# Common Tasks

This chapter presents several common tasks you perform when you work with the Yocto Project Linux kernel. These tasks include preparing your host development system for kernel development, preparing a layer, modifying an existing recipe, patching the kernel, configuring the kernel, iterative development, working with your own sources, and incorporating out-of-tree modules.

The examples presented in this chapter work with the Yocto Project 2.4 Release and forward.

## Preparing the Build Host to Work on the Kernel

Before you can do any kernel development, you need to be sure your build host is set up to use the Yocto Project. For information on how to get set up, see the "Preparing the Build Host" section in the Yocto Project Development Tasks Manual. Part of preparing the system is creating a local Git repository of the Source Directory (`poky`) on your system. Follow the steps in the "Cloning the `poky` Repository" section in the Yocto Project Development Tasks Manual to set up your Source Directory.

Be sure you check out the appropriate development branch or you create your local branch by checking out a specific tag to get the desired version of Yocto Project. See the "Checking Out by Branch in Poky" and "Checking Out by Tag in Poky" sections in the Yocto Project Development Tasks Manual for more information.

Kernel development is best accomplished using `devtool` and not through traditional kernel workflow methods. The remainder of this section provides information for both scenarios.

### Getting Ready to Develop Using `devtool`

Follow these steps to prepare to update the kernel image using `devtool`. Completing this procedure leaves you with a clean kernel image and ready to make modifications as described in the "Using `devtool` to Patch the Kernel" section:

1. Initialize the BitBake Environment: Before building an extensible SDK, you need to initialize the BitBake build environment by sourcing the build environment script (i.e. `oe-init-build-env`):

            $ cd ~/poky
            $ source oe-init-build-env

   The previous commands assume the Source Repositories (i.e. `poky`) have been cloned using Git and the local repository is named "poky".

2. Prepare Your `local.conf` File: By default, the `MACHINE` variable is set to "qemux86", which is fine if you are building for the QEMU emulator in 32-bit mode. However, if you are not, you need to set the `MACHINE` variable appropriately in your `conf/local.conf` file found in the Build Directory (i.e. `~/poky/build` in this example).

   Also, since you are preparing to work on the kernel image, you need to set the `MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS` variable to include kernel modules.

   This example uses the default "qemux86" for the `MACHINE` variable but needs to add the "kernel-modules":

            MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += "kernel-modules"

3. Create a Layer for Patches: You need to create a layer to hold patches created for the kernel image. You can use the `bitbake-layers create-layer` command as follows:

            $ cd ~/poky/build
            $ bitbake-layers create-layer ../../meta-mylayer
            NOTE: Starting bitbake server...
            Add your new layer with 'bitbake-layers add-layer ../../meta-mylayer'
            $

   For background information on working with common and BSP layers, see the "Understanding and Creating Layers" section in the Yocto Project Development Tasks Manual and the "BSP Layers" section in the Yocto Project Board Support (BSP) Developer's Guide, respectively. For information on how to use the `bitbake-layers create-layer` command to quickly set up a layer, see the "Creating a General Layer Using the `bitbake-layers` Script" section in the Yocto Project Development Tasks Manual.

4. Inform the BitBake Build Environment About Your Layer: As directed when you created your layer, you need to add the layer to the `BBLAYERS` variable in the `bblayers.conf` file as follows:

            $ cd ~/poky/build
            $ bitbake-layers add-layer ../../meta-mylayer
            NOTE: Starting bitbake server...
            $

5. Build the Extensible SDK: Use BitBake to build the extensible SDK specifically for use with images to be run using QEMU:

            $ cd ~/poky/build
            $ bitbake core-image-minimal -c populate_sdk_ext

   Once the build finishes, you can find the SDK installer file (i.e. `*.sh` file) in the following directory:

            ~/poky/build/tmp/deploy/sdk

   For this example, the installer file is named `poky-glibc-x86_64-core-image-minimal-i586-toolchain-ext-&DISTRO;.sh`

6. Install the Extensible SDK: Use the following command to install the SDK. For this example, install the SDK in the default `~/poky_sdk` directory:

            $ cd ~/poky/build/tmp/deploy/sdk
            $ ./poky-glibc-x86_64-core-image-minimal-i586-toolchain-ext-&DISTRO;.sh
            Poky (Yocto Project Reference Distro) Extensible SDK installer version &DISTRO;
            ============================================================================
            Enter target directory for SDK (default: ~/poky_sdk):
            You are about to install the SDK to "/home/scottrif/poky_sdk". Proceed [Y/n]? Y
            Extracting SDK......................................done
            Setting it up...
            Extracting buildtools...
            Preparing build system...
            Parsing recipes: 100% |#################################################################| Time: 0:00:52
            Initializing tasks: 100% |############## ###############################################| Time: 0:00:04
            Checking sstate mirror object availability: 100% |######################################| Time: 0:00:00
            Parsing recipes: 100% |#################################################################| Time: 0:00:33
            Initializing tasks: 100% |##############################################################| Time: 0:00:00
            done
            SDK has been successfully set up and is ready to be used.
            Each time you wish to use the SDK in a new shell session, you need to source the environment setup script e.g.
             $ . /home/scottrif/poky_sdk/environment-setup-i586-poky-linux

7. Set Up a New Terminal to Work With the Extensible SDK: You must set up a new terminal to work with the SDK. You cannot use the same BitBake shell used to build the installer.

   After opening a new shell, run the SDK environment setup script as directed by the output from installing the SDK:

            $ source ~/poky_sdk/environment-setup-i586-poky-linux
            "SDK environment now set up; additionally you may now run devtool to perform development tasks.
            Run devtool --help for further details.

   If you get a warning about attempting to use the extensible SDK in an environment set up to run BitBake, you did not use a new shell.

8. Build the Clean Image: The final step in preparing to work on the kernel is to build an initial image using `devtool` in the new terminal you just set up and initialized for SDK work:

            $ devtool build-image
            Parsing recipes: 100% |##########################################| Time: 0:00:05
            Parsing of 830 .bb files complete (0 cached, 830 parsed). 1299 targets, 47 skipped, 0 masked, 0 errors.
            WARNING: No packages to add, building image core-image-minimal unmodified
            Loading cache: 100% |############################################| Time: 0:00:00
            Loaded 1299 entries from dependency cache.
            NOTE: Resolving any missing task queue dependencies
            Initializing tasks: 100% |#######################################| Time: 0:00:07
            Checking sstate mirror object availability: 100% |###############| Time: 0:00:00
            NOTE: Executing SetScene Tasks
            NOTE: Executing RunQueue Tasks
            NOTE: Tasks Summary: Attempted 2866 tasks of which 2604 didn't need to be rerun and all succeeded.
            NOTE: Successfully built core-image-minimal. You can find output files in /home/scottrif/poky_sdk/tmp/deploy/images/qemux86

   If you were building for actual hardware and not for emulation, you could flash the image to a USB stick on `/dev/sdd` and boot your device. For an example that uses a Minnowboard, see the TipsAndTricks/KernelDevelopmentWithEsdk Wiki page.

At this point you have set up to start making modifications to the kernel by using the extensible SDK. For a continued example, see the "Using `devtool` to Patch the Kernel" section.

