# 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.

You can use the `yocto-kernel` script found in the Source Directory under `scripts` to manage kernel patches and configuration. See the "Managing kernel Patches and Config Items with yocto-kernel" section in the Yocto Project Board Support Packages (BSP) Developer's Guide for more information.

## 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 Source Directory for the poky Git repository

- Local copies of the `poky-extras` Git repository placed within the Source Directory.

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

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

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 Source Directory: This area contains all the metadata that supports building images using the OpenEmbedded build system. In this example, the Source Directory also contains the Build Directory, which contains the configuration directory that lets you control the build. Also in this example, the Source Directory contains local copies of the `poky-extras` Git repository.

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

- Local copies of the `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 Yocto Project (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 Yocto Project 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 "Yocto Project Kernel" for information on how to set up the bare clone.

- Copy of the Yocto Project 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 "Yocto Project 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 Source Directory

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

Once you have Source Directory set up, you have many development branches from which you can work. From inside the local repository you can see the branch names and the tag names used in the upstream Git repository by using either of the following 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 Local poky-extras Git Repository

This example creates a local copy of the `poky-extras` Git repository inside the `poky` Source Directory. See the bulleted item "The `poky-extras` Git Repository" for information on how to set up a local copy of 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-extras`. The following commands create and checkout the local branch you are using for the `&DISTRO_NAME;` branch:

         $ cd ~/poky/poky-extras
         $ 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-3.4` 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 "Yocto Project 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-3.4-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-3.4-work
         $ git checkout -b standard-common-pc-base origin/standard/common-pc/base
         Branch standard-common-pc-base set up to track remote branch
            standard/common-pc/base from origin.
         Switched to a new branch 'standard-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_FILE;` 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_FILE;
         You had no conf/local.conf file. This configuration file has therefore been
         created for you with some default values. You may wish to edit it to use a
         different MACHINE (target hardware) or enable parallel build options to take
         advantage of multiple cores for example. See the file for more information as
         common configuration options are commented.

         The Yocto Project has extensive documentation about OE including a reference manual
         which can be found at:
             http://yoctoproject.org/documentation

         For more information about OpenEmbedded see their website:
             http://www.openembedded.org/

         You had no conf/bblayers.conf file. The configuration file has been created for
         you with some default values. To add additional metadata layers into your
         configuration please add entries to this file.

         The Yocto Project has extensive documentation about OE including a reference manual
         which can be found at:
             http://yoctoproject.org/documentation

         For more information about OpenEmbedded see their website:
             http://www.openembedded.org/

         ### 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-3.4-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 OpenEmbedded build system will not pick up the changed source files.

The following command pushes the changes to the bare clone:

         $ git push origin standard-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 OpenEmbedded 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/meta-yocto-bsp \
             /home/scottrif/poky/poky-extras/meta-kernel-dev \
             "

- Identify Your Source Files: In the `linux-yocto_3.4.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-3.4.git`. To do this, set the `KSRC_linux_yocto` variable to point to your local `linux-yocto-3.4.git` Git repository by adding the following statement. Also, be sure the `SRC_URI` variable is pointing to your kernel source files by removing the comment. Finally, be sure to substitute your user information in the statement:

           KSRC_linux_yocto_3_4 ?= "/home/scottrif/linux-yocto-3.4.git"
           SRC_URI = "git://${KSRC_linux_yocto_3_4};protocol=file;nocheckout=1;branch=${KBRANCH},meta;name=machine,meta"

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, or simply remove (or rename) all the files except the one your are using for the build (i.e. `linux-yocto_3.4.bbappend` in this example).

If you do not make one of these two adjustments, your machine will be compatible with all the kernel recipes in the `meta-kernel-dev` layer. When your machine is comapatible with all the kernel recipes, the build attempts to build all kernels in the layer. You could end up with build errors blocking your work.

### 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_FILE;` script. If it isn't, source the script again from `poky`.

            $ cd ~/poky
            $ source &OE_INIT_FILE;

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 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.