Selecting a Kernel

To use a different kernel than the default for the machine, set the kernel using PREFERRED_PROVIDER. This can either be set in local.conf for testing/development or in machine/<machine>.conf to set the default kernel for that machine.

PREFERRED_PROVIDER_virtual/kernel = "<kernel>"

Note the old kernel should be cleaned before switching to a new kernel. Neglecting to do so may cause build errors.

$ bitbake virtual/kernel -c clean 

Adding a Kernel as a Recipe

Poky provides a template recipe for kernels in its meta-skeleton layer . The recipe can be found in poky/meta-skeleton/recipes-kernel/linux-yocto-custom.bb.

Canonically, kernels recipes live in recipes-kernel/linux/ and the recipe name starts with linux, for example linux-yocto or linux-toradex.

The kernel recipe - like any recipe - needs a layer to live in.

Copy the template from meta-skeleton to your custom layer into recipes-kernel/linux/ and rename it appropriately.

$ cd <layer dir> 
$ mkdir -p recipes-kernel/linux 
$ cp ../poky/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb recipes-kernel/linux/linux-<name>.bb 

Customizing the Recipe

Now the recipe needs to modify the kernel in question. The key fields in need of modification are listed below:

SRC_URI = "<kernel source>"

LINUX_VERSION ?= "<kernel version>"
LINUX_VERSION_EXTENSION_append = "-custom"

SRCREV = "<commit hash or tag if git repo>"

COMPATIBLE_MACHINE = "<machine>"

  • SRC_URI needs to include the kernel source as well as any patches if needed

  • LINUX_VERSION needs to contain the kernel version, e.g. 4.14

  • LINUX_VERSION_EXTENSION_append can be used to the kernel name

  • SRCREV needs to contain the commit hash or tag if the kernel source is a git repository

  • COMPATIBLE_MACHINE needs to include all machines that this kernel is compatible with

The kernel can now be set as PREFERRED_PROVIDER_virtual/kernel.

Working with the Kernel

Here are some common operations when dealing with the kernel:

commanddescription
bitbake virtual/kernel -c unpackunpack kernel
bitbake virtual/kernel -c patchapply patches (if any)
bitbake virtual/kernel -c compilebuild kernel
bitbake virtual/kernel -c configureconfigure kernel*
bitbake virtual/kernel -c menuconfigmenuconfig, like make menuconfig
bitbake virtual/kernel -c cleanclean kernel package, like make clean
bitbake virtual/kernelbuild and install kernel package

*: configure also extracts the kernel into tmp/work-shared/<machine>/kernel-source. This is convenient when needing to modify the kernel, for example to create patches.

Note that bitbake will also run all commands that need to be run to execute the specified command. For example, compile will also cause unpack and patch to be run, since these need to happen before the build.

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