As mentioned earlier in section "Yocto Project Source Repositories", the Yocto Project maintains source repositories at http://git.yoctoproject.org/cgit.cgi. If you look at this web-interface of the repositories, each item is a separate Git repository.

Git repositories use branching techniques that track content change (not files) within a project (e.g. a new feature or updated documentation). Creating a tree-like structure based on project divergence allows for excellent historical information over the life of a project. This methodology also allows for an environment in which you can do lots of local experimentation on a project as you develop changes or new features.

A Git repository represents all development efforts for a given project. For example, the Git repository poky contains all changes and developments for Poky over the course of its entire life. That means that all changes that make up all releases are captured. The repository maintains a complete history of changes.

You can create a local copy of any repository by "cloning" it with the Git clone command. When you clone a Git repository, you end up with an identical copy of the repository on your development system. Once you have a local copy of a repository, you can take steps to develop locally. For examples on how to clone Git repositories, see the section "Getting Set Up" earlier in this manual.

It is important to understand that Git tracks content change and not files. Git uses "branches" to organize different development efforts. For example, the poky repository has laverne, bernard, edison, denzil and master branches among others. You can see all the branches by going to http://git.yoctoproject.org/cgit.cgi/poky/ and clicking on the [...] link beneath the "Branch" heading.

Each of these branches represents a specific area of development. The master branch represents the current or most recent development. All other branches represent off-shoots of the master branch.

When you create a local copy of a Git repository, the copy has the same set of branches as the original. This means you can use Git to create a local working area (also called a branch) that tracks a specific development branch from the source Git repository. in other words, you can define your local Git environment to work on any development branch in the repository. To help illustrate, here is a set of commands that creates a local copy of the poky Git repository and then creates and checks out a local Git branch that tracks the Yocto Project 1.2 Release (denzil) development:

     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git checkout denzil -b denzil
            

In this example, the name of the top-level directory of your local Yocto Project Files Git repository is poky. And, the name of the local working area (or local branch) you have created and checked out is named denzil. The files in your repository now reflect the same files that are in the denzil development branch of the Yocto Project's poky repository. It is important to understand that when you create and checkout a local working branch based on a branch name, your local environment matches the "tip" of that development branch at the time you created your local branch, which could be different than the files at the time of a similarly named release. In other words, creating and checking out a local branch based on the denzil branch name is not the same as creating and checking out a local branch based on the denzil-1.2 release. Keep reading to see how you create a local snapshot of a Yocto Project Release.

Git uses "tags" to mark specific changes in a repository. Typically, a tag is used to mark a special point such as the final change before a project is released. You can see the tags used with the poky Git repository by going to http://git.yoctoproject.org/cgit.cgi/poky/ and clicking on the [...] link beneath the "Tag" heading.

Some key tags are laverne-4.0, bernard-5.0, and denzil-7.0. These tags represent Yocto Project releases.

When you create a local copy of the Git repository, you also have access to all the tags. Similar to branches, you can create and checkout a local working Git branch based on a tag name. When you do this, you get a snapshot of the Git repository that reflects the state of the files when the change was made associated with that tag. The most common use is to checkout a working branch that matches a specific Yocto Project release. Here is an example:

     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git checkout denzil-7.0 -b denzil-7.0
            

In this example, the name of the top-level directory of your local Yocto Project Files Git repository is poky. And, the name of the local branch you have created and checked out is denzil-7.0. The files in your repository now exactly match the Yocto Project 1.2 Release tag (denzil). It is important to understand that when you create and checkout a local working branch based on a tag, your environment matches a specific point in time and not a development branch.

Git has an extensive set of commands that lets you manage changes and perform collaboration over the life of a project. Conveniently though, you can manage with a small set of basic operations and workflows once you understand the basic philosophy behind Git. You do not have to be an expert in Git to be functional. A good place to look for instruction on a minimal set of Git commands is here. If you need to download Git, you can do so here.

If you don’t know much about Git, we suggest you educate yourself by visiting the links previously mentioned.

The following list briefly describes some basic Git operations as a way to get started. As with any set of commands, this list (in most cases) simply shows the base command and omits the many arguments they support. See the Git documentation for complete descriptions and strategies on how to use these commands:

The basic flow for pushing a change to an upstream "contrib" Git repository is as follows:

You can find general Git information on how to push a change upstream in the Git Community Book.