### Getting Ready for Traditional Kernel Development

Getting ready for traditional kernel development using the Yocto Project involves many of the same steps as described in the previous section. However, you need to establish a local copy of the kernel source since you will be editing these files.

Follow these steps to prepare to update the kernel image using traditional kernel development flow with the Yocto Project. Completing this procedure leaves you ready to make modifications to the kernel source as described in the "Using Traditional Kernel Development to Patch the Kernel" section:

1. Initialize the BitBake Environment: Before you can do anything using BitBake, you need to initialize the BitBake build environment by sourcing the build environment script (i.e. `oe-init-build-env`). Also, for this example, be sure that the local branch you have checked out for `poky` is the Yocto Project &DISTRO_NAME; branch. If you need to checkout out the &DISTRO_NAME; branch, see the "Checking out by Branch in Poky" section in the Yocto Project Development Tasks Manual.

            $ cd ~/poky
            $ git branch
            master
            * &DISTRO_NAME;
            $ source oe-init-build-env

   The previous commands assume the Source Repositories (i.e. `poky`) have been cloned using Git and the local repository is named "poky".

2. Prepare Your `local.conf` File: By default, the `MACHINE` variable is set to "qemux86", which is fine if you are building for the QEMU emulator in 32-bit mode. However, if you are not, you need to set the `MACHINE` variable appropriately in your `conf/local.conf` file found in the Build Directory (i.e. `~/poky/build` in this example).

   Also, since you are preparing to work on the kernel image, you need to set the `MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS` variable to include kernel modules.

   This example uses the default "qemux86" for the `MACHINE` variable but needs to add the "kernel-modules":

            MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += "kernel-modules"

3. Create a Layer for Patches: You need to create a layer to hold patches created for the kernel image. You can use the `bitbake-layers create-layer` command as follows:

            $ cd ~/poky/build
            $ bitbake-layers create-layer ../../meta-mylayer
            NOTE: Starting bitbake server...
            Add your new layer with 'bitbake-layers add-layer ../../meta-mylayer'

   For background information on working with common and BSP layers, see the "Understanding and Creating Layers" section in the Yocto Project Development Tasks Manual and the "BSP Layers" section in the Yocto Project Board Support (BSP) Developer's Guide, respectively. For information on how to use the `bitbake-layers create-layer` command to quickly set up a layer, see the "Creating a General Layer Using the `bitbake-layers` Script" section in the Yocto Project Development Tasks Manual.

4. Inform the BitBake Build Environment About Your Layer: As directed when you created your layer, you need to add the layer to the `BBLAYERS` variable in the `bblayers.conf` file as follows:

            $ cd ~/poky/build
            $ bitbake-layers add-layer ../../meta-mylayer
            NOTE: Starting bitbake server ...
            $

5. Create a Local Copy of the Kernel Git Repository: You can find Git repositories of supported Yocto Project kernels organized under "Yocto Linux Kernel" in the Yocto Project Source Repositories at .

   For simplicity, it is recommended that you create your copy of the kernel Git repository outside of the Source Directory, which is usually named `poky`. Also, be sure you are in the `standard/base` branch.

   The following commands show how to create a local copy of the `linux-yocto-4.12` kernel and be in the `standard/base` branch.

   The `linux-yocto-4.12` kernel can be used with the Yocto Project 2.4 release and forward. You cannot use the `linux-yocto-4.12` kernel with releases prior to Yocto Project 2.4:

            $ cd ~
            $ git clone git://git.yoctoproject.org/linux-yocto-4.12 --branch standard/base
            Cloning into 'linux-yocto-4.12'...
            remote: Counting objects: 6097195, done.
            remote: Compressing objects: 100% (901026/901026), done.
            remote: Total 6097195 (delta 5152604), reused 6096847 (delta 5152256)
            Receiving objects: 100% (6097195/6097195), 1.24 GiB | 7.81 MiB/s, done.
            Resolving deltas: 100% (5152604/5152604), done.
            Checking connectivity... done.
            Checking out files: 100% (59846/59846), done.

6. Create a Local Copy of the Kernel Cache Git Repository: For simplicity, it is recommended that you create your copy of the kernel cache Git repository outside of the Source Directory, which is usually named `poky`. Also, for this example, be sure you are in the `yocto-4.12` branch.

   The following commands show how to create a local copy of the `yocto-kernel-cache` and be in the `yocto-4.12` branch:

            $ cd ~
            $ git clone git://git.yoctoproject.org/yocto-kernel-cache --branch yocto-4.12
            Cloning into 'yocto-kernel-cache'...
            remote: Counting objects: 22639, done.
            remote: Compressing objects: 100% (9761/9761), done.
            remote: Total 22639 (delta 12400), reused 22586 (delta 12347)
            Receiving objects: 100% (22639/22639), 22.34 MiB | 6.27 MiB/s, done.
            Resolving deltas: 100% (12400/12400), done.
            Checking connectivity... done.

At this point, you are ready to start making modifications to the kernel using traditional kernel development steps. For a continued example, see the "Using Traditional Kernel Development to Patch the Kernel" section.

## Creating and Preparing a Layer

If you are going to be modifying kernel recipes, it is recommended that you create and prepare your own layer in which to do your work. Your layer contains its own BitBake append files (`.bbappend`) and provides a convenient mechanism to create your own recipe files (`.bb`) as well as store and use kernel patch files. For background information on working with layers, see the "Understanding and Creating Layers" section in the Yocto Project Development Tasks Manual.

### Tip

The Yocto Project comes with many tools that simplify tasks you need to perform. One such tool is the `bitbake-layers create-layer` command, which simplifies creating a new layer. See the "Creating a General Layer Using the `bitbake-layers` Script" section in the Yocto Project Development Tasks Manual for information on how to use this script to quick set up a new layer.

To better understand the layer you create for kernel development, the following section describes how to create a layer without the aid of tools. These steps assume creation of a layer named `mylayer` in your home directory:

1. Create Structure: Create the layer's structure:

            $ cd $HOME
            $ mkdir meta-mylayer
            $ mkdir meta-mylayer/conf
            $ mkdir meta-mylayer/recipes-kernel
            $ mkdir meta-mylayer/recipes-kernel/linux
            $ mkdir meta-mylayer/recipes-kernel/linux/linux-yocto

   The `conf` directory holds your configuration files, while the `recipes-kernel` directory holds your append file and eventual patch files.

2. Create the Layer Configuration File: Move to the `meta-mylayer/conf` directory and create the `layer.conf` file as follows:

            # We have a conf and classes directory, add to BBPATH
            BBPATH .= ":${LAYERDIR}"

            # We have recipes-* directories, add to BBFILES
            BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
                        ${LAYERDIR}/recipes-*/*/*.bbappend"

            BBFILE_COLLECTIONS += "mylayer"
            BBFILE_PATTERN_mylayer = "^${LAYERDIR}/"
            BBFILE_PRIORITY_mylayer = "5"

   Notice `mylayer` as part of the last three statements.

