Raspberry Pi/Kernel Compilation

The Raspberry Pi cannot run a vanilla Linux kernel. A patched version of the kernel is maintained by the Raspberry Pi Foundation and is available from their GitHub page.

Note
The official Raspberry Pi Foundation kernels are built 32-bit, which is appropriate for Raspberry Pi 1, 2, and 3 (running in 32-bit mode; recommended). This guide does not cover building a 64-bit kernel for the Raspberry Pi 3 (issues / unstable / not recommended).

Prerequisites

To compile a kernel, dev-vcs/git is required to download the source code when not using sys-kernel/raspberrypi-sources and also (optional) genkernel to manage the build process.

root #emerge --ask dev-vcs/git sys-kernel/genkernel

Get the kernel source

root #emerge --ask sys-kernel/raspberrypi-sources

or manually:

root #cd /opt

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

root #ln -s /opt/linux /usr/src/linux

Compile and install the kernel with genkernel

Note
Compiling the kernel will take about 6 hours.

genkernel can build a Linux kernel with support for many different features. Follow one of the examples below that has the required features.

Default kernel

In this example, the configuration options from the running kernel are used to compile the new kernel.

root #genkernel --kernel-config=/proc/config.gz kernel

After the kernel has compiled, it will be installed into the /boot folder.

Kernel with initramfs

This example will run menuconfig before compiling the kernel to enable any extra modules needed. Using a kernel with an initramfs allows loading modules, decrypt partitions and other more complex task that maybe require early in the boot process.

root #genkernel --kernname=rpi --menuconfig all

To support initramfs, the following options need to be enabled in menuconfig:

KERNEL

General setup  --->       
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support
        ()    Initramfs source file(s) (NEW)
        [*]   Support initial ramdisks compressed using gzip (NEW)
        [ ]   Support initial ramdisks compressed using bzip2 (NEW)

After the kernel has compiled, it and the initramfs be installed into the /boot folder, add it to bootloader (skip to Adding new kernel to the bootloader)

Compile and install the kernel without genkernel

The first time configuring the kernel sources, create a default .config file (for Raspberry Pi2 use bcm2709_defconfig):

root #cd /usr/src/linux

root #make bcm2835_defconfig

After that, modify this default configuration (a good idea is to add .config support):

root #cd /usr/src/linux

root #make menuconfig

An example config can be found at https://github.com/modulix/raspggen/blob/master/kernel.conf

Then, try to compile/install it:

root #cd /usr/src/linux

root #mount /boot

root #mkdir /boot/overlays

root #make -j4 Image modules dtbs

root #make modules_install dtbs_install

for 32-bit kernels:

root #gzip -9cf arch/arm/boot/Image > /boot/kernel8.img

for 64-bit kernels:

root #gzip -9cf arch/arm/boot/Image > /boot/kernel7.img

Note
If this kernel is called kernel7.img, adding it in /boot/config.txt file is not necessary.

For now, to have WiFi work, download firmware:

root #wget https://github.com/RPi-Distro/firmware-nonfree/blob/master/brcm80211/brcm/brcmfmac43430-sdio.bin -O /lib/firmware/brcm/brcmfmac43430-sdio.bin

root #wget https://github.com/RPi-Distro/firmware-nonfree/blob/master/brcm80211/brcm/brcmfmac43430-sdio.txt -O /lib/firmware/brcm/brcmfmac43430-sdio.txt

Adding new kernel to the bootloader

By default, the Raspberry Pi looks for a kernel in /boot/kernel.img. This is changed in the configuration file /boot/config.txt to load the new kernel.

FILE /boot/config.txtExample config.txt

kernel=kernel-genkernel-arm-3.2.27+

If using an initramfs, add that to the config.txt as well:

FILE /boot/config.txtExample config.txt with an initramfs

kernel=kernel-rpi-arm-3.2.27+
initramfs initramfs-rpi-arm-3.2.27+

Now, the Raspberry Pi can be rebooted and should make use of the new kernel. If for some reason the new kernel does not load or gives errors, the kernel entry in the /boot/config.txt can be removed. Then, on the next reboot, the default kernel.img will be loaded.

Detailed step-by-step guide

Upon encountering problems building or deploying the kernel, try following the detailed kernel building guide for clues on resolving the problems. Additionally The Raspberry Pi foundation provides these build guides to assist in Kernel compilation.