# Kernel Modification Example

Kernel modification 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 appendix presents simple examples that modify the kernel source code, change the kernel configuration, and add a kernel source recipe.

## Modifying the Kernel Source Code

This example adds some simple QEMU emulator console output at boot time by adding `printk` statements to the kernel's `calibrate.c` source code file. Booting the modified image causes the added messages to appear on the emulator's console.

### Understanding the Files You Need

Before you modify the kernel, you need to know what Git repositories and file structures you need. Briefly, you need the following:

- A local Yocto Project Files Git repository

- The `poky-extras` Git repository placed within the local Yocto Project files Git repository

- A bare clone of the Linux Yocto Kernel upstream Git repository to which you want to push your modifications.

- A copy of that bare clone in which you make your source modifcations

The following figure summarizes these four areas. Within each rectangular that represents a data structure, a host development directory pathname appears at the lower left-hand corner of the box. These pathnames are the locations used in this example. The figure also provides key statements and commands used during the kernel modification process:

Here is a brief description of the four areas:

- Local Yocto Project Files Git Repository: This area contains all the metadata that supports building images in the Yocto Project build environment - the local Yocto Project files. In this example, the local Yocto Project files Git repository also contains the build directory, which contains the configuration directory that lets you control the build. In this example, the repository also contains the `poky-extras` Git repository.

  See the bulleted item "Yocto Project Release" for information on how to get these files.

- `poky-extras` Git Repository: This area contains the `meta-kernel-dev` layer, which is where you make changes that append the kernel build recipes. You edit `.bbappend` files to locate your local kernel source files and to identify the kernel being built. This Git repository is a gathering place for extensions to the Linux Yocto (or really any) kernel recipes that faciliate the creation and development of kernel features, BSPs or configurations.

  See the bulleted item "The `poky-extras` Git Repository" for information on how to get these files.

- Bare Clone of the Linux Yocto kernel: This bare Git repository tracks the upstream Git repository of the Linux Yocto kernel source code you are changing. When you modify the kernel you must work through a bare clone. All source code changes you make to the kernel must be committed and pushed to the bare clone using Git commands. As mentioned, the `.bbappend` file in the `poky-extras` repository points to the bare clone so that the build process can locate the locally changed source files.

  See the bulleted item "Linux Yocto Kernel" for information on how to set up the bare clone.

- Copy of the Linux Yocto Kernel Bare Clone: This Git repository contains the actual source files that you modify. Any changes you make to files in this location need to ultimately be pushed to the bare clone using the `git push` command.

  See the bulleted item "Linux Yocto Kernel" for information on how to set up the bare clone.

  Typically, Git workflows follow a scheme where changes made to a local area are pulled into a Git repository. However, because the `git pull` command does not work with bare clones, this workflow pushes changes to the repository even though you could use other more complicated methods to get changes into the bare clone.

### Setting Up the Local Yocto Project Files Git Repository

You can get the local Yocto Project files through tarball extraction or by cloning the `poky` Git repository. This example uses `poky` as the root directory of the local Yocto Project files Git repository. See the bulleted item "Yocto Project Release" for information on how to get these files.

Once you have the repository set up, you have many development branches from which you can work. From inside the repository you can see the branch names and the tag names used in the Git repository using either of the following two commands:

         $ cd poky
         $ git branch -a
         $ git tag -l

This example uses the Yocto Project &DISTRO; Release code named "&DISTRO_NAME;", which maps to the `&DISTRO_NAME;` branch in the repository. The following commands create and checkout the local `&DISTRO_NAME;` branch:

         $ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
         Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
         Switched to a new branch '&DISTRO_NAME;'

### Setting Up the poky-extras Git Repository

This example places the `poky-extras` Git repository inside of `poky`. See the bulleted item "The `poky-extras` Git Repository" for information on how to get the `poky-extras` repository.

Because this example uses the Yocto Project &DISTRO; Release code named "&DISTRO_NAME;", which maps to the `&DISTRO_NAME;` branch in the repository, you need to be sure you are using that branch for `poky-extra`. The following commands create and checkout the local branch you are using for the `&DISTRO_NAME;` branch:

         $ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
         Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
         Switched to a new branch '&DISTRO_NAME;'

### Setting Up the Bare Clone and its Copy

This example modifies the `linux-yocto-3.2` kernel. Thus, you need to create a bare clone of that kernel and then make a copy of the bare clone. See the bulleted item "Linux Yocto Kernel" for information on how to do that.

