Yocto Project Linux Kernel Development Manual

You create this file in your custom layer. You also name it accordingly based on the linux-yocto recipe you are using. For example, if you are modifying the meta/recipes-kernel/linux/linux-yocto_3.4.bb recipe, the append file will typical be located as follows within your custom layer:

     <your-layer>/recipes-kernel/linux/linux-yocto_3.4.bbappend
                

The append file should initially extend the FILESPATH search path by prepending the directory that contains your files to the FILESEXTRAPATHS variable as follows:

     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
                

The path ${THISDIR}/${PN} expands to "linux-yocto" in the current directory for this example. If you add any new files that modify the kernel recipe and you have extended FILESPATH as described above, you must place the files in your layer in the following area:

     <your-layer>/recipes-kernel/linux/linux-yocto/
                

If you have a single patch or a small series of patches that you want to apply to the Linux kernel source, you can do so just as you would with any other recipe. You first copy the patches to the path added to FILESEXTRAPATHS in your .bbappend file as described in the previous section, and then reference them in SRC_URI statements.

For example, you can apply a three-patch series by adding the following lines to your linux-yocto .bbappend file in your layer:

     SRC_URI += "file://0001-first-change.patch"
     SRC_URI += "file://0002-first-change.patch"
     SRC_URI += "file://0003-first-change.patch"
                

The next time you run BitBake to build the Linux kernel, BitBake detects the change in the recipe and fetches and applies the patches before building the kernel.

For a detailed example showing how to patch the kernel, see the "Patching the Kernel" section in the Yocto Project Development Manual.

You can make wholesale or incremental changes to the Linux kernel .config file by including a defconfig and by specifying configuration fragments in the SRC_URI.

If you have a final Linux kernel .config file you want to use, copy it to a directory named files, which must be in your layer's recipes-kernel/linux directory, and name the file "defconfig". Then, add the following lines to your linux-yocto .bbappend file in your layer:

     FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
     SRC_URI += "file://defconfig"
                

The SRC_URI tells the build system how to search for the file, while the FILESEXTRAPATHS extends the FILESPATH variable (search directories) to include the files directory you created for the configuration changes.

Generally speaking, the preferred approach is to determine the incremental change you want to make and add that as a configuration fragment. For example, if you want to add support for a basic serial console, create a file named 8250.cfg in the files directory with the following content (without indentation):

     CONFIG_SERIAL_8250=y
     CONFIG_SERIAL_8250_CONSOLE=y
     CONFIG_SERIAL_8250_PCI=y
     CONFIG_SERIAL_8250_NR_UARTS=4
     CONFIG_SERIAL_8250_RUNTIME_UARTS=4
     CONFIG_SERIAL_CORE=y
     CONFIG_SERIAL_CORE_CONSOLE=y
                

Next, include this configuration fragment and extend the FILESPATH variable in your .bbappend file:

     FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
     SRC_URI += "file://8250.cfg"
                

The next time you run BitBake to build the Linux kernel, BitBake detects the change in the recipe and fetches and applies the new configuration before building the kernel.

For a detailed example showing how to configure the kernel, see the "Configuring the Kernel" section in the Yocto Project Development Manual.

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 above Git command 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 force a pickup and commit of all such pending changes, enter the following:

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

Next, rebuild the kernel.

You can manipulate the .config file used to build a linux-yocto recipe with the menuconfig command as follows:

     $ bitbake linux-yocto -c menuconfig
                

This command starts the Linux kernel configuration tool, which allows you to prepare a new .config file for the build. When you exit the tool, be sure to save your changes at the prompt.

The resulting .config file is located in ${WORKDIR} under the linux-${MACHINE}-${KTYPE}-build directory. You can use the entire .config file as the defconfig file as described in the "Changing the Configuration" section.

A better method is to create a configuration fragment using the differences between two configuration files: one previously created and saved, and one freshly created using the menuconfig tool.

To create a configuration fragment using this method, follow these steps:

Ultimately, the configuration fragment file needs to be a list of Linux kernel CONFIG_ assignments. It cannot be in diff format. Here is an example of a command that creates your configuration fragment file. Regardless of the exact command you use, plan on reviewing the output as you can usually remove some of the defaults:

     $ diff -Nurp config.orig .config | sed -n "s/^\+//p" > frag.cfg
                

See the "Changing the Configuration" section for information on how to use the output as a configuration fragment.