If you have a just a few changes, you can commit them and then submit them as an email to the maintainer. Here is a general procedure:

The Yocto Project contains several layers right out of the box. You can easily identify a layer in the Yocto Project by the name of its folder. Folders that are layers begin with the string meta. For example, when you set up the Yocto Project Files structure, you will see several layers: meta, meta-demoapps, meta-skeleton, and meta-yocto. Each of these folders is a layer.

Furthermore, if you set up a local copy of the meta-intel Git repository and then explore that folder, you will discover many BSP layers within the meta-intel layer. For more information on BSP layers, see the "BSP Layers" section in the Yocto Project Board Support Package (BSP) Developer's Guide.

It is very easy to create your own layer to use with the Yocto Project. Follow these general steps to create your layer:

To create layers that are easier to maintain, you should consider the following:

We also recommend the following:

Following these recommendations keeps your Yocto Project files area and its configuration entirely inside the Yocto Project's core base.

Before the Yocto Project build system can use your new layer, you need to enable it. To enable your layer, simply add your layer's path to the BBLAYERS variable in your conf/bblayers.conf file, which is found in the Yocto Project Build Directory. The following example shows how to enable a layer named meta-mylayer:

     LCONF_VERSION = "1"

     BBFILES ?= ""
     BBLAYERS = " \
       /path/to/poky/meta \
       /path/to/poky/meta-yocto \
       /path/to/poky/meta-mylayer \
       "
                

BitBake parses each conf/layer.conf file as specified in the BBLAYERS variable within the conf/bblayers.conf file. During the processing of each conf/layer.conf file, BitBake adds the recipes, classes and configurations contained within the particular layer to the Yocto Project.

Recipes used to append metadata to other recipes are called BitBake append files. BitBake append files use the .bbappend file type suffix, while underlying recipes to which metadata is being appended use the .bb file type suffix.

A .bbappend file allows your layer to make additions or changes to the content of another layer's recipe without having to copy the other recipe into your layer. Your .bbappend file resides in your layer, while the underlying .bb recipe file to which you are appending metadata resides in a different layer.

Append files files must have the same name as the underlying recipe. For example, the append file someapp_1.2.bbappend must apply to someapp_1.2.bb. This means the original recipe and append file names are version number specific. If the underlying recipe is renamed to update to a newer version, the corresponding .bbappend file must be renamed as well. During the build process, BitBake displays an error on starting if it detects a .bbappend file that does not have an underlying recipe with a matching name.

Being able to append information to an existing recipe not only avoids duplication, but also automatically applies recipe changes in a different layer to your layer. If you were copying recipes, you would have to manually merge changes as they occur.

As an example, consider the main formfactor recipe and a corresponding formfactor append file both from the Yocto Project Files. Here is the main formfactor recipe, which is named formfactor_0.0.bb and located in the meta layer at meta/bsp-recipes/formfactor:

     DESCRIPTION = "Device formfactor information"
     SECTION = "base"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COREBASE}/LICENSE;md5=3f40d7994397109285ec7b81fdeb3b58 \
                    file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
     PR = "r19"

     SRC_URI = "file://config file://machconfig"
     S = "${WORKDIR}"

     PACKAGE_ARCH = "${MACHINE_ARCH}"
     INHIBIT_DEFAULT_DEPS = "1"

     do_install() {
     	# Only install file if it has a contents
             install -d ${D}${sysconfdir}/formfactor/
             install -m 0644 ${S}/config ${D}${sysconfdir}/formfactor/
     	if [ -s "${S}/machconfig" ]; then
     	        install -m 0644 ${S}/machconfig ${D}${sysconfdir}/formfactor/
	     fi
     }
                

Here is the append file, which is named formfactor_0.0.bbappend and is from the Crown Bay BSP Layer named meta-intel/meta-crownbay:

     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
 
     PRINC = "1"
                

This example adds or overrides files in SRC_URI within a .bbappend by extending the path BitBake uses to search for files. The most reliable way to do this is by prepending the FILESEXTRAPATHS variable. For example, if you have your files in a directory that is named the same as your package (PN), you can add this directory by adding the following to your .bbappend file:

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

Using the immediate expansion assignment operator := is important because of the reference to THISDIR. The trailing colon character is important as it ensures that items in the list remain colon-separated.

For another example on how to use a .bbappend file, see the "Changing recipes-kernel" section.

Each layer is assigned a priority value. Priority values control which layer takes precedence if there are recipe files with the same name in multiple layers. For these cases, the recipe file from the layer with a higher priority number taking precedence. Priority values also affect the order in which multiple .bbappend files for the same recipe are applied. You can either specify the priority manually, or allow the build system to calculate it based on the layer's dependencies.

