# Working with the Yocto Project Kernel

## Introduction

This chapter describes how to accomplish tasks involving the kernel's tree structure. This information is designed to help the developer that wants to modify the Yocto Project kernel and contribute changes upstream to the Yocto Project. The information covers the following:

- Tree construction

- Build strategies

- Workflow examples

## Tree Construction

The Yocto Project kernel repository, as shipped with the product, is created by compiling and executing the set of feature descriptions for every BSP/feature in the product. Those feature descriptions list all necessary patches, configuration, branching, tagging and feature divisions found in the kernel. Thus, the Yocto Project kernel repository (or tree) is built. The existence of this tree allows you to build images based on your 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 kernel Git tree. For example, the following command clones the Yocto Project baseline kernel that branched off of linux.org version 2.6.37:

         $ git clone http://git.yoctoproject.org/cgit/cgit.cgi/linux-yocto-2.6.37

After you switch to the `meta` branch within the repository you can see a snapshot of all the kernel configuration and feature descriptions that are used to build the kernel repository. These descriptions are in the form of `.scc` files.

You should realize, however, that browsing the snapshot of 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. For examples showing how to use Git to inspect kernel commits, see the following sections in this chapter.

Ground up reconstruction of the complete kernel tree is an action only taken by the Yocto Project team during an active development cycle. Creating a project simply clones this tree to make it efficiently available for building and development.

The following steps describe what happens during tree construction 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:

1. A top-level kernel feature is passed to the kernel build subsystem. Normally, this is a BSP for a particular kernel type.

2. 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

3. 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.

4. Extra features are appended to the top-level feature description. These features can come from the `KERNEL_FEATURES` variable in recipes.

5. Each extra feature is located, compiled and appended to the script as described in step three.

6. The script is executed to produce a meta-series. The meta-series is a description 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.

7. The base repository is cloned, and the actions listed in the meta-series are applied to the tree.

8. The Git repository is left with the desired branch checked out and any required branching, patching and tagging has been performed.

The tree is now ready for configuration and compilation.

The end-user generated meta-series 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 is the combination of all supported boards and configurations.

This technique is flexible and allows for seamless blending of an immutable history with additional deployment specific patches. Any additions to the kernel become an integrated part of the branches.

## Build Strategy

There are some prerequisites that must be met before starting the compilation phase of the kernel build system:

- There must be a kernel Git repository indicated in the SRC_URI.

- There must be a BSP build branch - <bsp name>-<kernel type> in 0.9 or <kernel type>/<bsp name> in 1.0.

You can typically meet these prerequisites by running the tree construction/patching phase of the build system. However, other means do exist. For examples of alternate workflows such as bootstrapping a BSP, see the Workflow Examples section in this manual.

Before building a kernel it is configured 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 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 the Kernel compilation is started and a kernel image is produced.

The other thing that you will first see once you configure a kernel is that it will generate a build tree that is separate from your Git source tree. This build tree has the name using the following form:

         linux-<BSPname>-<kerntype>-build

"kerntype" is one of the standard kernel types.

The existing support in the kernel.org tree achieves this default functionality.

What this means, is that all the generated files for a particular BSP are now in this directory. The files include the final `.config`, all the `.o` files, the `.a` files, and so forth. Since each BSP has its own separate build directory in its own separate branch of the Git tree you can easily switch between different BSP builds.

## Workflow Examples

As previously noted, the Yocto Project kernel has built in Git integration. However, these utilities are not the only way to work with the kernel repository. Yocto Project has not made changes to Git or to other tools that would invalidate alternate workflows. Additionally, the way the kernel repository is constructed results in using only core Git functionality thus allowing any number of tools or front ends to use the resulting tree.

This section contains several workflow examples.

### Change Inspection: Kernel Changes/Commits

A common question when working with a BSP or kernel is: "What changes have been applied to this tree?"

In projects that have a collection of directories that contain patches to the kernel it is possible to inspect or "grep" the contents of the directories to get a general feel for the changes. This sort of patch inspection is not an efficient way to determine what has been done to the kernel. The reason it is inefficient is because there are many optional patches that are selected based on the kernel type and the feature description. Additionally, patches could exist in directories that are not included in the search.

A more efficient way to determine what has changed in the kernel is to use Git and inspect or search the kernel tree. This method gives you a full view of not only the source code modifications, but also provides the reasons for the changes.