The kernel tools also provide configuration validation. You can use these tools to produce warnings for when a requested configuration does not appear in the final .config file or when you override a policy configuration in a hardware configuration fragment. Here is an example with some sample output of the command that runs these tools:

     $ bitbake linux-yocto -c kernel_configcheck -f

     ...

     NOTE: validating kernel configuration
     This BSP sets 3 invalid/obsolete kernel options.
     These config options are not offered anywhere within this kernel.
     The full list can be found in your kernel src dir at:
     meta/cfg/standard/mybsp/invalid.cfg

     This BSP sets 21 kernel options that are possibly non-hardware related.
     The full list can be found in your kernel src dir at:
     meta/cfg/standard/mybsp/specified_non_hdw.cfg

     WARNING: There were 2 hardware options requested that do not
              have a corresponding value present in the final ".config" file.
              This probably means you are not't getting the config you wanted.
              The full list can be found in your kernel src dir at:
              meta/cfg/standard/mybsp/mismatch.cfg
                

The output describes the various problems that you can encounter along with where to find the offending configuration items. You can use the information in the logs to adjust your configuration files and then repeat the kernel_configme and kernel_configcheck commands until they produce no warnings.

For more information on how to use the menuconfig tool, see the "Using menuconfig" section in the Yocto Project Development Manual.

You can experiment with source code changes and create a simple patch without leaving the BitBake environment. To get started, be sure to complete a build at least through the kernel configuration task:

     $ bitbake linux-yocto -c kernel_configme -f
                

Taking this step ensures you have the sources prepared and the configuration completed. You can find the sources in the ${WORKDIR}/linux directory.

You can edit the sources as you would any other Linux source tree. However, keep in mind that you will lose changes if you trigger the fetch task for the recipe. You can avoid triggering this task by not issuing BitBake's cleanall, cleansstate, or forced fetch commands. Also, do not modify the recipe itself while working with temporary changes or BitBake might run the fetch command depending on the changes to the recipe.

To test your temporary changes, instruct BitBake to run the compile again. The -f option forces the command to run even though BitBake might think it has already done so:

     $ bitbake linux-yocto -c compile -f
                

If the compile fails, you can update the sources and repeat the compile. Once compilation is successful, you can inspect and test the resulting build (i.e. kernel, modules, and so forth) from the Build Directory:

     ${WORKDIR}/linux-${MACHINE}-${KTYPE}-build
                

Alternatively, you can run the deploy command to place the kernel image in the tmp/deploy/images directory:

	$ bitbake linux-yocto -c deploy
                

And, of course, you can perform the remaining installation and packaging steps by issuing:

	$ bitbake linux-yocto
                

For rapid iterative development, the edit-compile-repeat loop described in this section is preferable to rebuilding the entire recipe because the installation and packaging tasks are very time consuming.

Once you are satisfied with your source code modifications, you can make them permanent by generating patches and applying them to the SRC_URI statement as described in section "Applying Patches" section. If you are not familiar with generating patches, refer to the "Creating the Patch" section in the Yocto Project Development Manual.

Following are a few examples that show how to use Git commands to examine changes. These examples are by no means the only way to see changes.

To see a full range of the changes, use the git whatchanged command and specify a commit range for the branch (<commit>..<commit>).

Here is an example that looks at what has changed in the emenlow branch of the linux-yocto-3.4 kernel. The lower commit range is the commit associated with the standard/base branch, while the upper commit range is the commit associated with the standard/emenlow branch.

     $ git whatchanged origin/standard/base..origin/standard/emenlow
                

To see short, one line summaries of changes use the git log command:

     $ git log --oneline origin/standard/base..origin/standard/emenlow
                

Use this command to see code differences for the changes:

     $ git diff origin/standard/base..origin/standard/emenlow
                

Use this command to see the commit log messages and the text differences:

     $ git show origin/standard/base..origin/standard/emenlow
                

Use this command to create individual patches for each change. Here is an example that that creates patch files for each commit and places them in your Documents directory:

     $ git format-patch -o $HOME/Documents origin/standard/base..origin/standard/emenlow
                