The bare clone exists for the kernel build tools and simply as the receiving end of `git push` commands after you make edits and commits inside the copy of the clone. The copy (`my-linux-yocto-3.2-work` in this example) has to have a local branch created and checked out for your work. This example uses `common-pc-base` as the local branch. The following commands create and checkout the branch:

         $ cd ~/my-linux-yocto-3.2-work
         $ git checkout -b common-pc-base origin/standard/default/common-pc/base
         Checking out files: 100% (532/532), done.
         Branch common-pc-base set up to track remote branch
            standard/default/common-pc/base from origin.
         Switched to a new branch 'common-pc-base'

### Building and Booting the Default QEMU Kernel Image

Before we make changes to the kernel source files, this example first builds the default image and then boots it inside the QEMU emulator.

Because a full build can take hours, you should check two variables in the `build` directory that is created after you source the `oe-init-build-env` script. You can find these variables `BB_NUMBER_THREADS` and `PARALLEL_MAKE` in the `build/conf` directory in the `local.conf` configuration file. By default, these variables are commented out. If your host development system supports multi-core and multi-thread capabilities, you can uncomment these statements and set the variables to significantly shorten the full build time. As a guideline, set both `BB_NUMBER_THREADS` and `PARALLEL_MAKE` to twice the number of cores your machine supports.

The following two commands `source` the build environment setup script and build the default `qemux86` image. If necessary, the script creates the build directory:

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

              ### Shell environment set up for builds. ###

         You can now run 'bitbake <target>'

         Common targets are:
             core-image-minimal
             core-image-sato
             meta-toolchain
             meta-toolchain-sdk
             adt-installer
             meta-ide-support

         You can also run generated qemu images with a command like 'runqemu qemux86'

The following `bitbake` command starts the build:

         $ bitbake -k core-image-minimal

Be sure to check the settings in the `local.conf` before starting the build.

After the build completes, you can start the QEMU emulator using the resulting image `qemux86` as follows:

         $ runqemu qemux86

As the image boots in the emulator, console message and status output appears across the terminal window. Because the output scrolls by quickly, it is difficult to read. To examine the output, you log into the system using the login `root` with no password. Once you are logged in, issue the following command to scroll through the console output:

         # dmesg | less

Take note of the output as you will want to look for your inserted print command output later in the example.

### Changing the Source Code and Pushing it to the Bare Clone

The file you change in this example is named `calibrate.c` and is located in the `my-linux-yocto-3.2-work` Git repository (the copy of the bare clone) in `init`. This example simply inserts several `printk` statements at the beginning of the `calibrate_delay` function.

Here is the unaltered code at the start of this function:

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

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

Here is the altered code showing five new `printk` statements near the top of the function:

         void __cpuinit 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)) {
                   .
                   .
                   .

After making and saving your changes, you need to stage them for the push. The following Git commands are one method of staging and committing your changes:

         $ git add calibrate.c
         $ git commit --signoff

Once the source code has been modified, you need to use Git to push the changes to the bare clone. If you do not push the changes, then the Yocto Project build system will not pick up the changed source files.

The following command pushes the changes to the bare clone:

         $ git push origin common-pc-base:standard/default/common-pc/base

### Changing Build Parameters for Your Build

At this point, the source has been changed and pushed. The example now defines some variables used by the Yocto Project build system to locate your kernel source. You essentially need to identify where to find the kernel recipe and the changed source code. You also need to be sure some basic configurations are in place that identify the type of machine you are building and to help speed up the build should your host support multiple-core and thread capabilities.

Do the following to make sure the build parameters are set up for the example. Once you set up these build parameters, they do not have to change unless you change the target architecture of the machine you are building or you move the bare clone, copy of the clone, or the `poky-extras` repository:

- Build for the Correct Target Architecture: The `local.conf` file in the build directory defines the build's target architecture. By default, `MACHINE` is set to `qemux86`, which specifies a 32-bit Intel Architecture target machine suitable for the QEMU emulator. In this example, `MACHINE` is correctly configured.

- Optimize Build Time: Also in the `local.conf` file are two variables that can speed your build time if your host supports multi-core and multi-thread capabilities: `BB_NUMBER_THREADS` and `PARALLEL_MAKE`. If the host system has multiple cores then you can optimize build time by setting both these variables to twice the number of cores.