3. Create the Kernel Recipe Append File: Move to the `meta-mylayer/recipes-kernel/linux` directory and create the kernel's append file. This example uses the `linux-yocto-4.12` kernel. Thus, the name of the append file is `linux-yocto_4.12.bbappend`:

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

            SRC_URI_append += "file://patch-file-one"
            SRC_URI_append += "file://patch-file-two"
            SRC_URI_append += "file://patch-file-three"

   The `FILESEXTRAPATHS` and `SRC_URI` statements enable the OpenEmbedded build system to find patch files. For more information on using append files, see the "Using .bbappend Files in Your Layer" section in the Yocto Project Development Tasks Manual.

## Modifying an Existing Recipe

In many cases, you can customize an existing linux-yocto recipe to meet the needs of your project. Each release of the Yocto Project provides a few Linux kernel recipes from which you can choose. These are located in the Source Directory in `meta/recipes-kernel/linux`.

Modifying an existing recipe can consist of the following:

- Creating the append file

- Applying patches

- Changing the configuration

Before modifying an existing recipe, be sure that you have created a minimal, custom layer from which you can work. See the "Creating and Preparing a Layer" section for information.

### Creating the Append File

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_4.12.bb` recipe, the append file will typically be located as follows within your custom layer:

         your-layer/recipes-kernel/linux/linux-yocto_4.12.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 are working on a new machine Board Support Package (BSP), be sure to refer to the Yocto Project Board Support Package (BSP) Developer's Guide.

As an example, consider the following append file used by the BSPs in `meta-yocto-bsp`:

         meta-yocto-bsp/recipes-kernel/linux/linux-yocto_4.12.bbappend

The following listing shows the file. Be aware that the actual commit ID strings in this example listing might be different than the actual strings in the file from the `meta-yocto-bsp` layer upstream.

         KBRANCH_genericx86  = "standard/base"
         KBRANCH_genericx86-64  = "standard/base"

         KMACHINE_genericx86 ?= "common-pc"
         KMACHINE_genericx86-64 ?= "common-pc-64"
         KBRANCH_edgerouter = "standard/edgerouter"
         KBRANCH_beaglebone = "standard/beaglebone"
         KBRANCH_mpc8315e-rdb = "standard/fsl-mpc8315e-rdb"

         SRCREV_machine_genericx86    ?= "d09f2ce584d60ecb7890550c22a80c48b83c2e19"
         SRCREV_machine_genericx86-64 ?= "d09f2ce584d60ecb7890550c22a80c48b83c2e19"
         SRCREV_machine_edgerouter ?= "b5c8cfda2dfe296410d51e131289fb09c69e1e7d"
         SRCREV_machine_beaglebone ?= "b5c8cfda2dfe296410d51e131289fb09c69e1e7d"
         SRCREV_machine_mpc8315e-rdb ?= "2d1d010240846d7bff15d1fcc0cb6eb8a22fc78a"

         COMPATIBLE_MACHINE_genericx86 = "genericx86"
         COMPATIBLE_MACHINE_genericx86-64 = "genericx86-64"
         COMPATIBLE_MACHINE_edgerouter = "edgerouter"
         COMPATIBLE_MACHINE_beaglebone = "beaglebone"
         COMPATIBLE_MACHINE_mpc8315e-rdb = "mpc8315e-rdb"

         LINUX_VERSION_genericx86 = "4.12.7"
         LINUX_VERSION_genericx86-64 = "4.12.7"
         LINUX_VERSION_edgerouter = "4.12.10"
         LINUX_VERSION_beaglebone = "4.12.10"
         LINUX_VERSION_mpc8315e-rdb = "4.12.10"

This append file contains statements used to support several BSPs that ship with the Yocto Project. The file defines machines using the `COMPATIBLE_MACHINE` variable and uses the `KMACHINE` variable to ensure the machine name used by the OpenEmbedded build system maps to the machine name used by the Linux Yocto kernel. The file also uses the optional `KBRANCH` variable to ensure the build process uses the appropriate kernel branch.

Although this particular example does not use it, the `KERNEL_FEATURES` variable could be used to enable features specific to the kernel. The append file points to specific commits in the Source Directory Git repository and the `meta` Git repository branches to identify the exact kernel needed to build the BSP.

One thing missing in this particular BSP, which you will typically need when developing a BSP, is the kernel configuration file (`.config`) for your BSP. When developing a BSP, you probably have a kernel configuration file or a set of kernel configuration files that, when taken together, define the kernel configuration for your BSP. You can accomplish this definition by putting the configurations in a file or a set of files inside a directory located at the same level as your kernel's append file and having the same name as the kernel's main recipe file. With all these conditions met, simply reference those files in the `SRC_URI` statement in the append file.

For example, suppose you had some configuration options in a file called `network_configs.cfg`. You can place that file inside a directory named `linux-yocto` and then add a `SRC_URI` statement such as the following to the append file. When the OpenEmbedded build system builds the kernel, the configuration options are picked up and applied.

         SRC_URI += "file://network_configs.cfg"

To group related configurations into multiple files, you perform a similar procedure. Here is an example that groups separate configurations specifically for Ethernet and graphics into their own files and adds the configurations by using a `SRC_URI` statement like the following in your append file:

         SRC_URI += "file://myconfig.cfg \
                     file://eth.cfg \
                     file://gfx.cfg"

Another variable you can use in your kernel recipe append file is the `FILESEXTRAPATHS` variable. When you use this statement, you are extending the locations used by the OpenEmbedded system to look for files and patches as the recipe is processed.

Other methods exist to accomplish grouping and defining configuration options. For example, if you are working with a local clone of the kernel repository, you could checkout the kernel's `meta` branch, make your changes, and then push the changes to the local bare clone of the kernel. The result is that you directly add configuration options to the `meta` branch for your BSP. The configuration options will likely end up in that location anyway if the BSP gets added to the Yocto Project.

In general, however, the Yocto Project maintainers take care of moving the `SRC_URI`-specified configuration options to the kernel's `meta` branch. Not only is it easier for BSP developers to not have to worry about putting those configurations in the branch, but having the maintainers do it allows them to apply 'global' knowledge about the kinds of common configuration options multiple BSPs in the tree are typically using. This allows for promotion of common configurations into common features.

### Applying Patches

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-second-change.patch"
         SRC_URI += "file://0003-third-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 using `devtool`, see the "Using `devtool` to Patch the Kernel" and "Using Traditional Kernel Development to Patch the Kernel" sections.

### Changing the Configuration

You can make wholesale or incremental changes to the final `.config` file used for the eventual Linux kernel configuration by including a `defconfig` file and by specifying configuration fragments in the `SRC_URI` to be applied to that file.

If you have a complete, working Linux kernel `.config` file you want to use for the configuration, as before, copy that file to the appropriate `${PN}` directory in your layer's `recipes-kernel/linux` directory, and rename the copied file to "defconfig". Then, add the following lines to the linux-yocto `.bbappend` file in your layer:

         FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
         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 `${PN}` directory you created to hold the configuration changes.

The build system applies the configurations from the `defconfig` file before applying any subsequent configuration fragments. The final kernel configuration is a combination of the configurations in the `defconfig` file and any configuration fragments you provide. You need to realize that if you have any configuration fragments, the build system applies these on top of and after applying the existing `defconfig` file configurations.

