Kernel modification involves changing or adding configurations to an existing kernel, or adding recipes to the kernel that are needed to support specific hardware features. The process is similar to creating a Board Support Package (BSP) except that it does not involve a BSP layer. This section presents a brief overview of the kernel structure and then provides a simple example that shows how to modify the kernel.

When one thinks of the source files for a kernel they usually think of a fixed structure of files that contain kernel patches. The Yocto Project, however, employs mechanisims that in a sense result in a kernel source generator. The Yocto Project uses the source code management (SCM) tool Git to manage and track Yocto Project files. Git employs branching strategies that effectively produce a tree-like structure whose branches represent diversions from more general code. For example, suppose two kernels are basically identical with the exception of a couple different features in each. In the Yocto Project source repositories managed by Git a main branch can contain the common or shared parts of the kernel source and two branches that diverge from that common branch can each contain the features specific to the respective kernel. The result is a managed tree whose "leaves" represent the end of a specific path that yields a set of kernel source files necessary for a specific piece of hardware and its features. A big advantage to this scheme is the sharing of common features by keeping them in "larger" branches that are further up the tree. This practice eliminates redundant storage of similar features shared among kernels. When you build the kernel on your development system all files needed for the build are taken from the Yocto Project source repositories pointed to by the SRC_URI variable and gathered in a temporary work area where they are subsequently used to create the unique kernel. Thus, in a sense, the process constructs a local source tree specific to your kernel to generate the new kernel image - a source generator if you will. For a complete discussion of the Yocto Project kernel's architcture and its branching strategy, see the The Yocto Project Kernel Architecture and Use Manual. You can find a web interface to the Yocto Project source repository at . Within the interface you will see groups of related source code, each of which can be cloned using Git to result in a working Git repository on your local system (referred to as the "local Yocto Project files" in this manual). The Yocto Project supports four types of kernels in its source repositories at : linux-yocto-2.6.34 - The stable Linux Yocto kernel that is based on the Linux 2.6.34 release. linux-yocto-2.6.37 - The current Linux Yocto kernel that is based on the Linux 2.6.37 release. linux-yocto-dev - A development kernel based on the Linux 2.6.39-rc1 release. linux-2.6 - A kernel based on minimal Linux mainline tracking. [WRITER'S NOTE: I don't know which Git repository the user needs to clone to get this repository on their development system.]

This section presents a simple example that illustrates kernel modification based on the linux-yocto-2.6.37 kernel. The example uses the audio and mixer capabilities supported by the Advanced Linux Sound Architecture (ALSA) Project. As the example progresses you will see how to do the following: Iteratively modify a base kernel locally. Provide a recipe-based solution for your modified kernel. Proved an "in-tree" solution for your modified kernel (i.e. make the modifcations part of the Yocto Project). The example flows as follows: Be sure your host development system is set up to support development using the Yocto Project. See The Linux Distributions section and The Packages section both in the Yocto Project Quick Start for requirements. You will also need a release of Yocto Project installed on the host. Set up your environment for optimal local kernel development. Create a layer to isolate your kernel work. Next item. Next item. Next item. Next item.

You need to have the Yocto Project files available on your host system. The process is identical to that described in the "Getting Setup" section earlier in this manual. Be sure to either set up a local Git repository for poky or download and unpack the Yocto Project release tarball.

Everytime you change a configuration or add a recipe to the kernel you need to do a fetch from the Linux Yocto kernel source repositories. This can get tedious and time consuming if you need to fetch the entire Linux Yocto 2.6.37 Git repository down from the Internet everytime you make a change to the kernel. You can get around this by setting up a meta-kernel-dev area on your local system. This area contains "append" files for every kernel recipe, which also include a KSRC statement that points to the kernel source files. You can set up the environment so that the KSRC points to the meta-kernel-dev, thus pulling source from a local area. This setup can speed up development time. To get set up you need to do two things: create a local Git repository of the poky-extras repository, and create a bare clone of the Linux Yocto 2.6.37 kernel Git repository. The following transcript shows how to clone the poky-extras Git repository into the current working directory, which is poky in this example. The command creates the repository in a directory named poky-extras: $ git clone git://git.yoctoproject.org/poky-extras Initialized empty Git repository in /home/scottrif/poky/poky-extras/.git/ remote: Counting objects: 532, done. remote: Compressing objects: 100% (472/472), done. remote: Total 532 (delta 138), reused 307 (delta 39) Receiving objects: 100% (532/532), 534.28 KiB | 362 KiB/s, done. Resolving deltas: 100% (138/138), done. This transcript shows how to clone a bare Git repository of the Linux Yocto 2.6.37 kernel: $ git clone --bare git://git.yoctoproject.org/linux-yocto-2.6.37 Initialized empty Git repository in /home/scottrif/linux-yocto-2.6.37.git/ remote: Counting objects: 1886034, done. remote: Compressing objects: 100% (314326/314326), done. remote: Total 1886034 (delta 1570202), reused 1870335 (delta 1554798) Receiving objects: 100% (1886034/1886034), 401.51 MiB | 714 KiB/s, done. Resolving deltas: 100% (1570202/1570202), done. The bare clone of the Linux Yocto 2.6.37 kernel on your local system mirrors the upstream repository of the kernel. You can effectively point to this local clone now during development to avoid having to fetch the entire Linux Yocto 2.6.37 kernel every time you make a kernel change.

