2 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.

Note

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

2.1 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 “Setting Up to Use the Yocto Project” 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.

Note

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.

2.1.1 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:

Initialize the BitBake Environment: 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
```
Note

The previous commands assume the Yocto Project Source Repositories (i.e. poky) have been cloned using Git and the local repository is named “poky”.
Prepare Your local.conf File: By default, the MACHINE variable is set to “qemux86-64”, which is fine if you are building for the QEMU emulator in 64-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.

In this example we wish to build for qemux86 so we must set the MACHINE variable to “qemux86” and also add the “kernel-modules”. As described we do this by appending to conf/local.conf:
```
MACHINE = "qemux86"
MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += "kernel-modules"
```
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'
$
```
Note

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.
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...
$
```

Build the Clean Image: The final step in preparing to work on the kernel is to build an initial image using bitbake:

$ bitbake core-image-minimal
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.

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. For a continued example, see the “Using devtool to Patch the Kernel” section.

2.1.2 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:

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 Styhead branch. If you need to checkout out the Styhead branch, see the “Checking Out by Branch in Poky” section in the Yocto Project Development Tasks Manual:
```
$ cd poky
$ git branch
master
* styhead
$ source oe-init-build-env
```
Note

The previous commands assume the Yocto Project Source Repositories (i.e. poky) have been cloned using Git and the local repository is named “poky”.
Prepare Your local.conf File: By default, the MACHINE variable is set to “qemux86-64”, which is fine if you are building for the QEMU emulator in 64-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.

In this example we wish to build for qemux86 so we must set the MACHINE variable to “qemux86” and also add the “kernel-modules”. As described we do this by appending to conf/local.conf:
```
MACHINE = "qemux86"
MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += "kernel-modules"
```
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'
```
Note

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.
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 ...
$
```
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 https://git.yoctoproject.org/.

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:
```
$ 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.
```
Note

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.

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 switch to 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.

2.2 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.

Note

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:

Create Structure: Create the layer’s structure:
```
$ mkdir -p meta-mylayer/conf 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.

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.

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 += "file://patch-file-one.patch"
SRC_URI += "file://patch-file-two.patch"
SRC_URI += "file://patch-file-three.patch"
```
The FILESEXTRAPATHS and SRC_URI statements enable the OpenEmbedded build system to find patch files. For more information on using append files, see the “Appending Other Layers Metadata With Your Layer” section in the Yocto Project Development Tasks Manual.

2.3 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.

2.3.1 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_6.1.bb recipe, the append file will typically be located as follows within your custom layer:

your-layer/recipes-kernel/linux/linux-yocto_6.1.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/

Note

If you are working on a new machine Board Support Package (BSP), be sure to refer to the Yocto Project Board Support Package 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_6.1.bbappend

Here are the contents of this 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  = "v6.1/standard/base"
KBRANCH:genericx86-64  = "v6.1/standard/base"
KBRANCH:beaglebone-yocto = "v6.1/standard/beaglebone"

KMACHINE:genericx86 ?= "common-pc"
KMACHINE:genericx86-64 ?= "common-pc-64"
KMACHINE:beaglebone-yocto ?= "beaglebone"

SRCREV_machine:genericx86 ?= "6ec439b4b456ce929c4c07fe457b5d6a4b468e86"
SRCREV_machine:genericx86-64 ?= "6ec439b4b456ce929c4c07fe457b5d6a4b468e86"
SRCREV_machine:beaglebone-yocto ?= "423e1996694b61fbfc8ec3bf062fc6461d64fde1"

COMPATIBLE_MACHINE:genericx86 = "genericx86"
COMPATIBLE_MACHINE:genericx86-64 = "genericx86-64"
COMPATIBLE_MACHINE:beaglebone-yocto = "beaglebone-yocto"

LINUX_VERSION:genericx86 = "6.1.30"
LINUX_VERSION:genericx86-64 = "6.1.30"
LINUX_VERSION:beaglebone-yocto = "6.1.20"

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.

Note

There are other ways of 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 not to have to put 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.

2.3.2 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.

2.3.3 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.

You can also use a regular defconfig file, as generated by the do_savedefconfig task instead of a complete .config file. This only specifies the non-default configuration values. You need to additionally set KCONFIG_MODE in the linux-yocto .bbappend file in your layer:

KCONFIG_MODE = "alldefconfig"

Note

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.

2.3.4 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:<machine> ?= "defconfig_file"

Here is an example that assigns the KBUILD_DEFCONFIG variable utilizing an override for the “raspberrypi2” MACHINE and provides the path to the “in-tree” defconfig file to be used for a Raspberry Pi 2, which is based on the Broadcom 2708/2709 chipset:

KBUILD_DEFCONFIG:raspberrypi2 ?= "bcm2709_defconfig"

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.

2.4 Using `devtool` to Patch the Kernel

The steps in this procedure show you how you can patch the kernel using devtool.

Note

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.

Check Out the Kernel Source Files: First you must use devtool to checkout the kernel source code in its workspace.

Note

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
```
Note

During the checkout operation, there is a bug that could cause errors such as the following:
```
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.

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

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
```

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)) {
          .
          .
          .