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 `${PN}` 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 := "${THISDIR}/${PN}:"
         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.

### Using an "In-Tree" `defconfig` File

It might be desirable to have kernel configuration fragment support through a `defconfig` file that is pulled from the kernel source tree for the configured machine. By default, the OpenEmbedded build system looks for `defconfig` files in the layer used for Metadata, which is "out-of-tree", and then configures them using the following:

         SRC_URI += "file://defconfig"

If you do not want to maintain copies of `defconfig` files in your layer but would rather allow users to use the default configuration from the kernel tree and still be able to add configuration fragments to the `SRC_URI` through, for example, append files, you can direct the OpenEmbedded build system to use a `defconfig` file that is "in-tree".

To specify an "in-tree" `defconfig` file, use the following statement form:

         KBUILD_DEFCONFIG_KMACHINE ?= defconfig_file

Here is an example that appends the `KBUILD_DEFCONFIG` variable with "common-pc" and provides the path to the "in-tree" `defconfig` file:

         KBUILD_DEFCONFIG_common-pc ?= "/home/scottrif/configfiles/my_defconfig_file"

Aside from modifying your kernel recipe and providing your own `defconfig` file, you need to be sure no files or statements set `SRC_URI` to use a `defconfig` other than your "in-tree" file (e.g. a kernel's `linux-`machine`.inc` file). In other words, if the build system detects a statement that identifies an "out-of-tree" `defconfig` file, that statement will override your `KBUILD_DEFCONFIG` variable.

See the `KBUILD_DEFCONFIG` variable description for more information.

## Using `devtool` to Patch the Kernel

The steps in this procedure show you how you can patch the kernel using the extensible SDK and `devtool`.

Before attempting this procedure, be sure you have performed the steps to get ready for updating the kernel as described in the "Getting Ready to Develop Using `devtool`" section.

Patching the kernel involves changing or adding configurations to an existing kernel, changing or adding recipes to the kernel that are needed to support specific hardware features, or even altering the source code itself.

This example creates a simple patch by adding some QEMU emulator console output at boot time through `printk` statements in the kernel's `calibrate.c` source code file. Applying the patch and booting the modified image causes the added messages to appear on the emulator's console. The example is a continuation of the setup procedure found in the "Getting Ready to Develop Using `devtool`" Section.

1. Check Out the Kernel Source Files: First you must use `devtool` to checkout the kernel source code in its workspace. Be sure you are in the terminal set up to do work with the extensible SDK.

   See this step in the "Getting Ready to Develop Using `devtool`" section for more information.

   Use the following `devtool` command to check out the code:

            $ devtool modify linux-yocto

   During the checkout operation, a bug exists that could cause errors such as the following to appear:

            ERROR: Taskhash mismatch 2c793438c2d9f8c3681fd5f7bc819efa versus
                   be3a89ce7c47178880ba7bf6293d7404 for
                   /path/to/esdk/layers/poky/meta/recipes-kernel/linux/linux-yocto_4.10.bb.do_unpack

   You can safely ignore these messages. The source code is correctly checked out.

2. Edit the Source Files Follow these steps to make some simple changes to the source files:

   1. Change the working directory: In the previous step, the output noted where you can find the source files (e.g. `~/poky_sdk/workspace/sources/linux-yocto`). Change to where the kernel source code is before making your edits to the `calibrate.c` file:

               $ cd ~/poky_sdk/workspace/sources/linux-yocto

   2. Edit the source file: Edit the `init/calibrate.c` file to have the following changes:

               void calibrate_delay(void)
               {
                   unsigned long lpj;
                   static bool printed;
                   int this_cpu = smp_processor_id();

                   printk("*************************************\n");
                   printk("*                                   *\n");
                   printk("*        HELLO YOCTO KERNEL         *\n");
                   printk("*                                   *\n");
                   printk("*************************************\n");

               	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
                         .
                         .
                         .

3. Build the Updated Kernel Source: To build the updated kernel source, use `devtool`:

            $ devtool build linux-yocto

4. Create the Image With the New Kernel: Use the `devtool build-image` command to create a new image that has the new kernel.

   If the image you originally created resulted in a Wic file, you can use an alternate method to create the new image with the updated kernel. For an example, see the steps in the TipsAndTricks/KernelDevelopmentWithEsdk Wiki Page.

            $ cd ~
            $ devtool build-image core-image-minimal

5. Test the New Image: For this example, you can run the new image using QEMU to verify your changes:

   1. Boot the image: Boot the modified image in the QEMU emulator using this command:

               $ runqemu qemux86

   2. Verify the changes: Log into the machine using `root` with no password and then use the following shell command to scroll through the console's boot output.

               # dmesg | less

      You should see the results of your `printk` statements as part of the output when you scroll down the console window.

6. Stage and commit your changes: Within your eSDK terminal, change your working directory to where you modified the `calibrate.c` file and use these Git commands to stage and commit your changes:

            $ cd ~/poky_sdk/workspace/sources/linux-yocto
            $ git status
            $ git add init/calibrate.c
            $ git commit -m "calibrate: Add printk example"

7. Export the Patches and Create an Append File: To export your commits as patches and create a `.bbappend` file, use the following command in the terminal used to work with the extensible SDK. This example uses the previously established layer named `meta-mylayer`.

   See Step 3 of the "Getting Ready to Develop Using devtool" section for information on setting up this layer.

            $ devtool finish linux-yocto ~/meta-mylayer

   Once the command finishes, the patches and the `.bbappend` file are located in the `~/meta-mylayer/recipes-kernel/linux` directory.

8. Build the Image With Your Modified Kernel: You can now build an image that includes your kernel patches. Execute the following command from your Build Directory in the terminal set up to run BitBake:

            $ cd ~/poky/build
            $ bitbake core-image-minimal

## Using Traditional Kernel Development to Patch the Kernel

The steps in this procedure show you how you can patch the kernel using traditional kernel development (i.e. not using `devtool` and the extensible SDK as described in the "Using `devtool` to Patch the Kernel" section).

Before attempting this procedure, be sure you have performed the steps to get ready for updating the kernel as described in the "Getting Ready for Traditional Kernel Development" section.

Patching the kernel involves changing or adding configurations to an existing kernel, changing or adding recipes to the kernel that are needed to support specific hardware features, or even altering the source code itself.

The example in this section creates a simple patch by adding some QEMU emulator console output at boot time through `printk` statements in the kernel's `calibrate.c` source code file. Applying the patch and booting the modified image causes the added messages to appear on the emulator's console. The example is a continuation of the setup procedure found in the "Getting Ready for Traditional Kernel Development" Section.

1. Edit the Source Files Prior to this step, you should have used Git to create a local copy of the repository for your kernel. Assuming you created the repository as directed in the "Getting Ready for Traditional Kernel Development" section, use the following commands to edit the `calibrate.c` file:

   1. Change the working directory: You need to locate the source files in the local copy of the kernel Git repository: Change to where the kernel source code is before making your edits to the `calibrate.c` file:

               $ cd ~/linux-yocto-4.12/init

   2. Edit the source file: Edit the `calibrate.c` file to have the following changes:

               void calibrate_delay(void)
               {
                   unsigned long lpj;
                   static bool printed;
                   int this_cpu = smp_processor_id();

                   printk("*************************************\n");
                   printk("*                                   *\n");
                   printk("*        HELLO YOCTO KERNEL         *\n");
                   printk("*                                   *\n");
                   printk("*************************************\n");

               	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
                         .
                         .
                         .