To specify the layer's priority manually, use the BBFILE_PRIORITY variable. For example:

     BBFILE_PRIORITY := "1"
                

You can use the BitBake layer management tool to provide a view into the structure of recipes across a multi-layer project. Being able to generate output that reports on configured layers with their paths and priorities and on .bbappend files and their applicable recipes can help to reveal potential problems.

Use the following form when running the layer management tool.

     $ bitbake-layers <command> [arguments]
                

The following list describes the available commands:

One way to get additional software into an image is to create a custom image. The following example shows the form for the two lines you need:

     IMAGE_INSTALL = "task-core-x11-base package1 package2"

     inherit core-image
                

By creating a custom image, a developer has total control over the contents of the image. It is important to use the correct names of packages in the IMAGE_INSTALL variable. You must use the OpenEmbedded notation and not the Debian notation for the names (e.g. eglibc-dev instead of libc6-dev).

The other method for creating a custom image is to modify an existing image. For example, if a developer wants to add strace into the core-image-sato image, they can use the following recipe:

     require core-image-sato.bb

     IMAGE_INSTALL += "strace"
                

For complex custom images, the best approach is to create a custom task package that is used to build the image or images. A good example of a tasks package is meta/recipes-core/tasks/task-core-boot.bb The PACKAGES variable lists the task packages to build along with the complementary -dbg and -dev packages. For each package added, you can use RDEPENDS and RRECOMMENDS entries to provide a list of packages the parent task package should contain. Following is an example:

     DESCRIPTION = "My Custom Tasks"

     PACKAGES = "\
         task-custom-apps \
         task-custom-apps-dbg \
         task-custom-apps-dev \
         task-custom-tools \
         task-custom-tools-dbg \
         task-custom-tools-dev \
         "

     RDEPENDS_task-custom-apps = "\
         dropbear \
         portmap \
         psplash"

     RDEPENDS_task-custom-tools = "\
         oprofile \
         oprofileui-server \
         lttng-control \
         lttng-viewer"

     RRECOMMENDS_task-custom-tools = "\
         kernel-module-oprofile"
                

In the previous example, two task packages are created with their dependencies and their recommended package dependencies listed: task-custom-apps, and task-custom-tools. To build an image using these task packages, you need to add task-custom-apps and/or task-custom-tools to IMAGE_INSTALL. For other forms of image dependencies see the other areas of this section.

Ultimately users might want to add extra image features to the set used by Yocto Project with the IMAGE_FEATURES variable. To create these features, the best reference is meta/classes/core-image.bbclass, which shows how the Yocto Project achieves this. In summary, the file looks at the contents of the IMAGE_FEATURES variable and then maps that into a set of tasks or packages. Based on this information the IMAGE_INSTALL variable is generated automatically. Users can add extra features by extending the class or creating a custom class for use with specialized image .bb files. You can also add more features by configuring the EXTRA_IMAGE_FEATURES variable in the local.conf file found in the Yocto Project files located in the build directory.

The Yocto Project ships with two SSH servers you can use in your images: Dropbear and OpenSSH. Dropbear is a minimal SSH server appropriate for resource-constrained environments, while OpenSSH is a well-known standard SSH server implementation. By default, the core-image-sato image is configured to use Dropbear. The core-image-basic and core-image-lsb images both include OpenSSH. To change these defaults, edit the IMAGE_FEATURES variable so that it sets the image you are working with to include ssh-server-dropbear or ssh-server-openssh.

It is possible to customize image contents by using variables from your local configuration in your conf/local.conf file. Because it is limited to local use, this method generally only allows you to add packages and is not as flexible as creating your own customized image. When you add packages using local variables this way, you need to realize that these variable changes affect all images at the same time and might not be what you require.

The simplest way to add extra packages to all images is by using the IMAGE_INSTALL variable with the _append operator:

     IMAGE_INSTALL_append = " strace"
                

Use of the syntax is important. Specifically, the space between the quote and the package name, which is strace in this example. This space is required since the _append operator does not add the space.

Furthermore, you must use _append instead of the += operator if you want to avoid ordering issues. The reason for this is because doing so unconditionally appends to the variable and avoids ordering problems due to the variable being set in image recipes and .bbclass files with operators like ?=. Using _append ensures the operation takes affect.