- Identify Your `meta-kernel-dev` Layer: The `BBLAYERS` variable in the `bblayers.conf` file found in the `poky/build/conf` directory needs to have the path to your local `meta-kernel-dev` layer. By default, the `BBLAYERS` variable contains paths to `meta` and `meta-yocto` in the `poky` Git repository. Add the path to your `meta-kernel-dev` location. Be sure to substitute your user information in the statement. Here is an example:

           BBLAYERS = " \
             /home/scottrif/poky/meta \
             /home/scottrif/poky/meta-yocto \
             /home/scottrif/poky/poky-extras/meta-kernel-dev \
             "

- Identify Your Source Files: In the `linux-yocto_3.2.bbappend` file located in the `poky-extras/meta-kernel-dev/recipes-kernel/linux` directory, you need to identify the location of the local source code, which in this example is the bare clone named `linux-yocto-3.2.git`. To do this, set the `KSRC_linux_yocto` variable to point to your local `linux-yocto-3.2.git` Git repository by adding the following statement. Be sure to substitute your user information in the statement:

           KSRC_linux_yocto_3_2 ?= "/home/scottrif/linux-yocto-3.2.git"

Before attempting to build the modified kernel, there is one more set of changes you need to make in the `meta-kernel-dev` layer. Because all the kernel `.bbappend` files are parsed during the build process regardless of whether you are using them or not, you should either comment out the `COMPATIBLE_MACHINE` statements in all unused `.bbappend` files. Alternatively, you can simply remove all the files except the one your are using for the build (i.e. `linux-yocto_3.2.bbappend` in this example).

### Building and Booting the Modified QEMU Kernel Image

Next, you need to build the modified image. Do the following:

1. Your environment should be set up since you previously sourced the `oe-init-build-env` script. If it isn't, source the script again from `poky`.

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

2. Be sure old images are cleaned out by running the `cleanall` BitBake task as follows from your build directory:

            $ bitbake -c cleanall linux-yocto

   Never remove any files by hand from the `tmp/deploy` directory insided the local Yocto Project files build directory. Always use the BitBake `cleanall` task to clear out previous builds.

3. Next, build the kernel image using this command:

            $ bitbake -k core-image-minimal

4. Finally, boot the modified image in the QEMU emulator using this command:

            $ runqemu qemux86

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.

## Changing the Kernel Configuration

This example changes the default behavior, which is "on", of the Symmetric Multi-processing Support (`CONFIG_SMP`) to "off". It is a simple example that demonstrates how to reconfigure the kernel.

### Getting Set Up to Run this Example

If you took the time to work through the example that modifies the kernel source code in "Modifying the Kernel Source Code" you should already have the Yocto Project files set up on your host machine. If this is the case, go to the next section, which is titled "Examining the Default `CONFIG_SMP` Behavior", and continue with the example.

If you don't have the Yocto Project files established on your system, you can get them through tarball extraction or by cloning the `poky` Git repository. This example uses `poky` as the root directory of the local Yocto Project Files Git repository. See the bulleted item "Yocto Project Release" for information on how to get these files.

Once you have the repository set up, you have many development branches from which you can work. From inside the repository you can see the branch names and the tag names used in the Git repository using either of the following two commands:

         $ cd poky
         $ git branch -a
         $ git tag -l

This example uses the Yocto Project &DISTRO; Release code named "&DISTRO_NAME;", which maps to the `&DISTRO_NAME;` branch in the repository. The following commands create and checkout the local `&DISTRO_NAME;` branch:

         $ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;
         Branch &DISTRO_NAME; set up to track remote branch &DISTRO_NAME; from origin.
         Switched to a new branch '&DISTRO_NAME;'

Next, you need to build the default `qemux86` image that you can boot using QEMU.

Because a full build can take hours, you should check two variables in the `build` directory that is created after you source the `oe-init-build-env` script. You can find these variables `BB_NUMBER_THREADS` and `PARALLEL_MAKE` in the `build/conf` directory in the `local.conf` configuration file. By default, these variables are commented out. If your host development system supports multi-core and multi-thread capabilities, you can uncomment these statements and set the variables to significantly shorten the full build time. As a guideline, set `BB_NUMBER_THREADS` to twice the number of cores your machine supports and set `PARALLEL_MAKE` to one and a half times the number of cores your machine supports.