#### What Changed in a BSP?

Following are a few examples that show how to use Git to examine changes. Note that because the Yocto Project Git repository does not break existing Git functionality and because there exists many permutations of these types of commands there are many more methods to discover changes.

Unless you provide a commit range (<kernel-type>..<bsp>-<kernel-type>), kernel.org history is blended with Yocto Project changes.

         # full description of the changes
         > git whatchanged <kernel type>..<kernel type>/<bsp>
            > eg: git whatchanged yocto/standard/base..yocto/standard/common-pc/base

         # summary of the changes
         > git log --pretty=oneline --abbrev-commit <kernel type>..<kernel type>/<bsp>

         # source code changes (one combined diff)
         > git diff <kernel type>..<kernel type>/<bsp>
         > git show <kernel type>..<kernel type>/<bsp>

         # dump individual patches per commit
         > git format-patch -o <dir> <kernel type>..<kernel type>/<bsp>

         # determine the change history of a particular file
         > git whatchanged <path to file>

         # determine the commits which touch each line in a file
         > git blame <path to file>

#### Show a Particular Feature or Branch Change

Significant features or branches are tagged in the Yocto Project tree to divide changes. Remember to first determine (or add) the tag of interest.

Because BSP branch, kernel.org, and feature tags are all present, there are many tags.

         # show the changes tagged by a feature
         > git show <tag>
            > eg: git show yaffs2

         # determine which branches contain a feature
         > git branch --contains <tag>

         # show the changes in a kernel type
         > git whatchanged yocto/base..<kernel type>
            > eg: git whatchanged yocto/base..yocto/standard/base

You can use many other comparisons to isolate BSP changes. For example, you can compare against kernel.org tags (e.g. v2.6.27.18, etc), or you can compare against subsystems (e.g. `git whatchanged mm`).

### Development: Saving Kernel Modifications

Another common operation is to build a BSP supplied by Yocto Project, make some changes, rebuild and then test. Those local changes often need to be exported, shared or otherwise maintained.

Since the Yocto Project kernel source tree is backed by Git, this activity is much easier as compared to with previous releases. Because Git tracks file modifications, additions and deletions, it is easy to modify the code and later realize that the changes should be saved. It is also easy to determine what has changed. This method also provides many tools to commit, undo and export those modifications.

There are many ways to save kernel modifications. The technique employed depends on the destination for the patches:

- Bulk storage

- Internal sharing either through patches or by using Git

- External submissions

- Exporting for integration into another SCM

Because of the following list of issues, the destination of the patches also influences the method for gathering them:

- Bisectability

- Commit headers

- Division of subsystems for separate submission or review

#### Bulk Export

This section describes how you can export in "bulk" changes that have not been separated or divided. This situation works well when you are simply storing patches outside of the kernel source repository, either permanently or temporarily, and you are not committing incremental changes during development.

This technique is not appropriate for full integration of upstream submission because changes are not properly divided and do not provide an avenue for per-change commit messages. Therefore, this example assumes that changes have not been committed incrementally during development and that you simply must gather and export them.

         # bulk export of ALL modifications without separation or division
         # of the changes

         > git add .
         > git commit -s -a -m >commit message<
            or
         > git commit -s -a # and interact with $EDITOR

The previous operations capture all the local changes in the project source tree in a single Git commit. And, that commit is also stored in the project's source tree.

Once the changes are exported, you can restore them manually using a template or through integration with the `default_kernel`.

#### Incremental/Planned Sharing

This section describes how to save modifications when you are making incremental commits or practicing planned sharing. The examples in this section assume that changes have been incrementally committed to the tree during development and now need to be exported. The sections that follow describe how you can export your changes internally through either patches or by using Git commands.

During development the following commands are of interest. For full Git documentation, refer to the Git man pages or to an online resource such as .

         # edit a file
         > vi >path</file
         # stage the change
         > git add >path</file
         # commit the change
         > git commit -s
         # remove a file
         > git rm >path</file
         # commit the change
         > git commit -s

         ... etc.

Distributed development with git is possible when you use a universally agreed-upon unique commit identifier (set by the creator of the commit) that maps to a specific change set with a specific parent. This identifier is created for you when you create a commit, and is re-created when you amend, alter or re-apply a commit. As an individual in isolation, this is of no interest. However, if you intend to share your tree with normal git push and pull operations for distributed development, you should consider the ramifications of changing a commit that you have already shared with others.