As shown in its simplest use, IMAGE_INSTALL_append affects all images. It is possible to extend the syntax so that the variable applies to a specific image only. Here is an example:

     IMAGE_INSTALL_append_pn-core-image-minimal = " strace"
                

This example adds strace to core-image-minimal only.

You can add packages using a similar approach through the CORE_IMAGE_EXTRA_INSTALL variable. If you use this variable, only core-image-* images are affected.

Building an application from a single file that is stored locally (e.g. under files/) requires a recipe that has the file listed in the SRC_URI variable. Additionally, you need to manually write the do_compile and do_install tasks. The S variable defines the directory containing the source code, which is set to WORKDIR in this case - the directory BitBake uses for the build.

     DESCRIPTION = "Simple helloworld application"
     SECTION = "examples"
     LICENSE = "MIT"
     LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"
     PR = "r0"

     SRC_URI = "file://helloworld.c"

     S = "${WORKDIR}"

     do_compile() {
     	${CC} helloworld.c -o helloworld
     }

     do_install() {
     	install -d ${D}${bindir}
     	install -m 0755 helloworld ${D}${bindir}
     }
                

By default, the helloworld, helloworld-dbg, and helloworld-dev packages are built. For information on how to customize the packaging process, see the "Splitting an Application into Multiple Packages" section.

Applications that use Autotools such as autoconf and automake require a recipe that has a source archive listed in SRC_URI and also inherits Autotools, which instructs BitBake to use the autotools.bbclass file, which contains the definitions of all the steps needed to build an Autotool-based application. The result of the build is automatically packaged. And, if the application uses NLS for localization, packages with local information are generated (one package per language). Following is one example: (hello_2.3.bb)

     DESCRIPTION = "GNU Helloworld application"
     SECTION = "examples"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=751419260aa954499f7abaabaa882bbe"
     PR = "r0"

     SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.gz"

     inherit autotools gettext
                 

The variable LIC_FILES_CHKSUM is used to track source license changes as described in the "Track License Change" section. You can quickly create Autotool-based recipes in a manner similar to the previous example.

Applications that use GNU make also require a recipe that has the source archive listed in SRC_URI. You do not need to add a do_compile step since by default BitBake starts the make command to compile the application. If you need additional make options you should store them in the EXTRA_OEMAKE variable. BitBake passes these options into the make GNU invocation. Note that a do_install task is still required. Otherwise BitBake runs an empty do_install task by default.

Some applications might require extra parameters to be passed to the compiler. For example, the application might need an additional header path. You can accomplish this by adding to the CFLAGS variable. The following example shows this:

     CFLAGS_prepend = "-I ${S}/include "
                

In the following example, mtd-utils is a makefile-based package:

     DESCRIPTION = "Tools for managing memory technology devices."
     SECTION = "base"
     DEPENDS = "zlib lzo e2fsprogs util-linux"
     HOMEPAGE = "http://www.linux-mtd.infradead.org/"
     LICENSE = "GPLv2+"
     LIC_FILES_CHKSUM = "file://COPYING;md5=0636e73ff0215e8d672dc4c32c317bb3 \
                    file://include/common.h;beginline=1;endline=17;md5=ba05b07912a44ea2bf81ce409380049c"

     SRC_URI = "git://git.infradead.org/mtd-utils.git;protocol=git;tag=995cfe51b0a3cf32f381c140bf72b21bf91cef1b \
	     	file://add-exclusion-to-mkfs-jffs2-git-2.patch"

     S = "${WORKDIR}/git/"

     PR = "r1"

     EXTRA_OEMAKE = "'CC=${CC}' 'RANLIB=${RANLIB}' 'AR=${AR}' \
        'CFLAGS=${CFLAGS} -I${S}/include -DWITHOUT_XATTR' 'BUILDDIR=${S}'"

     do_install () {
	     oe_runmake install DESTDIR=${D} SBINDIR=${sbindir} MANDIR=${mandir} \
            INCLUDEDIR=${includedir}
	     install -d ${D}${includedir}/mtd/
	     for f in ${S}/include/mtd/*.h; do
	     	install -m 0644 $f ${D}${includedir}/mtd/
	     done
     }

     PARALLEL_MAKE = ""

     BBCLASSEXTEND = "native"
                

If your sources are available as a tarball instead of a Git repository, you will need to provide the URL to the tarball as well as an md5 or sha256 sum of the download. Here is an example:

     SRC_URI="ftp://ftp.infradead.org/pub/mtd-utils/mtd-utils-1.4.9.tar.bz2"
     SRC_URI[md5sum]="82b8e714b90674896570968f70ca778b"
                

You can generate the md5 or sha256 sums by using the md5sum or sha256sum commands with the target file as the only argument. Here is an example:

     $ md5sum mtd-utils-1.4.9.tar.bz2 
     82b8e714b90674896570968f70ca778b mtd-utils-1.4.9.tar.bz2
                

You can use the variables PACKAGES and FILES to split an application into multiple packages.