The following two commands `source` the build environment setup script and build the default `qemux86` image. If necessary, the script creates the build directory:

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

              ### Shell environment set up for builds. ###

         You can now run 'bitbake <target>'

         Common targets are:
             core-image-minimal
             core-image-sato
             meta-toolchain
             meta-toolchain-sdk
             adt-installer
             meta-ide-support

         You can also run generated qemu images with a command like 'runqemu qemux86'

The following `bitbake` command starts the build:

         $ bitbake -k core-image-minimal

Be sure to check the settings in the `local.conf` before starting the build.

### Examining the Default `CONFIG_SMP` Behavior

By default, `CONFIG_SMP` supports multiple processor machines. To see this default setting from within the QEMU emulator, boot your image using the emulator as follows:

         $ runqemu qemux86 qemuparams="-smp 4"

Login to the machine using `root` with no password. After logging in, enter the following command to see how many processors are being supported in the emulator. The emulator reports support for the number of processors you specified using the `-smp` option, four in this case:

         # cat /proc/cpuinfo | grep processor
         processor       : 0
         processor       : 1
         processor       : 2
         processor       : 3
         #

To check the setting for `CONFIG_SMP`, you can use the following command:

         zcat /proc/config.gz | grep CONFIG_SMP

The console returns the following showing that multi-processor machine support is set:

         CONFIG_SMP=y

Logout of the emulator using the `exit` command and then close it down.

### Changing the `CONFIG_SMP` Configuration Using `menuconfig`

The `menuconfig` tool provides an interactive method with which to set kernel configurations. You need to run `menuconfig` inside the Yocto BitBake environment. Thus, the environment must be set up using the `oe-init-build-env` script found in the Yocto Project files Git repository build directory. If you have not sourced this script do so with the following commands:

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

After setting up the environment to run `menuconfig`, you are ready to use the tool to interactively change the kernel configuration. In this example, we are basing our changes on the `linux-yocto-3.2` kernel. The Yocto Project build environment recognizes this kernel as `linux-yocto`. Thus, the following commands from the shell in which you previously sourced the environment initialization script cleans the shared state memory and the `WORKDIR` directory and then builds and launches `menuconfig`:

         $ bitbake linux-yocto -c cleansstate
         $ bitbake linux-yocto -c menuconfig

Due to a bug in the release, it is necessary to clean the shared state memory in order for configurations made using `menuconfig` to take effect. For information on the bug, see

Once `menuconfig` launches, navigate through the user interface to find the `CONFIG_SMP` configuration setting. You can find it at `Processor Type and Features`. The configuration selection is `Symmetric Multi-processing Support`. After using the arrow keys to highlight this selection, press "n" to turn it off. Then, exit out and save your selections.

Once you save the selection, the `.config` configuration file is updated. This is the file that the build system uses to configure the Linux Yocto kernel when it is built. You can find and examine this file in the Yocto Project Files Git repository in the build directory. This example uses the following:

         ~/poky/build/tmp/work/qemux86-poky-linux/linux-yocto-3.2.11+git1+84f...
            ...656ed30-r1/linux-qemux86-standard-build

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 slightly.

Within the `.config` file, you can see the following setting:

         # 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 an 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` and don't just rename it. The Yocto Project build system needs an existing `.config` from which to work.

### Recompiling the Kernel and Testing the New Configuration

At this point, you are ready to recompile your kernel image with the new setting in effect using the BitBake commands below:

         $ bitbake linux-yocto -c compile -f
         $ bitbake linux-yocto

Manually turning off a kernel configuration setting such as `CONFIG_SMP` can cause the kernel configuration audit to issue warnings during the build. In this example, warnings appear telling you that the expected value `CONFIG_SMP` does not appear in the `.config` file. Because in this example you specifically turned off `CONFIG_SMP`, you can safely ignore the apparent conflict.

Now run the QEMU emulator and pass it the same multi-processor option as before:

         $ runqemu qemux86 qemuparams="-smp 4"

Login to the machine using `root` with no password and test for the number of processors the kernel supports:

         # cat /proc/cpuinfo | grep processor
         processor       : 0
         #

From the output, you can see that the kernel no longer supports multi-processor systems. The output indicates support for a single processor. You can verify the `CONFIG_SMP` setting by using this command:

         zcat /proc/config.gz | grep CONFIG_SMP

The console returns the following output:

         # CONFIG_SMP is not set

You have successfully reconfigured the kernel.

## Adding Kernel Recipes

A future release of this manual will present an example that adds kernel recipes, which provide new functionality to the kernel.