Assuming that the changes have not been pushed upstream, or pulled into another repository, you can update both the commit content and commit messages associated with development by using the following commands:

         > Git add >path</file
         > Git commit --amend
         > Git rebase or Git rebase -i

Again, assuming that the changes have not been pushed upstream, and that no pending works-in-progress exist (use `git status` to check) then you can revert (undo) commits by using the following commands:

         # remove the commit, update working tree and remove all
         # traces of the change
         > git reset --hard HEAD^
         # remove the commit, but leave the files changed and staged for re-commit
         > git reset --soft HEAD^
         # remove the commit, leave file change, but not staged for commit
         > git reset --mixed HEAD^

You can create branches, "cherry-pick" changes or perform any number of Git operations until the commits are in good order for pushing upstream or for pull requests. After a push or pull, commits are normally considered "permanent" and you should not modify them. If they need to be changed you can incrementally do so with new commits. These practices follow the standard Git workflow and the kernel.org best practices, which Yocto Project recommends.

It is recommended to tag or branch before adding changes to a Yocto Project BSP or before creating a new one. The reason for this recommendation is because the branch or tag provides a reference point to facilitate locating and exporting local changes.

##### Exporting Changes Internally by Using Patches

This section describes how you can extract committed changes from a working directory by exporting them as patches. Once extracted, you can use the patches for upstream submission, place them in a Yocto Project template for automatic kernel patching, or apply them in many other common uses.

This example shows how to create a directory with sequentially numbered patches. Once the directory is created, you can apply it to a repository using the `git am` command to reproduce the original commit and all the related information such as author, date, commit log, and so forth.

The new commit identifiers (ID) will be generated upon re-application. This action reflects that the commit is now applied to an underlying commit with a different ID.

         # <first-commit> can be a tag if one was created before development
         # began. It can also be the parent branch if a branch was created
         # before development began.

         > git format-patch -o <dir> <first commit>..<last commit>

In other words:

         # Identify commits of interest.

         # If the tree was tagged before development
         > git format-patch -o <save dir> <tag>

         # If no tags are available
         > git format-patch -o <save dir> HEAD^  # last commit
         > git format-patch -o <save dir> HEAD^^ # last 2 commits
         > git whatchanged # identify last commit
         > git format-patch -o <save dir> <commit id>
         > git format-patch -o <save dir> <rev-list>

##### Exporting Changes Internally by Using Git

This section describes how you can export changes from a working directory by pushing the changes into a master repository or by making a pull request. Once you have pushed the changes in the master repository you can then pull those same changes into a new kernel build at a later time.

Use this command form to push the changes:

         > git push ssh://<master_server>/<path_to_repo>
            <local_branch>:<remote_branch>

For example, the following command pushes the changes from your local branch `yocto/standard/common-pc/base` to the remote branch with the same name in the master repository `//git.mycompany.com/pub/git/kernel-2.6.37`.

         > git push ssh://git.mycompany.com/pub/git/kernel-2.6.37 \
            yocto/standard/common-pc/base:yocto/standard/common-pc/base

A pull request entails using `git request-pull` to compose an email to the maintainer requesting that a branch be pulled into the master repository, see for an example.

Other commands such as `git stash` or branching can also be used to save changes, but are not covered in this document.

#### Exporting Changes for External (Upstream) Submission

This section describes how to export changes for external upstream submission. If the patch series is large or the maintainer prefers to pull changes, you can submit these changes by using a pull request. However, it is common to sent patches as an email series. This method allows easy review and integration of the changes.

Before sending patches for review be sure you understand the community standards for submitting and documenting changes and follow their best practices. For example, kernel patches should follow standards such as:

- Documentation/SubmittingPatches (in any linux kernel source tree)

The messages used to commit changes are a large part of these standards. Consequently, be sure that the headers for each commit have the required information. If the initial commits were not properly documented or do not meet those standards, you can re-base by using the `git rebase -i` command to manipulate the commits and get them into the required format. Other techniques such as branching and cherry-picking commits are also viable options.

Once you complete the commits, you can generate the email that sends the patches to the maintainer(s) or lists that review and integrate changes. The command `git send-email` is commonly used to ensure that patches are properly formatted for easy application and avoid mailer-induced patch damage.