Following is an example that uses the libXpm recipe. By default, this recipe generates a single package that contains the library along with a few binaries. You can modify the recipe to split the binaries into separate packages:

     require xorg-lib-common.inc

     DESCRIPTION = "X11 Pixmap library"
     LICENSE = "X-BSD"
     LIC_FILES_CHKSUM = "file://COPYING;md5=3e07763d16963c3af12db271a31abaa5"
     DEPENDS += "libxext libsm libxt"
     PR = "r3"
     PE = "1"

     XORG_PN = "libXpm"

     PACKAGES =+ "sxpm cxpm"
     FILES_cxpm = "${bindir}/cxpm"
     FILES_sxpm = "${bindir}/sxpm"
                

In the previous example, we want to ship the sxpm and cxpm binaries in separate packages. Since bindir would be packaged into the main PN package by default, we prepend the PACKAGES variable so additional package names are added to the start of list. This results in the extra FILES_* variables then containing information that define which files and directories go into which packages. Files included by earlier packages are skipped by latter packages. Thus, the main PN package does not include the above listed files.

If you are building a library and the library offers static linking, you can control which static library files (*.a files) get included in the built library.

The PACKAGES and FILES_* variables in the meta/conf/bitbake.conf configuration file define how files installed by the do_install task are packaged. By default, the PACKAGES variable contains ${PN}-staticdev, which includes all static library files.

Following, is part of the BitBake configuration file. You can see where the static library files are defined:

     PACKAGES = "${PN}-dbg ${PN} ${PN}-doc ${PN}-dev ${PN}-staticdev ${PN}-locale"
     PACKAGES_DYNAMIC = "${PN}-locale-*"
     FILES = ""

     FILES_${PN} = "${bindir}/* ${sbindir}/* ${libexecdir}/* ${libdir}/lib*${SOLIBS} \
                 ${sysconfdir} ${sharedstatedir} ${localstatedir} \
                 ${base_bindir}/* ${base_sbindir}/* \
                 ${base_libdir}/*${SOLIBS} \
                 ${datadir}/${BPN} ${libdir}/${BPN}/* \
                 ${datadir}/pixmaps ${datadir}/applications \
                 ${datadir}/idl ${datadir}/omf ${datadir}/sounds \
                 ${libdir}/bonobo/servers"

     FILES_${PN}-doc = "${docdir} ${mandir} ${infodir} ${datadir}/gtk-doc \
                 ${datadir}/gnome/help"
     SECTION_${PN}-doc = "doc"
     
     FILES_${PN}-dev = "${includedir} ${libdir}/lib*${SOLIBSDEV} ${libdir}/*.la \
                     ${libdir}/*.o ${libdir}/pkgconfig ${datadir}/pkgconfig \
                     ${datadir}/aclocal ${base_libdir}/*.o"
     SECTION_${PN}-dev = "devel"
     ALLOW_EMPTY_${PN}-dev = "1"
     RDEPENDS_${PN}-dev = "${PN} (= ${EXTENDPKGV})"
     
     FILES_${PN}-staticdev = "${libdir}/*.a ${base_libdir}/*.a"
     SECTION_${PN}-staticdev = "devel"
     RDEPENDS_${PN}-staticdev = "${PN}-dev (= ${EXTENDPKGV})"
                

To add a post-installation script to a package, add a pkg_postinst_PACKAGENAME() function to the .bb file and use PACKAGENAME as the name of the package you want to attach to the postinst script. Normally PN can be used, which automatically expands to PACKAGENAME. A post-installation function has the following structure:

     pkg_postinst_PACKAGENAME () {
     #!/bin/sh -e
     # Commands to carry out
     }
                

The script defined in the post-installation function is called when the root filesystem is created. If the script succeeds, the package is marked as installed. If the script fails, the package is marked as unpacked and the script is executed when the image boots again.