Tags in the Yocto Project kernel tree divide changes for significant features or branches. The git show <tag> command shows changes based on a tag. Here is an example that shows systemtap changes:

     $ git show systemtap
                

You can use the git branch --contains <tag> command to show the branches that contain a particular feature. This command shows the branches that contain the systemtap feature:

     $ git branch --contains systemtap
                

When stored in recipe-space, the kernel Metadata files reside in a directory hierarchy below FILESEXTRAPATHS. For a linux-yocto recipe or for a Linux kernel recipe derived by copying and modifying oe-core/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb to a recipe in your layer, FILESEXTRAPATHS is typically set to ${THISDIR}/${PN}. See the "Modifying an Existing Recipe" section for more information.

Here is an example that shows a trivial tree of kernel Metadata stored in recipe-space within a BSP layer:

     meta-my_bsp_layer/
     `-- recipes-kernel
         `-- linux
             `-- linux-yocto
                 |-- bsp-standard.scc
                 |-- bsp.cfg
                 `-- standard.cfg
            

When the Metadata is stored in recipe-space, you must take steps to ensure BitBake has the necessary information to decide what files to fetch and when they need to be fetched again. It is only necessary to specify the .scc files on the SRC_URI. BitBake parses them and fetches any files referenced in the .scc files by the include, patch, or kconf commands. Because of this, it is necessary to bump the recipe PR value when changing the content of files not explicitly listed in the SRC_URI.

When stored in-tree, the kernel Metadata files reside in the meta directory of the Linux kernel sources. The meta directory can be present in the same repository branch as the sources, such as "master", or meta can be its own orphan branch.

For the purposes of this document, we will discuss all in-tree Metadata as residing below the meta/cfg/kernel-cache directory.

Following is an example that shows how a trivial tree of Metadata is stored in a custom Linux kernel Git repository:

     meta/
     `-- cfg
         `-- kernel-cache
             |-- bsp-standard.scc
             |-- bsp.cfg
             `-- standard.cfg
            

To use a branch different from where the sources reside, specify the branch in the KMETA variable in your Linux kernel recipe. Here is an example:

     KMETA = "meta"
            

To use the same branch as the sources, set KMETA to an empty string:

     KMETA = ""
            

If you are working with your own sources and want to create an orphan meta branch, use these commands from within your Linux kernel Git repository:

     $ git checkout --orphan meta
     $ git rm -rf .
     $ git commit --allow-empty -m "Create orphan meta branch"
            

If you modify the Metadata in the linux-yocto meta branch, you must not forget to update the SRCREV statements in the kernel's recipe. In particular, you need to update the SRCREV_meta variable to match the commit in the KMETA branch you wish to use. Changing the data in these branches and not updating the SRCREV statements to match will cause the build to fetch an older commit.

The simplest unit of kernel Metadata is the configuration-only feature. This feature consists of one or more Linux kernel configuration parameters in a configuration fragment file (.cfg) and an .scc file that describes the fragment.

The Symmetric Multi-Processing (SMP) fragment included in the linux-yocto-3.4 Git repository consists of the following two files:

     cfg/smp.scc:
        define KFEATURE_DESCRIPTION "Enable SMP"
        kconf hardware smp.cfg

     cfg/smp.cfg:
        CONFIG_SMP=y
        CONFIG_SCHED_SMT=y
            

You can find information on configuration fragment files in the "Creating Configuration Fragments" section of the Yocto Project Development Manual and in the "Generating Configuration Files" section earlier in this manual.

KFEATURE_DESCRIPTION provides a short description of the fragment. Higher level kernel tools use this description.

The kconf command is used to include the actual configuration fragment in an .scc file, and the "hardware" keyword identifies the fragment as being hardware enabling, as opposed to general policy, which would use the "non-hardware" keyword. The distinction is made for the benefit of the configuration validation tools, which warn you if a hardware fragment overrides a policy set by a non-hardware fragment.

As described in the "Generating Configuration Files" section, you can use the following BitBake command to audit your configuration:

     $ bitbake linux-yocto -c kernel_configcheck -f
            

Patch descriptions are very similar to configuration fragment descriptions, which are described in the previous section. However, instead of a .cfg file, these descriptions work with source patches.