The following is an example of dumping patches for external submission:

         # dump the last 4 commits
         > git format-patch --thread -n -o ~/rr/ HEAD^^^^
         > git send-email --compose --subject '[RFC 0/N] <patch series summary>' \
          --to foo@yoctoproject.org --to bar@yoctoproject.org \
          --cc list@yoctoproject.org  ~/rr
         # the editor is invoked for the 0/N patch, and when complete the entire
         # series is sent via email for review

#### Exporting Changes for Import into Another SCM

When you want to export changes for import into another Source Code Manager (SCM) you can use any of the previously discussed techniques. However, if the patches are manually applied to a secondary tree and then that tree is checked into the SCM you can lose change information such as commit logs. Yocto Project does not recommend this process.

Many SCMs can directly import Git commits, or can translate Git patches so that information is not lost. Those facilities are SCM-dependent and you should use them whenever possible.

### Working with the Yocto Project Kernel in Another SCM

This section describes kernel development in another SCM, which is not the same as exporting changes to another SCM. For this scenario you use the Yocto Project build system to develop the kernel in a different SCM. The following must be true for you to accomplish this:

- The delivered Yocto Project kernel must be exported into the second SCM.

- Development must be exported from that secondary SCM into a format that can be used by the Yocto Project build system.

#### Exporting the Delivered Kernel to the SCM

Depending on the SCM it might be possible to export the entire Yocto Project kernel Git repository, branches and all, into a new environment. This method is preferred because it has the most flexibility and potential to maintain the meta data associated with each commit.

When a direct import mechanism is not available, it is still possible to export a branch (or series of branches) and check them into a new repository.

The following commands illustrate some of the steps you could use to import the yocto/standard/common-pc/base kernel into a secondary SCM:

         > git checkout yocto/standard/common-pc/base
         > cd .. ; echo linux/.git > .cvsignore
         > cvs import -m "initial import" linux MY_COMPANY start

You could now relocate the CVS repository and use it in a centralized manner.

The following commands illustrate how you can condense and merge two BSPs into a second SCM:

         > git checkout yocto/standard/common-pc/base
         > git merge yocto/standard/common-pc-64/base
         # resolve any conflicts and commit them
         > cd .. ; echo linux/.git > .cvsignore
         > cvs import -m "initial import" linux MY_COMPANY start

#### Importing Changes for the Build

Once development has reached a suitable point in the second development environment, you need to export the changes as patches. To export them place the changes in a recipe and automatically apply them to the kernel during patching.

### Creating a BSP Based on an Existing Similar BSP

This section overviews the process of creating a BSP based on an existing similar BSP. The information is introductory in nature and does not provide step-by-step examples. For detailed information on how to create a BSP given an existing similar BSP see the Yocto Project Development Manual [NEED LINK] or the wiki page.

The basic steps you need to follow are:

1. Make sure you have the Yocto Project source tree available. You should either create a Yocto Project Git repository (recommended), or you should get the Yocto Project release tarball and extract it.

2. Choose an existing BSP available with the Yocto Project. Try to map your board features as closely to the features of a BSP that is already supported and exists in the Yocto Project. Starting with something as close as possible to your board makes developing your BSP easier. You can find all the BSPs that are supported and ship with the Yocto Project on the Yocto Project's Download page at .

3. Be sure you have the Base BSP. You need to either have the Yocto Project Git repository set up or download the tarball of the base BSP. Either method gives you access to the BSP source files.

4. Make a copy of the existing BSP, thus isolating your new BSP work. Copying the existing BSP structure gives you a new area in which to work.

5. Make configuration and recipe changes to your new BSP. Configuration changes involve the files in the BSP's `conf` directory. Changes include creating a machine-specific configuration file and editing the `layer.conf` file. The configuration changes identify the kernel you will be using. Recipe changes include removing, modifying, or adding new recipe files that instruct the build process on what features to include in the image.

6. Prepare for the build. Before you actually initiate the build you need to set up the build environment by sourcing the environment initialization script. After setting up the environment you need to make some build configuration changes to the `local.conf` and `bblayers.conf` files.

7. Build the image. The Yocto Project uses the BitBake tool to create the image. You need to decide on the type of image you are going to build (e.g. minimal, base, core, sato, and so forth) and then start the build using the `bitbake` command.

### "-dirty" String

If kernel images are being built with "-dirty" on the end of the version string, this simply means that modifications in the source directory have not been committed.

         > git status

You can use the Git command above to report modified, removed, or added files. You should commit those changes to the tree regardless of whether they will be saved, exported, or used. Once you commit the changes you need to rebuild the kernel.

To brute force pickup and commit all such pending changes enter the following:

         > git add .
         > git commit -s -a -m "getting rid of -dirty"

Next, rebuild the kernel.