Sometimes it is necessary for the execution of a post-installation script to be delayed until the first boot. For example, the script might need to be executed on the device itself. To delay script execution until boot time, use the following structure in the post-installation script:

     pkg_postinst_PACKAGENAME () {
     #!/bin/sh -e
     if [ x"$D" = "x" ]; then
          # Actions to carry out on the device go here
     else
          exit 1
     fi
     }
                

The previous example delays execution until the image boots again because the D variable points to the directory containing the image when the root filesystem is created at build time but is unset when executed on the first boot.

To add a machine configuration you need to add a .conf file with details of the device being added to the conf/machine/ file. The name of the file determines the name the Yocto Project uses to reference the new machine.

The most important variables to set in this file are as follows:

You might also need these variables:

You can find full details on these variables in the reference section. You can leverage many existing machine .conf files from meta/conf/machine/.

The Yocto Project needs to be able to build a kernel for the machine. You need to either create a new kernel recipe for this machine, or extend an existing recipe. You can find several kernel examples in the Yocto Project file's meta/recipes-kernel/linux directory that you can use as references.

If you are creating a new recipe, normal recipe-writing rules apply for setting up a SRC_URI. Thus, you need to specify any necessary patches and set S to point at the source code. You need to create a configure task that configures the unpacked kernel with a defconfig. You can do this by using a make defconfig command or, more commonly, by copying in a suitable defconfig file and and then running make oldconfig. By making use of inherit kernel and potentially some of the linux-*.inc files, most other functionality is centralized and the the defaults of the class normally work well.

If you are extending an existing kernel, it is usually a matter of adding a suitable defconfig file. The file needs to be added into a location similar to defconfig files used for other machines in a given kernel. A possible way to do this is by listing the file in the SRC_URI and adding the machine to the expression in COMPATIBLE_MACHINE:

     COMPATIBLE_MACHINE = '(qemux86|qemumips)'
                

A formfactor configuration file provides information about the target hardware for which the Yocto Project is building and information that the Yocto Project cannot obtain from other sources such as the kernel. Some examples of information contained in a formfactor configuration file include framebuffer orientation, whether or not the system has a keyboard, the positioning of the keyboard in relation to the screen, and the screen resolution.

The Yocto Project uses reasonable defaults in most cases, but if customization is necessary you need to create a machconfig file in the Yocto Project file's meta/recipes-bsp/formfactor/files directory. This directory contains directories for specific machines such as qemuarm and qemux86. For information about the settings available and the defaults, see the meta/recipes-bsp/formfactor/files/config file found in the same area. Following is an example for qemuarm:

     HAVE_TOUCHSCREEN=1
     HAVE_KEYBOARD=1

     DISPLAY_CAN_ROTATE=0
     DISPLAY_ORIENTATION=0
     #DISPLAY_WIDTH_PIXELS=640
     #DISPLAY_HEIGHT_PIXELS=480
     #DISPLAY_BPP=16
     DISPLAY_DPI=150
     DISPLAY_SUBPIXEL_ORDER=vrgb
                

During a build, the unpacked temporary source code used by recipes to build packages is available in the Yocto Project Build Directory as defined by the S variable. Below is the default value for the S variable as defined in the meta/conf/bitbake.conf configuration file in the Yocto Project Files:

     S = ${WORKDIR}/${BP}
                

You should be aware that many recipes override the S variable. For example, recipes that fetch their source from Git usually set S to ${WORKDIR}/git.

     BP = ${BPN}-${PV}
                    

The path to the work directory for the recipe (WORKDIR) depends on the package name and the architecture of the target device. For example, here is the work directory for packages whose targets are not device-dependent:

     ${TMPDIR}/work/${PACKAGE_ARCH}-poky-${TARGET_OS}/${PN}-${PV}-${PR}
                

Let's look at an example without variables. Assuming a Yocto Project Files top-level directory named poky and a default Yocto Project Build Directory of poky/build, the following is the work directory for the acl package:

     ~/poky/build/tmp/work/i586-poky-linux/acl-2.2.51-r3
                

If your package is dependent on the target device, the work directory varies slightly:

     ${TMPDIR}/work/${MACHINE}-poky-${TARGET_OS}/${PN}-${PV}-${PR}
                

Again, assuming a Yocto Project Files top-level directory named poky and a default Yocto Project Build Directory of poky/build, the following is the work directory for the acl package that is being built for a MIPS-based device:

     ~/poky/build/tmp/work/mips-poky-linux/acl-2.2.51-r2
                

Now that you know where to locate the directory that has the temporary source code, you can use a Quilt or Git workflow to make your edits, test the changes, and preserve the changes in the form of patches.