2. Stage and Commit Your Changes: Use standard Git commands to stage and commit the changes you just made:

            $ git add calibrate.c
            $ git commit -m "calibrate.c - Added some printk statements"

   If you do not stage and commit your changes, the OpenEmbedded Build System will not pick up the changes.

3. Update Your `local.conf` File to Point to Your Source Files: In addition to your `local.conf` file specifying to use "kernel-modules" and the "qemux86" machine, it must also point to the updated kernel source files. Add `SRC_URI` and `SRCREV` statements similar to the following to your `local.conf`:

            $ cd ~/poky/build/conf

   Add the following to the `local.conf`:

            SRC_URI_pn-linux-yocto = "git:///path-to/linux-yocto-4.12;protocol=file;name=machine;branch=standard/base; \
                                      git:///path-to/yocto-kernel-cache;protocol=file;type=kmeta;name=meta;branch=yocto-4.12;destsuffix=${KMETA}"
            SRCREV_meta_qemux86 = "${AUTOREV}"
            SRCREV_machine_qemux86 = "${AUTOREV}"

   Be sure to replace path-to with the pathname to your local Git repositories. Also, you must be sure to specify the correct branch and machine types. For this example, the branch is `standard/base` and the machine is "qemux86".

4. Build the Image: With the source modified, your changes staged and committed, and the `local.conf` file pointing to the kernel files, you can now use BitBake to build the image:

            $ cd ~/poky/build
            $ bitbake core-image-minimal

5. Boot the image: Boot the modified image in the QEMU emulator using this command. When prompted to login to the QEMU console, use "root" with no password:

            $ cd ~/poky/build
            $ runqemu qemux86

6. Look for Your Changes: As QEMU booted, you might have seen your changes rapidly scroll by. If not, use these commands to see your changes:

            # dmesg | less

   You should see the results of your `printk` statements as part of the output when you scroll down the console window.

7. Generate the Patch File: Once you are sure that your patch works correctly, you can generate a `*.patch` file in the kernel source repository:

            $ cd ~/linux-yocto-4.12/init
            $ git format-patch -1
            0001-calibrate.c-Added-some-printk-statements.patch

8. Move the Patch File to Your Layer: In order for subsequent builds to pick up patches, you need to move the patch file you created in the previous step to your layer `meta-mylayer`. For this example, the layer created earlier is located in your home directory as `meta-mylayer`. When the layer was created using the `yocto-create` script, no additional hierarchy was created to support patches. Before moving the patch file, you need to add additional structure to your layer using the following commands:

            $ cd ~/meta-mylayer
            $ mkdir recipes-kernel
            $ mkdir recipes-kernel/linux
            $ mkdir recipes-kernel/linux/linux-yocto

   Once you have created this hierarchy in your layer, you can move the patch file using the following command:

            $ mv ~/linux-yocto-4.12/init/0001-calibrate.c-Added-some-printk-statements.patch ~/meta-mylayer/recipes-kernel/linux/linux-yocto

9. Create the Append File: Finally, you need to create the `linux-yocto_4.12.bbappend` file and insert statements that allow the OpenEmbedded build system to find the patch. The append file needs to be in your layer's `recipes-kernel/linux` directory and it must be named `linux-yocto_4.12.bbappend` and have the following contents:

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

            SRC_URI_append = " file://0001-calibrate.c-Added-some-printk-statements.patch"

   The `FILESEXTRAPATHS` and `SRC_URI` statements enable the OpenEmbedded build system to find the patch file.

   For more information on append files and patches, see the "Creating the Append File" and "Applying Patches" sections. You can also see the "Using .bbappend Files in Your Layer"" section in the Yocto Project Development Tasks Manual.

   To build `core-image-minimal` again and see the effects of your patch, you can essentially eliminate the temporary source files saved in `poky/build/tmp/work/...` and residual effects of the build by entering the following sequence of commands:

            $ cd ~/poky/build
            $ bitbake -c cleanall yocto-linux
            $ bitbake core-image-minimal -c cleanall
            $ bitbake core-image-minimal
            $ runqemu qemux86

## Configuring the Kernel

Configuring the Yocto Project kernel consists of making sure the `.config` file has all the right information in it for the image you are building. You can use the `menuconfig` tool and configuration fragments to make sure your `.config` file is just how you need it. You can also save known configurations in a `defconfig` file that the build system can use for kernel configuration.

This section describes how to use `menuconfig`, create and use configuration fragments, and how to interactively modify your `.config` file to create the leanest kernel configuration file possible.

For more information on kernel configuration, see the "Changing the Configuration" section.

### Using `menuconfig`

The easiest way to define kernel configurations is to set them through the `menuconfig` tool. This tool provides an interactive method with which to set kernel configurations. For general information on `menuconfig`, see .

To use the `menuconfig` tool in the Yocto Project development environment, you must do the following:

- Because you launch `menuconfig` using BitBake, you must be sure to set up your environment by running the `&OE_INIT_FILE;` script found in the Build Directory.

- You must be sure of the state of your build's configuration in the Source Directory.

- Your build host must have the following two packages installed:

           libncurses5-dev
           libtinfo-dev

The following commands initialize the BitBake environment, run the `do_kernel_configme` task, and launch `menuconfig`. These commands assume the Source Directory's top-level folder is `~/poky`:

         $ cd poky
         $ source oe-init-build-env
         $ bitbake linux-yocto -c kernel_configme -f
         $ bitbake linux-yocto -c menuconfig

Once `menuconfig` comes up, its standard interface allows you to interactively examine and configure all the kernel configuration parameters. After making your changes, simply exit the tool and save your changes to create an updated version of the `.config` configuration file.

You can use the entire `.config` file as the `defconfig` file. For information on `defconfig` files, see the "Changing the Configuration", "Using an In-Tree `defconfig` File, and "Creating a `defconfig` File" sections.

Consider an example that configures the "CONFIG_SMP" setting for the `linux-yocto-4.12` kernel.

The OpenEmbedded build system recognizes this kernel as `linux-yocto` through Metadata (e.g. `PREFERRED_VERSION` `_linux-yocto ?= "12.4%"`).

Once `menuconfig` launches, use the interface to navigate through the selections to find the configuration settings in which you are interested. For this example, you deselect "CONFIG_SMP" by clearing the "Symmetric Multi-Processing Support" option. Using the interface, you can find the option under "Processor Type and Features". To deselect "CONFIG_SMP", use the arrow keys to highlight "Symmetric Multi-Processing Support" and enter "N" to clear the asterisk. When you are finished, exit out and save the change.

