This section describes construction of the Yocto Project kernel source repositories as accomplished by the Yocto Project team to create kernel repositories. These kernel repositories are found under the heading "Yocto Linux Kernel" at http://git.yoctoproject.org/cgit.cgi and can be shipped as part of a Yocto Project release. The team creates these repositories by compiling and executing the set of feature descriptions for every BSP and feature in the product. Those feature descriptions list all necessary patches, configuration, branching, tagging and feature divisions found in a kernel. Thus, the Yocto Project kernel repository (or tree) is built.
The existence of this tree allows you to access and clone a particular Yocto Project kernel repository and use it to build images based on their configurations and features.
You can find the files used to describe all the valid features and BSPs
in the Yocto Project kernel in any clone of the Yocto Project kernel source repository
Git tree.
For example, the following command clones the Yocto Project baseline kernel that
branched off of linux.org
version 3.19:
$ git clone git://git.yoctoproject.org/linux-yocto-3.19
For another example of how to set up a local Git repository of the Yocto Project kernel files, see the "Yocto Project Kernel" bulleted item in the Yocto Project Development Manual.
Once you have cloned the kernel Git repository on your local machine, you can
switch to the meta
branch within the repository.
Here is an example that assumes the local Git repository for the kernel is in
a top-level directory named linux-yocto-3.19
:
$ cd linux-yocto-3.19 $ git checkout -b meta origin/meta
Once you have checked out and switched to the meta
branch,
you can see a snapshot of all the kernel configuration and feature descriptions that are
used to build that particular kernel repository.
These descriptions are in the form of .scc
files.
You should realize, however, that browsing your local kernel repository for feature descriptions and patches is not an effective way to determine what is in a particular kernel branch. Instead, you should use Git directly to discover the changes in a branch. Using Git is an efficient and flexible way to inspect changes to the kernel.
The following steps describe what happens when the Yocto Project Team constructs the Yocto Project kernel source Git repository (or tree) found at http://git.yoctoproject.org/cgit.cgi given the introduction of a new top-level kernel feature or BSP. These are the actions that effectively create the tree that includes the new feature, patch or BSP:
A top-level kernel feature is passed to the kernel build subsystem. Normally, this feature is a BSP for a particular kernel type.
The file that describes the top-level feature is located by searching these system directories:
The in-tree kernel-cache directories, which are located
in meta/cfg/kernel-cache
Areas pointed to by SRC_URI
statements
found in recipes
For a typical build, the target of the search is a
feature description in an .scc
file
whose name follows this format:
bsp_name
-kernel_type
.scc
Once located, the feature description is either compiled into a simple script of actions, or into an existing equivalent script that is already part of the shipped kernel.
Extra features are appended to the top-level feature description.
These features can come from the
KERNEL_FEATURES
variable in recipes.
Each extra feature is located, compiled and appended to the script as described in step three.
The script is executed to produce a series of meta-*
directories.
These directories are descriptions of all the branches, tags, patches and configurations that
need to be applied to the base Git repository to completely create the
source (build) branch for the new BSP or feature.
The base repository is cloned, and the actions
listed in the meta-*
directories are applied to the
tree.
The Git repository is left with the desired branch checked out and any required branching, patching and tagging has been performed.
The kernel tree is now ready for developer consumption to be locally cloned, configured, and built into a Yocto Project kernel specific to some target hardware.
The generated meta-*
directories add to the kernel
as shipped with the Yocto Project release.
Any add-ons and configuration data are applied to the end of an existing branch.
The full repository generation that is found in the
official Yocto Project kernel repositories at
http://git.yoctoproject.org/cgit.cgi
is the combination of all supported boards and configurations.
The technique the Yocto Project team uses is flexible and allows for seamless blending of an immutable history with additional patches specific to a deployment. Any additions to the kernel become an integrated part of the branches.