Quilt is a powerful tool that allows you to capture source code changes without having a clean source tree. This section outlines the typical workflow you can use to modify temporary source code, test changes, and then preserve the changes in the form of a patch all using Quilt.

Follow these general steps:

Git is an even more powerful tool that allows you to capture source code changes without having a clean source tree. This section outlines the typical workflow you can use to modify temporary source code, test changes, and then preserve the changes in the form of a patch all using Git. For general information on Git as it is used in the Yocto Project, see the "Git" section.

Follow these general steps:

User-specific requirements drive the Multilib feature, Consequently, there is no one "out-of-the-box" configuration that likely exists to meet your needs.

In order to enable Multilib, you first need to ensure your recipe is extended to support multiple libraries. Many standard recipes are already extended and support multiple libraries. You can check in the meta/conf/multilib.conf configuration file in the Yocto Project files directory to see how this is done using the BBCLASSEXTEND variable. Eventually, all recipes will be covered and this list will be unneeded.

For the most part, the Multilib class extension works automatically to extend the package name from ${PN} to ${MLPREFIX}${PN}, where MLPREFIX is the particular multilib (e.g. "lib32-" or "lib64-"). Standard variables such as DEPENDS, RDEPENDS, RPROVIDES, RRECOMMENDS, PACKAGES, and PACKAGES_DYNAMIC are automatically extended by the system. If you are extending any manual code in the recipe, you can use the ${MLPREFIX} variable to ensure those names are extended correctly. This automatic extension code resides in multilib.bbclass.

After you have set up the recipes, you need to define the actual combination of multiple libraries you want to build. You accomplish this through your local.conf configuration file in the Yocto Project Build Directory. An example configuration would be as follows:

     MACHINE = "qemux86-64"
     require conf/multilib.conf
     MULTILIBS = "multilib:lib32"
     DEFAULTTUNE_virtclass-multilib-lib32 = "x86"
     IMAGE_INSTALL = "lib32-connman"
                

This example enables an additional library named lib32 alongside the normal target packages. When combining these "lib32" alternatives, the example uses "x86" for tuning. For information on this particular tuning, see meta/conf/machine/include/ia32/arch-ia32.inc.

The example then includes lib32-connman in all the images, which illustrates one method of including a multiple library dependency. You can use a normal image build to include this dependency, for example:

     $ bitbake core-image-sato
                

You can also build Multilib packages specifically with a command like this:

     $  bitbake lib32-connman
                

Different packaging systems have different levels of native Multilib support. For the RPM Package Management System, the following implementation details exist:

For the IPK Package Management System, the following implementation details exist:

The easiest way to define kernel configurations is to set them through the menuconfig tool. For general information on menuconfig, see http://en.wikipedia.org/wiki/Menuconfig.

To use the menuconfig tool in the Yocto Project development environment, you must build the tool using BitBake. The following commands build and invoke menuconfig assuming the Yocto Project files top-level directory is ~/poky:

     $ cd ~/poky
     $ source oe-init-build-env
     $ bitbake linux-yocto -c menuconfig
                

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

For an example that shows how to change the SMP_CONFIG parameter using menuconfig, see the "Changing the CONFIG_SMP Configuration Using menuconfig" section.

Configuration fragments are simply kernel options that appear in a file. Syntactically, the configuration statement is identical to what would appear in the .config. For example, issuing the following from the shell would create a config fragment file named my_smp.cfg that enables multi-processor support within the kernel:

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

Where do you put your configuration files? You can place these configuration files in the same area pointed to by SRC_URI. The Yocto Project build process will pick up the configuration and add it to the kernel's configuration. For example, assume you add the following to your linux-yocto_3.0.bbappend file:

     file://my_smp.cfg
                

You would put the config fragment file my_smp.cfg in a sub-directory with the same root name (linux-yocto) beneath the directory that contains your linux-yocto_3.0.bbappend file and the build system will pick up and apply the fragment.

You can make sure the .config 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 Linux Yocto kernel build process, the 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:

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

To streamline the configuration, do the following:

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.

A BSP is a package of recipes that, when applied, during a build results in an image that you can run on a particular board. Thus, the package, when compiled into the new image, supports the operation of the board.

The remainder of this section presents the basic steps used to create a BSP based on an existing BSP that ships with the Yocto Project. You can reference the "BSP Development Example" appendix for a detailed example that uses the Crown Bay BSP as a base BSP from which to start.