It is always good to isolate your work using your own layer. Doing so allows you to experiment and easily start over should things go wrong. This example uses a layer named meta-amixer. When you set up a layer for kernel work you should follow the general layout guidelines as described for BSP layers. This layout is described in the Example Filesystem Layout section of the Board Support Package (BSP) Development Guide. In the standard layout you will notice a suggested structure for recipes and configuration information. [WRITER'S NOTE: The meta-elc example uses an images directory. Currently, images is not part of the standard BSP layout. I need to find out from Darren if this directory is required for kernel work.] [WRITER'S NOTE: I need a paragraph here describing how to set up the layer. I am not sure if you should copy an existing BSP layer and modify from there. Or, if you should just look at a BSP layer and then create your own files. Email to Darren on this but no answer yet.]

In the standard layer structure you have several areas that you need to examine or modify. For this example the layer contains four areas: conf - Contains the layer.conf that identifies the location of the recipe files. images - Contains the image recipe file. This recipe includes the base image you will be using and specifies other packages the image might need. recipes-bsp - Contains recipes specific to the hardware for which you are developing the kernel. recipes-kernel - Contains the "append" files that add information to the main recipe kernel. Let's take a look at the layer.conf in the conf directory first. This configuration file enables the Yocto Project build system to locate and use the information in your new layer. The variable BBPATH needs to include the path to your layer as follows: BBPATH := "${BBPATH}:${LAYERDIR}" And, the variable BBFILES needs to be modified to include your recipe and append files: BBFILES := "${BBFILES} ${LAYERDIR}/images/*.bb \ ${LAYERDIR}/images/*.bbappend \ ${LAYERDIR}/recipes-*/*/*.bb \ ${LAYERDIR}/recipes-*/*/*.bbappend" Finally, you need to be sure to use your layer name in these variables at the end of the file: BBFILE_COLLECTIONS += "elc" BBFILE_PATTERN_elc := "^${LAYERDIR}/" BBFILE_PRIORITY_elc = "9" The images directory contains an append file that helps further define the image. In our example, the base image is core-image-minimal. The image does, however, need some additional modules that we are using for this example. These modules support the amixer functionality. Here is the append file: require recipes-core/images/poky-image-minimal.bb IMAGE_INSTALL += "dropbear alsa-utils-aplay alsa-utils-alsamixer" IMAGE_INSTALL_append_qemux86 += " kernel-module-snd-ens1370 \ kernel-module-snd-rawmidi kernel-module-loop kernel-module-nls-cp437 \ kernel-module-nls-iso8859-1 qemux86-audio alsa-utils-amixer" LICENSE = "MIT" While the focus of this example is not on the BSP, it is worth mentioning that the recipes-bsp directory has the recipes and append files for features that the hardware requires. In this example, there is a script and a recipe to support the amixer functionality in QEMU. It is beyond the scope of this manual to go too deeply into the script. Suffice it to say that the script tests for the presence of the mixer, sets up default mixer values, enables the mixer, unmutes master and then sets the volume to 100. The recipe qemu86-audio.bb installs and runs the amixer when the system boots. Here is the recipe: SUMMARY = "Provide a basic init script to enable audio" DESCRIPTION = "Set the volume and unmute the Front mixer setting during boot." SECTION = "base" LICENSE = "MIT" LIC_FILES_CHKSUM = "file://${POKYBASE}/LICENSE;md5=3f40d7994397109285ec7b81fdeb3b58" PR = "r4" inherit update-rc.d RDEPENDS = "alsa-utils-amixer" SRC_URI = "file://qemux86-audio" INITSCRIPT_NAME = "qemux86-audio" INITSCRIPT_PARAMS = "defaults 90" do_install() { install -d ${D}${sysconfdir} \ ${D}${sysconfdir}/init.d install -m 0755 ${WORKDIR}/qemux86-audio ${D}${sysconfdir}/init.d cat ${WORKDIR}/${INITSCRIPT_NAME} | \ sed -e 's,/etc,${sysconfdir},g' \ -e 's,/usr/sbin,${sbindir},g' \ -e 's,/var,${localstatedir},g' \ -e 's,/usr/bin,${bindir},g' \ -e 's,/usr,${prefix},g' > ${D}${sysconfdir}/init.d/${INITSCRIPT_NAME} chmod 755 ${D}${sysconfdir}/init.d/${INITSCRIPT_NAME} } The last area to look at is recipes-kernel. This area holds configuration fragments and kernel append files. The append file must have the same name as the kernel recipe, which is linux-yocto-2.6.37 in this example. The file can SRC_URI statements to point to configuration fragments you might have in the layer. The file can also contain KERNEL_FEATURES statements that specify included kernel configurations that ship with the Yocto Project.