Once a local Git repository of the Yocto Project kernel exists on a development system, you can consider the compilation phase of kernel development - building a kernel image. Some prerequisites exist that are validated by the build process before compilation starts:
The
SRC_URI
points
to the kernel Git repository.
A BSP build branch exists. This branch has the following form:
<kernel_type>/<bsp_name>
The OpenEmbedded build system makes sure these conditions exist before attempting compilation. Other means, however, do exist, such as as bootstrapping a BSP.
Before building a kernel, the build process verifies the tree
and configures the kernel by processing all of the
configuration "fragments" specified by feature descriptions in the .scc
files.
As the features are compiled, associated kernel configuration fragments are noted
and recorded in the meta-*
series of directories in their compilation order.
The fragments are migrated, pre-processed and passed to the Linux Kernel
Configuration subsystem (lkc
) as raw input in the form
of a .config
file.
The lkc
uses its own internal dependency constraints to do the final
processing of that information and generates the final .config
file
that is used during compilation.
Using the board's architecture and other relevant values from the board's template, kernel compilation is started and a kernel image is produced.
The other thing that you notice once you configure a kernel is that
the build process generates a build tree that is separate from your kernel's local Git
source repository tree.
This build tree has a name that uses the following form, where
${MACHINE}
is the metadata name of the machine (BSP) and "kernel_type" is one
of the Yocto Project supported kernel types (e.g. "standard"):
linux-${MACHINE}-<kernel_type>-build
The existing support in the kernel.org
tree achieves this
default functionality.
This behavior means that all the generated files for a particular machine or BSP are now in
the build tree directory.
The files include the final .config
file, all the .o
files, the .a
files, and so forth.
Since each machine or BSP has its own separate
Build Directory
in its own separate branch
of the Git repository, you can easily switch between different builds.