Build the Updated Kernel Source: To build the updated kernel source, use devtool:
```
$ devtool build linux-yocto
```
Create the Image With the New Kernel: Use the devtool build-image command to create a new image that has the new kernel:
```
$ cd ~
$ devtool build-image core-image-minimal
```
Note

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.
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.
Stage and commit your changes: 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"
```
Export the Patches and Create an Append File: To export your commits as patches and create a .bbappend file, use the following command. This example uses the previously established layer named meta-mylayer:
```
$ devtool finish linux-yocto ~/meta-mylayer
```
Note

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

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

2.5 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 as described in the “Using devtool to Patch the Kernel” section).

Note

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.

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:

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
```

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)) {
          .
          .
          .

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.
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}"
```
Note

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.
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
```
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
```
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.
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
```
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 -p recipes-kernel 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
```
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 += "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 “Appending Other Layers Metadata With Your Layer” section in the Yocto Project Development Tasks Manual.
Note

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 linux-yocto
$ bitbake core-image-minimal -c cleanall
$ bitbake core-image-minimal
$ runqemu qemux86
```

2.6 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.

2.6.1 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 https://en.wikipedia.org/wiki/Menuconfig.

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-build-env 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.

Note

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.

Note

The OpenEmbedded build system recognizes this kernel as linux-yocto through Metadata (e.g. PREFERRED_VERSION_linux-yocto ?= "4.12%").

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

Note

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.

Note

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.

2.6.2 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.

Note

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.

Note

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.

2.6.3 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.

Note

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

Note

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:

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 there are situations where your build state might become unknown, it is best to run this task prior to starting menuconfig.
Launch menuconfig: Run the menuconfig command:
```
$ bitbake linux-yocto -c menuconfig
```
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.

Note

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"

2.6.4 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.

Here 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.

Note

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.

2.6.5 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 it into 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.

Note

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:

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.
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
```
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.
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.

2.7 Expanding Variables

Sometimes it is helpful to determine what a variable expands to during a build. You can 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.

2.8 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, it means there are uncommitted modifications in the kernel’s source directory. Follow these steps to clean up the version string:

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
```
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"
```
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.

2.9 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 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: meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb.

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

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).
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).
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, there is no guarantee 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).
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"
```
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.

2.10 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.

2.10.1 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.

2.10.2 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: 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:

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:

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.

2.11 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.

2.11.1 What Changed in a Kernel?

Here 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.

Note

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 https://git.yoctoproject.org/.

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 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

2.11.2 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

2.12 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.

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.
Note
- 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.
Add the Feature File to SRC_URI: Add the .scc file to the recipe’s SRC_URI statement:
```
SRC_URI += "file://test.scc"
```
The leading space before the path is important as the path is appended to the existing path.
Specify the Feature as a Kernel Feature: Use the KERNEL_FEATURES statement to specify the feature as a kernel feature:
```
KERNEL_FEATURES += "test.scc"
```
The OpenEmbedded build system processes the kernel feature when it builds the kernel.

Note

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

2 Common Tasks

2.1 Preparing the Build Host to Work on the Kernel

2.1.1 Getting Ready to Develop Using devtool

2.1.2 Getting Ready for Traditional Kernel Development

2.2 Creating and Preparing a Layer

2.3 Modifying an Existing Recipe

2.3.1 Creating the Append File

2.3.2 Applying Patches

2.3.3 Changing the Configuration

2.3.4 Using an “In-Tree” defconfig File

2.4 Using devtool to Patch the Kernel

2.5 Using Traditional Kernel Development to Patch the Kernel

2.6 Configuring the Kernel

2.6.1 Using menuconfig

2.6.2 Creating a defconfig File

2.6.3 Creating Configuration Fragments

2.6.4 Validating Configuration

2.6.5 Fine-Tuning the Kernel Configuration File

2.7 Expanding Variables

2.8 Working with a “Dirty” Kernel Version String

2.9 Working With Your Own Sources

2.10 Working with Out-of-Tree Modules

2.10.1 Building Out-of-Tree Modules on the Target

2.10.2 Incorporating Out-of-Tree Modules

2.11 Inspecting Changes and Commits

2.11.1 What Changed in a Kernel?

2.11.2 Showing a Particular Feature or Branch Change

2.12 Adding Recipe-Space Kernel Features

2.1.1 Getting Ready to Develop Using `devtool`

2.3.4 Using an “In-Tree” `defconfig` File

2.4 Using `devtool` to Patch the Kernel

2.6.1 Using `menuconfig`

2.6.2 Creating a `defconfig` File