Saving the selections updates the `.config` configuration file. This is the file that the OpenEmbedded build system uses to configure the kernel during the build. You can find and examine this file in the Build Directory in `tmp/work/`. The actual `.config` is located in the area where the specific kernel is built. For example, if you were building a Linux Yocto kernel based on the `linux-yocto-4.12` kernel and you were building a QEMU image targeted for `x86` architecture, the `.config` file would be:

         poky/build/tmp/work/qemux86-poky-linux/linux-yocto/4.12.12+gitAUTOINC+eda4d18...
         ...967-r0/linux-qemux86-standard-build/.config

The previous example directory is artificially split and many of the characters in the actual filename are omitted in order to make it more readable. Also, depending on the kernel you are using, the exact pathname might differ.

Within the `.config` file, you can see the kernel settings. For example, the following entry shows that symmetric multi-processor support is not set:

         # CONFIG_SMP is not set

A good method to isolate changed configurations is to use a combination of the `menuconfig` tool and simple shell commands. Before changing configurations with `menuconfig`, copy the existing `.config` and rename it to something else, use `menuconfig` to make as many changes as you want and save them, then compare the renamed configuration file against the newly created file. You can use the resulting differences as your base to create configuration fragments to permanently save in your kernel layer.

Be sure to make a copy of the `.config` file and do not just rename it. The build system needs an existing `.config` file from which to work.

### Creating a `defconfig` File

A `defconfig` file in the context of the Yocto Project is often a `.config` file that is copied from a build or a `defconfig` taken from the kernel tree and moved into recipe space. You can use a `defconfig` file to retain a known set of kernel configurations from which the OpenEmbedded build system can draw to create the final `.config` file.

Out-of-the-box, the Yocto Project never ships a `defconfig` or `.config` file. The OpenEmbedded build system creates the final `.config` file used to configure the kernel.

To create a `defconfig`, start with a complete, working Linux kernel `.config` file. Copy that file to the appropriate `${` `PN` `}` directory in your layer's `recipes-kernel/linux` directory, and rename the copied file to "defconfig" (e.g. `~/meta-mylayer/recipes-kernel/linux/linux-yocto/defconfig`). Then, add the following lines to the linux-yocto `.bbappend` file in your layer:

         FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
         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 `${PN}` directory you created to hold the configuration changes.

The build system applies the configurations from the `defconfig` file before applying any subsequent configuration fragments. The final kernel configuration is a combination of the configurations in the `defconfig` file and any configuration fragments you provide. You need to realize that if you have any configuration fragments, the build system applies these on top of and after applying the existing defconfig file configurations.

For more information on configuring the kernel, see the "Changing the Configuration" section.

### Creating Configuration Fragments

Configuration fragments are simply kernel options that appear in a file placed where the OpenEmbedded build system can find and apply them. The build system applies configuration fragments after applying configurations from a `defconfig` file. Thus, the final kernel configuration is a combination of the configurations in the `defconfig` file and then any configuration fragments you provide. The build system applies fragments on top of and after applying the existing defconfig file configurations.

Syntactically, the configuration statement is identical to what would appear in the `.config` file, which is in the Build Directory.

For more information about where the `.config` file is located, see the example in the "Using `menuconfig`" section.

It is simple to create a configuration fragment. One method is to use shell commands. For example, issuing the following from the shell creates a configuration fragment file named `my_smp.cfg` that enables multi-processor support within the kernel:

         $ echo "CONFIG_SMP=y" >> my_smp.cfg

All configuration fragment files must use the `.cfg` extension in order for the OpenEmbedded build system to recognize them as a configuration fragment.

Another 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:

1. Complete a Build Through Kernel Configuration: Complete a build at least through the kernel configuration task as follows:

            $ bitbake linux-yocto -c kernel_configme -f

   This step ensures that you create a `.config` file from a known state. Because situations exist where your build state might become unknown, it is best to run this task prior to starting `menuconfig`.

2. Launch `menuconfig`: Run the `menuconfig` command:

            $ bitbake linux-yocto -c menuconfig

3. Create the Configuration Fragment: Run the `diffconfig` command to prepare a configuration fragment. The resulting file `fragment.cfg` is placed in the `${` `WORKDIR` `}` directory:

            $ bitbake linux-yocto -c diffconfig

The `diffconfig` command creates a file that is a list of Linux kernel `CONFIG_` assignments. See the "Changing the Configuration" section for additional information on how to use the output as a configuration fragment.

You can also use this method to create configuration fragments for a BSP. See the "BSP Descriptions" section for more information.

Where do you put your configuration fragment files? You can place these files in an area pointed to by `SRC_URI` as directed by your `bblayers.conf` file, which is located in your layer. The OpenEmbedded build system picks up the configuration and adds it to the kernel's configuration. For example, suppose you had a set of configuration options in a file called `myconfig.cfg`. If you put that file inside a directory named `linux-yocto` that resides in the same directory as the kernel's append file within your layer and then add the following statements to the kernel's append file, those configuration options will be picked up and applied when the kernel is built:

         FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
         SRC_URI += "file://myconfig.cfg"

As mentioned earlier, you can group related configurations into multiple files and name them all in the `SRC_URI` statement as well. For example, you could group separate configurations specifically for Ethernet and graphics into their own files and add those by using a `SRC_URI` statement like the following in your append file:

         SRC_URI += "file://myconfig.cfg \
                file://eth.cfg \
                file://gfx.cfg"

### Validating Configuration

You can use the `do_kernel_configcheck` task to provide configuration validation:

         $ bitbake linux-yocto -c kernel_configcheck -f

Running this task produces 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.

In order to run this task, you must have an existing `.config` file. See the "Using `menuconfig`" section for information on how to create a configuration file.

Following is sample output from the `do_kernel_configcheck` task:

         Loading cache: 100% |########################################################| Time: 0:00:00
         Loaded 1275 entries from dependency cache.
         NOTE: Resolving any missing task queue dependencies

         Build Configuration:
             .
             .
             .

         NOTE: Executing SetScene Tasks
         NOTE: Executing RunQueue Tasks
         WARNING: linux-yocto-4.12.12+gitAUTOINC+eda4d18ce4_16de014967-r0 do_kernel_configcheck:
             [kernel config]: specified values did not make it into the kernel's final configuration:

         ---------- CONFIG_X86_TSC -----------------
         Config: CONFIG_X86_TSC
         From: /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/bsp/common-pc/common-pc-cpu.cfg
         Requested value:  CONFIG_X86_TSC=y
         Actual value:

         ---------- CONFIG_X86_BIGSMP -----------------
         Config: CONFIG_X86_BIGSMP
         From: /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/cfg/smp.cfg
               /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/defconfig
         Requested value:  # CONFIG_X86_BIGSMP is not set
         Actual value:

         ---------- CONFIG_NR_CPUS -----------------
         Config: CONFIG_NR_CPUS
         From: /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/cfg/smp.cfg
               /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/bsp/common-pc/common-pc.cfg
               /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/defconfig
         Requested value:  CONFIG_NR_CPUS=8
         Actual value:     CONFIG_NR_CPUS=1

         ---------- CONFIG_SCHED_SMT -----------------
         Config: CONFIG_SCHED_SMT
         From: /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/cfg/smp.cfg
               /home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/defconfig
         Requested value:  CONFIG_SCHED_SMT=y
         Actual value:

         NOTE: Tasks Summary: Attempted 288 tasks of which 285 didn't need to be rerun and all succeeded.

         Summary: There were 3 WARNING messages shown.

