This section describes construction of the Yocto Project kernel
source repositories as accomplished by the Yocto Project team to
create Yocto Linux kernel repositories.
These kernel repositories are found under the heading "Yocto Linux
Kernel" at
http://git.yoctoproject.org
and are 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,
configurations, branches, tags, and feature divisions found in a
Yocto Linux kernel.
Thus, the Yocto Project Linux kernel repository (or tree) and
accompanying Metadata in the
yocto-kernel-cache
are built.
The existence of these repositories allow you to access and clone a particular Yocto Project Linux 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 Linux kernel in any clone of the Yocto
Project Linux kernel source repository and
yocto-kernel-cache
Git trees.
For example, the following commands clone the Yocto Project
baseline Linux kernel that branches off
linux.org
version 4.12 and the
yocto-kernel-cache
, which contains stores of
kernel Metadata:
$ git clone git://git.yoctoproject.org/linux-yocto-4.12 $ git clone git://git.yoctoproject.org/linux-kernel-cache
For more information on how to set up a local Git repository of the Yocto Project Linux kernel files, see the "Preparing the Build Host to Work on the Kernel" section.
Once you have cloned the kernel Git repository and the cache of Metadata on your local machine, you can discover the branches that are available in the repository using the following Git command:
$ git branch -a
Checking out a branch allows you to work with a particular
Yocto Linux kernel.
For example, the following commands check out the
"standard/beagleboard" branch of the Yocto Linux kernel repository
and the "yocto-4.12" branch of the
yocto-kernel-cache
repository:
$ cd ~/linux-yocto-4.12 $ git checkout -b my-kernel-4.12 remotes/origin/standard/beagleboard $ cd ~/linux-kernel-cache $ git checkout -b my-4.12-metadata remotes/origin/yocto-4.12
yocto-kernel-cache
repository correspond to Yocto Linux kernel versions
(e.g. "yocto-4.12", "yocto-4.10", "yocto-4.9", and so forth).
Once you have checked out and switched to appropriate branches, you can see a snapshot of all the kernel source files used to used to build that particular Yocto Linux kernel for a particular board.
To see the features and configurations for a particular Yocto
Linux kernel, you need to examine the
yocto-kernel-cache
Git repository.
As mentioned, branches in the
yocto-kernel-cache
repository correspond to
Yocto Linux kernel versions (e.g. yocto-4.12
).
Branches contain descriptions in the form of
.scc
and .cfg
files.
You should realize, however, that browsing your local
yocto-kernel-cache
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 given the introduction of a new top-level kernel feature or BSP. The following actions effectively provide the Metadata and create the tree that includes the new feature, patch, or BSP:
Pass Feature to the OpenEmbedded Build System: A top-level kernel feature is passed to the kernel build subsystem. Normally, this feature is a BSP for a particular kernel type.
Locate Feature: 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 the
yocto-kernel-cache
repository organized under the "Yocto Linux Kernel"
heading in the
Yocto Project Source Repositories.
Areas pointed to by SRC_URI
statements found in kernel recipes
For a typical build, the target of the search is a
feature description in an .scc
file
whose name follows this format (e.g.
beaglebone-standard.scc
and
beaglebone-preempt-rt.scc
):
bsp_root_name
-kernel_type
.scc
Expand Feature: Once located, the feature description is either expanded into a simple script of actions, or into an existing equivalent script that is already part of the shipped kernel.
Append Extra Features:
Extra features are appended to the top-level feature
description.
These features can come from the
KERNEL_FEATURES
variable in recipes.
Locate, Expand, and Append Each Feature: Each extra feature is located, expanded and appended to the script as described in step three.
Execute the Script:
The script is executed to produce files
.scc
and .cfg
files in appropriate directories of the
yocto-kernel-cache
repository.
These files 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.
Clone Base Repository:
The base repository is cloned, and the actions
listed in the yocto-kernel-cache
directories are applied to the tree.
Perform Cleanup: The Git repositories are left with the desired branches checked out and any required branching, patching and tagging has been performed.
The kernel tree and cache are ready for developer consumption to be locally cloned, configured, and built into a Yocto Project kernel specific to some target hardware.
The generated yocto-kernel-cache
repository adds 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
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.
The full kernel tree that you see on
http://git.yoctoproject.org is
generated through repeating the above steps for all
valid BSPs.
The end result is a branched, clean history tree that
makes up the kernel for a given release.
You can see the script (kgit-scc
)
responsible for this in the
yocto-kernel-tools
repository.
The steps used to construct the full kernel tree are the same steps that BitBake uses when it builds a kernel image.