When we have a small amount of changes or we don't own the source repository, we may be forced to work with patches and use the Yocto build system to build the Linux kernel.

A typical workflow when working on a modification would be as follows:

1. Start the kernel package compilation from scratch:

$ cd /opt/yocto/fsl-community-bsp/
$ source setup-environment wandboard
$ bitbake -c cleanall virtual/kernel  

This will erase the build folder, shared state cache, and downloaded package source.

2. Configure the kernel as follows:

 $ bitbake -c configure virtual/kernel   

This will convert the machine defconfig file into a .config file and call oldconfig to validate the configuration with the kernel source.

3. You can optionally add your own configuration changes with the following:

$ bitbake -c menuconfig virtual/kernel  

4. Now, to modify the kernel source, start a development shell on the kernel:

$ bitbake -c devshell virtual/kernel    

This spawns a new shell with the environment ready for kernel compilation. The new shell will change to the kernel build directory, which contains a local Git repository.

We can then perform our modifications, including kernel configuration changes.

5. Go back to the Terminal with the sourced Yocto environment to compile the source without erasing our modifications as follows:

$ bitbake -C compile virtual/kernel

Note the capital C. This invokes the compile task but also all the tasks that follow it.

The kernel source is placed under the tmp/work-shared/wandboard folder. A kernel-source directory contains the checked-out source, and kernel-build-artifacts contains the build output, defconfig file, map file, and everything required to build external modules. The work-shared directory is used for packages that need to be kept outside of the shared state control.

The newly compiled kernel image is available under tmp/deploy/images/wandboard.