The previous output example has artificial line breaks to make it more readable.

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 `do_kernel_configme` and `do_kernel_configcheck` tasks until they produce no warnings.

For more information on how to use the `menuconfig` tool, see the "Using `menuconfig`" section.

### Fine-Tuning the Kernel Configuration File

You can make sure the `.config` file is as lean or efficient as possible by reading the output of the kernel configuration fragment audit, noting any issues, making changes to correct the issues, and then repeating.

As part of the kernel build process, the `do_kernel_configcheck` task runs. This task validates the kernel configuration by checking the final `.config` file against the input files. During the check, the task produces warning messages for the following issues:

- Requested options that did not make the final `.config` file.

- Configuration items that appear twice in the same configuration fragment.

- Configuration items tagged as "required" that were overridden.

- A board overrides a non-board specific option.

- Listed options not valid for the kernel being processed. In other words, the option does not appear anywhere.

The `do_kernel_configcheck` task can also optionally report if an option is overridden during processing.

For each output warning, a message points to the file that contains a list of the options and a pointer to the configuration fragment that defines them. Collectively, the files are the key to streamlining the configuration.

To streamline the configuration, do the following:

1. Use a Working Configuration: Start with a full configuration that you know works. Be sure the configuration builds and boots successfully. Use this configuration file as your baseline.

2. Run Configure and Check Tasks: Separately run the `do_kernel_configme` and `do_kernel_configcheck` tasks:

            $ bitbake linux-yocto -c kernel_configme -f
            $ bitbake linux-yocto -c kernel_configcheck -f

3. Process the Results: Take the resulting list of files from the `do_kernel_configcheck` task warnings and do the following:

   - Drop values that are redefined in the fragment but do not change the final `.config` file.

   - Analyze and potentially drop values from the `.config` file that override required configurations.

   - Analyze and potentially remove non-board specific options.

   - Remove repeated and invalid options.

4. Re-Run Configure and Check Tasks: After you have worked through the output of the kernel configuration audit, you can re-run the `do_kernel_configme` and `do_kernel_configcheck` tasks to see the results of your changes. If you have more issues, you can deal with them as described in the previous step.

Iteratively working through steps two through four eventually yields a minimal, streamlined configuration file. Once you have the best `.config`, you can build the Linux Yocto kernel.

## Expanding Variables

Sometimes it is helpful to determine what a variable expands to during a build. You can do examine the values of variables by examining the output of the `bitbake -e` command. The output is long and is more easily managed in a text file, which allows for easy searches:

         $ bitbake -e virtual/kernel > some_text_file

Within the text file, you can see exactly how each variable is expanded and used by the OpenEmbedded build system.

## Working with a "Dirty" Kernel Version String

If you build a kernel image and the version string has a "+" or a "-dirty" at the end, uncommitted modifications exist in the kernel's source directory. Follow these steps to clean up the version string:

1. Discover the Uncommitted Changes: Go to the kernel's locally cloned Git repository (source directory) and use the following Git command to list the files that have been changed, added, or removed:

            $ git status

2. Commit the Changes: You should commit those changes to the kernel source tree regardless of whether or not you will save, export, or use the changes:

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

3. Rebuild the Kernel Image: Once you commit the changes, rebuild the kernel.

   Depending on your particular kernel development workflow, the commands you use to rebuild the kernel might differ. For information on building the kernel image when using `devtool`, see the "Using `devtool` to Patch the Kernel" section. For information on building the kernel image when using Bitbake, see the "Using Traditional Kernel Development to Patch the Kernel" section.

## Working With Your Own Sources

If you cannot work with one of the Linux kernel versions supported by existing linux-yocto recipes, you can still make use of the Yocto Project Linux kernel tooling by working with your own sources. When you use your own sources, you will not be able to leverage the existing kernel Metadata and stabilization work of the linux-yocto sources. However, you will be able to manage your own Metadata in the same format as the linux-yocto sources. Maintaining format compatibility facilitates converging with linux-yocto on a future, mutually-supported kernel version.

To help you use your own sources, the Yocto Project provides a linux-yocto custom recipe (`linux-yocto-custom.bb`) that uses `kernel.org` sources and the Yocto Project Linux kernel tools for managing kernel Metadata. You can find this recipe in the `poky` Git repository of the Yocto Project Source Repository at:

         poky/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb

Here are some basic steps you can use to work with your own sources:

1. Create a Copy of the Kernel Recipe: Copy the `linux-yocto-custom.bb` recipe to your layer and give it a meaningful name. The name should include the version of the Yocto Linux kernel you are using (e.g. `linux-yocto-myproject_4.12.bb`, where "4.12" is the base version of the Linux kernel with which you would be working).

2. Create a Directory for Your Patches: In the same directory inside your layer, create a matching directory to store your patches and configuration files (e.g. `linux-yocto-myproject`).

3. Ensure You Have Configurations: Make sure you have either a `defconfig` file or configuration fragment files in your layer. When you use the `linux-yocto-custom.bb` recipe, you must specify a configuration. If you do not have a `defconfig` file, you can run the following:

            $ make defconfig

   After running the command, copy the resulting `.config` file to the `files` directory in your layer as "defconfig" and then add it to the `SRC_URI` variable in the recipe.

   Running the `make defconfig` command results in the default configuration for your architecture as defined by your kernel. However, no guarantee exists that this configuration is valid for your use case, or that your board will even boot. This is particularly true for non-x86 architectures.

   To use non-x86 `defconfig` files, you need to be more specific and find one that matches your board (i.e. for arm, you look in `arch/arm/configs` and use the one that is the best starting point for your board).

4. Edit the Recipe: Edit the following variables in your recipe as appropriate for your project:

   - `SRC_URI`: The `SRC_URI` should specify a Git repository that uses one of the supported Git fetcher protocols (i.e. `file`, `git`, `http`, and so forth). The `SRC_URI` variable should also specify either a `defconfig` file or some configuration fragment files. The skeleton recipe provides an example `SRC_URI` as a syntax reference.

   - `LINUX_VERSION`: The Linux kernel version you are using (e.g. "4.12").

   - `LINUX_VERSION_EXTENSION`: The Linux kernel `CONFIG_LOCALVERSION` that is compiled into the resulting kernel and visible through the `uname` command.

   - `SRCREV`: The commit ID from which you want to build.

   - `PR`: Treat this variable the same as you would in any other recipe. Increment the variable to indicate to the OpenEmbedded build system that the recipe has changed.

   - `PV`: The default `PV` assignment is typically adequate. It combines the `LINUX_VERSION` with the Source Control Manager (SCM) revision as derived from the `SRCPV` variable. The combined results are a string with the following form:

              3.19.11+git1+68a635bf8dfb64b02263c1ac80c948647cc76d5f_1+218bd8d2022b9852c60d32f0d770931e3cf343e2

     While lengthy, the extra verbosity in `PV` helps ensure you are using the exact sources from which you intend to build.