A typical patch includes a description file and the patch itself:

     patches/mypatch.scc:
        patch mypatch.patch

     patches/mypatch.patch:
        <typical-patch>
            

You can create the typical .patch file using diff -Nurp or git format-patch.

The description file can include multiple patch statements, one per patch.

Features are complex kernel Metadata types that consist of configuration fragments (kconf), patches (patch), and possibly other feature description files (include).

Here is an example that shows a feature description file:

     features/myfeature.scc
        define KFEATURE_DESCRIPTION "Enable myfeature"

        patch 0001-myfeature-core.patch
        patch 0002-myfeature-interface.patch

        include cfg/myfeature_dependency.scc
        kconf non-hardware myfeature.cfg
            

This example shows how the patch and kconf commands are used as well as how an additional feature description file is included.

Typically, features are less granular than configuration fragments and are more likely than configuration fragments and patches to be the types of things you want to specify in the KERNEL_FEATURES variable of the Linux kernel recipe. See the "Using Kernel Metadata in a Recipe" section earlier in the manual.

A kernel type defines a high-level kernel policy by aggregating non-hardware configuration fragments with patches you want to use when building a Linux kernels of a specific type. Syntactically, kernel types are no different than features as described in the "Features" section. The LINUX_KERNEL_TYPE variable in the kernel recipe selects the kernel type. See the "Using Kernel Metadata in a Recipe" section for more information.

As an example, the linux-yocto-3.4 tree defines three kernel types: "standard", "tiny", and "preempt-rt":

The "standard" kernel type is defined by standard.scc:

     # Include this kernel type fragment to get the standard features and
     # configuration values.

     # Include all standard features
     include standard-nocfg.scc

     kconf non-hardware standard.cfg

     # individual cfg block section
     include cfg/fs/devtmpfs.scc
     include cfg/fs/debugfs.scc
     include cfg/fs/btrfs.scc
     include cfg/fs/ext2.scc
     include cfg/fs/ext3.scc
     include cfg/fs/ext4.scc

     include cfg/net/ipv6.scc
     include cfg/net/ip_nf.scc
     include cfg/net/ip6_nf.scc
     include cfg/net/bridge.scc
            

As with any .scc file, a kernel type definition can aggregate other .scc files with include commands. These definitions can also directly pull in configuration fragments and patches with the kconf and patch commands, respectively.

BSP descriptions combine kernel types with hardware-specific features. The hardware-specific portion is typically defined independently, and then aggregated with each supported kernel type. Consider this simple BSP description that supports the "mybsp" machine:

     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386

        kconf mybsp.cfg
            

Every BSP description should define the KMACHINE, KTYPE, and KARCH variables. These variables allow the OpenEmbedded build system to identify the description as meeting the criteria set by the recipe being built. This simple example supports the "mybsp" machine for the "standard" kernel and the "i386" architecture.

Be aware that a hard link between the KTYPE variable and a kernel type description file does not exist. Thus, if you do not have kernel types defined in your kernel Metadata, you only need to ensure that the kernel recipe's LINUX_KERNEL_TYPE variable and the KTYPE variable in the BSP description file match.

If you did want to separate your kernel policy from your hardware configuration, you could do so by specifying a kernel type, such as "standard" and including that description file in the BSP description file. See the "Kernel Types" section for more information.

You might also have multiple hardware configurations that you aggregate into a single hardware description file that you could include in the BSP description file, rather than referencing a single .cfg file. Consider the following:

     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386

        include standard.scc
        include mybsp-hw.scc
            

In the above example, standard.scc aggregates all the configuration fragments, patches, and features that make up your standard kernel policy whereas mybsp-hw.scc aggregates all those necessary to support the hardware available on the "mybsp" machine. For information on how to break a complete .config file into the various configuration fragments, see the "Generating Configuration Files" section.

