt/fat32/$KERNEL.img mnt/fat32/$KERNEL-backup.img
sudo scripts/mkknlimg arch/arm/boot/zImage mnt/fat32/$KERNEL.img
sudo cp arch/arm/boot/dts/*.dtb mnt/fat32/
sudo cp arch/arm/boot/dts/overlays/*.dtb* mnt/fat32/overlays/
sudo cp arch/arm/boot/dts/overlays/README mnt/fat32/overlays/
sudo umount mnt/fat32
sudo umount mnt/ext4

Another option is to copy the kernel into the same place, but with a different filename - for instance, kernel-myconfig.img - rather than overwriting the kernel.img file. You can then edit the config.txt file to select the kernel that the Pi will boot into:

kernel=kernel-myconfig.img

This has the advantage of keeping your kernel separate from the kernel image managed by the system and any automatic update tools, and allowing you to easily revert to a stock kernel in the event that your kernel cannot boot.

Finally, plug the card into the Pi and boot it!