5 Kernel Maintenance

5.1 Tree Construction

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 https://git.yoctoproject.org/cgit/cgit.cgi/ 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

Note

Branches in the 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 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.

Note

Ground up reconstruction of the complete kernel tree is an action only taken by the Yocto Project team during an active development cycle. When you create a clone of the kernel Git repository, you are simply making it efficiently available for building and development.

The following steps describe what happens when the Yocto Project Team constructs the Yocto Project kernel source Git repository (or tree) found at https://git.yoctoproject.org/cgit/cgit.cgi/ 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.

Note

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 https://git.yoctoproject.org/cgit/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.
The full kernel tree that you see on https://git.yoctoproject.org/cgit/cgit.cgi/ 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.

5.2 Build Strategy

Once you have cloned a Yocto Linux kernel repository and the cache repository (yocto-kernel-cache) onto your development system, you can consider the compilation phase of kernel development, which is building a kernel image. Some prerequisites are validated by the build process before compilation starts:

The SRC_URI points to the kernel Git repository.
A BSP build branch with Metadata exists in the yocto-kernel-cache repository. The branch is based on the Yocto Linux kernel version and has configurations and features grouped under the yocto-kernel-cache/bsp directory. For example, features and configurations for the BeagleBone Board assuming a linux-yocto_4.12 kernel reside in the following area of the yocto-kernel-cache repository: yocto-kernel-cache/bsp/beaglebone

Note

In the previous example, the “yocto-4.12” branch is checked out in the yocto-kernel-cache repository.

The OpenEmbedded build system makes sure these conditions are satisfied before attempting compilation. Other means, however, do exist, such 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 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.