   - `COMPATIBLE_MACHINE`: A list of the machines supported by your new recipe. This variable in the example recipe is set by default to a regular expression that matches only the empty string, "(^$)". This default setting triggers an explicit build failure. You must change it to match a list of the machines that your new recipe supports. For example, to support the `qemux86` and `qemux86-64` machines, use the following form:

              COMPATIBLE_MACHINE = "qemux86|qemux86-64"

5. Customize Your Recipe as Needed: Provide further customizations to your recipe as needed just as you would customize an existing linux-yocto recipe. See the "Modifying an Existing Recipe" section for information.

## Working with Out-of-Tree Modules

This section describes steps to build out-of-tree modules on your target and describes how to incorporate out-of-tree modules in the build.

### Building Out-of-Tree Modules on the Target

While the traditional Yocto Project development model would be to include kernel modules as part of the normal build process, you might find it useful to build modules on the target. This could be the case if your target system is capable and powerful enough to handle the necessary compilation. Before deciding to build on your target, however, you should consider the benefits of using a proper cross-development environment from your build host.

If you want to be able to build out-of-tree modules on the target, there are some steps you need to take on the target that is running your SDK image. Briefly, the `kernel-dev` package is installed by default on all `*.sdk` images and the `kernel-devsrc` package is installed on many of the `*.sdk` images. However, you need to create some scripts prior to attempting to build the out-of-tree modules on the target that is running that image.

Prior to attempting to build the out-of-tree modules, you need to be on the target as root and you need to change to the `/usr/src/kernel` directory. Next, `make` the scripts:

         # cd /usr/src/kernel
         # make scripts

Because all SDK image recipes include `dev-pkgs`, the `kernel-dev` packages will be installed as part of the SDK image and the `kernel-devsrc` packages will be installed as part of applicable SDK images. The SDK uses the scripts when building out-of-tree modules. Once you have switched to that directory and created the scripts, you should be able to build your out-of-tree modules on the target.

### Incorporating Out-of-Tree Modules

While it is always preferable to work with sources integrated into the Linux kernel sources, if you need an external kernel module, the `hello-mod.bb` recipe is available as a template from which you can create your own out-of-tree Linux kernel module recipe.

This template recipe is located in the `poky` Git repository of the Yocto Project Source Repository at:

         poky/meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb

To get started, copy this recipe to your layer and give it a meaningful name (e.g. `mymodule_1.0.bb`). In the same directory, create a new directory named `files` where you can store any source files, patches, or other files necessary for building the module that do not come with the sources. Finally, update the recipe as needed for the module. Typically, you will need to set the following variables:

- `DESCRIPTION`

- `LICENSE*`

- `SRC_URI`

- `PV`

Depending on the build system used by the module sources, you might need to make some adjustments. For example, a typical module `Makefile` looks much like the one provided with the `hello-mod` template:

         obj-m := hello.o

         SRC := $(shell pwd)

         all:
             $(MAKE) -C $(KERNEL_SRC) M=$(SRC)

         modules_install:
             $(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules_install
         ...

The important point to note here is the `KERNEL_SRC` variable. The `module` class sets this variable and the `KERNEL_PATH` variable to `${`STAGING_KERNEL_DIR`}` with the necessary Linux kernel build information to build modules. If your module `Makefile` uses a different variable, you might want to override the `do_compile` step, or create a patch to the `Makefile` to work with the more typical `KERNEL_SRC` or `KERNEL_PATH` variables.

After you have prepared your recipe, you will likely want to include the module in your images. To do this, see the documentation for the following variables in the Yocto Project Reference Manual and set one of them appropriately for your machine configuration file:

- `MACHINE_ESSENTIAL_EXTRA_RDEPENDS`

- `MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS`

- `MACHINE_EXTRA_RDEPENDS`

- `MACHINE_EXTRA_RRECOMMENDS`

Modules are often not required for boot and can be excluded from certain build configurations. The following allows for the most flexibility:

         MACHINE_EXTRA_RRECOMMENDS += "kernel-module-mymodule"

The value is derived by appending the module filename without the `.ko` extension to the string "kernel-module-".

Because the variable is `RRECOMMENDS` and not a `RDEPENDS` variable, the build will not fail if this module is not available to include in the image.

## Inspecting Changes and Commits

A common question when working with a kernel is: "What changes have been applied to this tree?" Rather than using "grep" across directories to see what has changed, you can use Git to inspect or search the kernel tree. Using Git is an efficient way to see what has changed in the tree.

### What Changed in a Kernel?

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.

In the following examples, unless you provide a commit range, `kernel.org` history is blended with Yocto Project kernel changes. You can form ranges by using branch names from the kernel tree as the upper and lower commit markers with the Git commands. You can see the branch names through the web interface to the Yocto Project source repositories at .

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-3.19` 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

### Showing a Particular Feature or Branch Change

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

## Adding Recipe-Space Kernel Features

You can add kernel features in the recipe-space by using the `KERNEL_FEATURES` variable and by specifying the feature's `.scc` file path in the `SRC_URI` statement. When you add features using this method, the OpenEmbedded build system checks to be sure the features are present. If the features are not present, the build stops. Kernel features are the last elements processed for configuring and patching the kernel. Therefore, adding features in this manner is a way to enforce specific features are present and enabled without needing to do a full audit of any other layer's additions to the `SRC_URI` statement.

You add a kernel feature by providing the feature as part of the `KERNEL_FEATURES` variable and by providing the path to the feature's `.scc` file, which is relative to the root of the kernel Metadata. The OpenEmbedded build system searches all forms of kernel Metadata on the `SRC_URI` statement regardless of whether the Metadata is in the "kernel-cache", system kernel Metadata, or a recipe-space Metadata (i.e. part of the kernel recipe). See the "Kernel Metadata Location" section for additional information.

When you specify the feature's `.scc` file on the `SRC_URI` statement, the OpenEmbedded build system adds the directory of that `.scc` file along with all its subdirectories to the kernel feature search path. Because subdirectories are searched, you can reference a single `.scc` file in the `SRC_URI` statement to reference multiple kernel features.

Consider the following example that adds the "test.scc" feature to the build.

1. Create the Feature File: Create a `.scc` file and locate it just as you would any other patch file, `.cfg` file, or fetcher item you specify in the `SRC_URI` statement.

   ### Notes

   - You must add the directory of the `.scc` file to the fetcher's search path in the same manner as you would add a `.patch` file.

   - You can create additional `.scc` files beneath the directory that contains the file you are adding. All subdirectories are searched during the build as potential feature directories.

   Continuing with the example, suppose the "test.scc" feature you are adding has a `test.scc` file in the following directory:

            my_recipe
               |
               +-linux-yocto
                  |
                  +-test.cfg
                  +-test.scc

   In this example, the `linux-yocto` directory has both the feature `test.scc` file and a similarly named configuration fragment file `test.cfg`.

2. Add the Feature File to `SRC_URI`: Add the `.scc` file to the recipe's `SRC_URI` statement:

            SRC_URI_append = " file://test.scc"

   The leading space before the path is important as the path is appended to the existing path.

3. Specify the Feature as a Kernel Feature: Use the `KERNEL_FEATURES` statement to specify the feature as a kernel feature:

            KERNEL_FEATURES_append = " test.scc"

   The OpenEmbedded build system processes the kernel feature when it builds the kernel.

   If other features are contained below "test.scc", then their directories are relative to the directory containing the `test.scc` file.