Many real-world examples are more complex. Like any other .scc file, BSP descriptions can aggregate features. Consider the Fish River Island 2 (fri2) BSP definition from the linux-yocto-3.4 Git repository:

     fri2.scc:
        kconf hardware fri2.cfg

        include cfg/x86.scc
        include features/eg20t/eg20t.scc
        include cfg/dmaengine.scc
        include features/ericsson-3g/f5521gw.scc
        include features/power/intel.scc
        include cfg/efi.scc
        include features/usb/ehci-hcd.scc
        include features/usb/ohci-hcd.scc
        include features/iwlwifi/iwlwifi.scc
            

The fri2.scc description file includes a hardware configuration fragment (fri2.cfg) specific to the Fish River Island 2 BSP as well as several more general configuration fragments and features enabling hardware found on the machine. This description file is then included in each of the three "fri2" description files for the supported kernel types (i.e. "standard", "preempt-rt", and "tiny"). Consider the "fri2" description for the "standard" kernel type:

     fri2-standard.scc:
        define KMACHINE fri2
        define KTYPE standard
        define KARCH i386

        include ktypes/standard/standard.scc
        branch fri2

        git merge emgd-1.14

        include fri2.scc

        # Extra fri2 configs above the minimal defined in fri2.scc
        include cfg/efi-ext.scc
        include features/drm-emgd/drm-emgd.scc
        include cfg/vesafb.scc

        # default policy for standard kernels
        include cfg/usb-mass-storage.scc
            

The include command midway through the file includes the fri2.scc description that defines all hardware enablements for the BSP that is common to all kernel types. Using this command significantly reduces duplication.

This "fri2" standard description introduces a few more variables and commands that are worth further discussion. Notice the branch fri2 command, which creates a machine-specific branch into which source changes are applied. With this branch set up, the git merge command uses Git to merge in a feature branch named "emgd-1.14". You could also handle this with the patch command. However, for commonly used features such as this, feature branches are a convenient mechanism. See the "Feature Branches" section for more information.

Now consider the "fri2" description for the "tiny" kernel type:

     fri2-tiny.scc:
        define KMACHINE fri2
        define KTYPE tiny
        define KARCH i386

        include ktypes/tiny/tiny.scc
        branch fri2

        include fri2.scc
            

As you might expect, the "tiny" description includes quite a bit less. In fact, it includes only the minimal policy defined by the "tiny" kernel type and the hardware-specific configuration required for booting the machine along with the most basic functionality of the system as defined in the base "fri2" description file.

Notice again the three critical variables: KMACHINE, KTYPE, and KARCH. Of these variables, only the KTYPE has changed. It is now set to "tiny".

if you are reusing patches from an external tree and are not working on the patches, you might find the encapsulated feature to be appropriate. Given this scenario, you do not need to create any branches in the source repository. Rather, you just take the static patches you need and encapsulate them within a feature description. Once you have the feature description, you simply include that into the BSP description as described in the "BSP Descriptions" section.

You can find information on how to create patches and BSP descriptions in the "Patches" and "BSP Descriptions" sections.

When you have multiple machines and architectures to support, or you are actively working on board support, it is more efficient to create branches in the repository based on individual machines. Having machine branches allows common source to remain in the "master" branch with any features specific to a machine stored in the appropriate machine branch. This organization method frees you from continually reintegrating your patches into a feature.

Once you have a new branch, you can set up your kernel Metadata to use the branch a couple different ways. In the recipe, you can specify the new branch as the KBRANCH to use for the board as follows:

     KBRANCH = "mynewbranch"
            

Another method is to use the branch command in the BSP description:

     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386
        include standard.scc

        branch mynewbranch

        include mybsp-hw.scc
            

If you find yourself with numerous branches, you might consider using a hierarchical branching system similar to what the linux-yocto Linux kernel repositories use:

     <common>/<kernel_type>/<machine>
            

If you had two kernel types, "standard" and "small" for instance, and three machines, the branches in your Git repository might look like this:

     common/base
     common/standard/base
     common/standard/machine_a
     common/standard/machine_b
     common/standard/machine_c
     common/small/base
     common/small/machine_a
            

This organization can help clarify the branch relationships. In this case, common/standard/machine_a includes everything in common/base and common/standard/base. The "standard" and "small" branches add sources specific to those kernel types that for whatever reason are not appropriate for the other branches.

When you are actively developing new features, it can be more efficient to work with that feature as a branch, rather than as a set of patches that have to be regularly updated. The Yocto Project Linux kernel tools provide for this with the git merge command.

To merge a feature branch into a BSP, insert the git merge command after any branch commands:

     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386
        include standard.scc

        branch mynewbranch
        git merge myfeature

        include mybsp-hw.scc