The following illustration and list summarize the BSP creation general workflow.

You can view a video presentation on "Building Custom Embedded Images with Yocto" at Free Electrons. You can also find supplemental information in The Board Support Package (BSP) Development Guide. Finally, there is wiki page write up of the example also located here that you might find helpful.

Kernel modification involves changing the Linux Yocto kernel, which could involve changing configuration options as well as adding new kernel recipes. Configuration changes can be added in the form of configuration fragments, while recipe modification comes through the kernel's recipes-kernel area in a kernel layer you create.

The remainder of this section presents a high-level overview of the Linux Yocto kernel architecture and the steps to modify the Linux Yocto kernel. For a complete discussion of the kernel, see The Yocto Project Kernel Architecture and Use Manual. You can reference the appendix "Kernel Modification Example" for a detailed example that changes the configuration of a kernel.

5.1.2.1. Kernel Overview

Traditionally, when one thinks of a patched kernel, they think of a base kernel source tree and a fixed structure that contains kernel patches. The Yocto Project, however, employs mechanisms, that in a sense, result in a kernel source generator. By the end of this section, this analogy will become clearer.

You can find a web interface to the Linux Yocto kernel source repositories at http://git.yoctoproject.org. If you look at the interface, you will see to the left a grouping of Git repositories titled "Yocto Linux Kernel." Within this group, you will find the four different kernels supported by the Yocto Project:

• 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 stable Linux Yocto kernel that is based on the Linux 2.6.37 release.

• linux-yocto-3.0 - The stable Linux Yocto kernel that is based on the Linux 3.0 release.

• linux-yocto-3.0-1.1.x - The stable Linux Yocto kernel to use with the Yocto Project Release 1.1.x. This kernel is based on the Linux 3.0 release

• linux-yocto-3.2 - The stable Linux Yocto kernel to use with the Yocto Project Release 1.2. This kernel is based on the Linux 3.2 release

• linux-yocto-dev - A development kernel based on the latest upstream release candidate available.

The kernels are maintained using the Git revision control system that structures them using the familiar "tree", "branch", and "leaf" scheme. Branches represent diversions from general code to more specific code, while leaves represent the end-points for a complete and unique kernel whose source files when gathered from the root of the tree to the leaf accumulate to create the files necessary for a specific piece of hardware and its features. The following figure displays this concept:

Within the figure, the "Kernel.org Branch Point" represents the point in the tree where a supported base kernel is modified from the Linux kernel. For example, this could be the branch point for the linux-yocto-3.0 kernel. Thus, everything further to the right in the structure is based on the linux-yocto-3.0 kernel. Branch points to right in the figure represent where the linux-yocto-3.0 kernel is modified for specific hardware or types of kernels, such as real-time kernels. Each leaf thus represents the end-point for a kernel designed to run on a specific targeted device.

The overall result is a Git-maintained repository from which all the supported Yocto Project kernel types can be derived for all the supported Yocto Project devices. A big advantage to this scheme is the sharing of common features by keeping them in "larger" branches within the tree. This practice eliminates redundant storage of similar features shared among kernels.

Note

Keep in mind the figure does not take into account all four supported Linux Yocto kernel types, but rather shows a single generic kernel just for conceptual purposes. Also keep in mind that this structure represents the Yocto Project source repositories that are either pulled from during the build or established on the host development system prior to the build by either cloning a particular kernel's Git repository or by downloading and unpacking a tarball.

Storage of all the available kernel source code is one thing, while representing the code on your host development system is another. Conceptually, you can think of the Yocto Project kernel source repositories as all the source files necessary for all the supported kernels. As a developer, you are just interested in the source files for the kernel on on which you are working. And, furthermore, you need them available on your host system.

You make kernel source code available on your host development system by using Git to create a bare clone of the Linux Yocto kernel Git repository in which you are interested. Then, you use Git again to clone a copy of that bare clone. This copy represents the directory structure on your host system that is particular to the kernel you want. These are the files you actually modify to change the kernel. See the Linux Yocto Kernel item earlier in this manual for an example of how to set up the kernel source directory structure on your host system.

This next figure illustrates how the kernel source files might be arranged on your host system.

In the previous figure, the file structure on the left represents the bare clone set up to track the Yocto Project kernel Git repository. The structure on the right represents the copy of the bare clone. When you make modifcations to the kernel source code, this is the area in which you work. Once you make corrections, you must use Git to push the committed changes to the bare clone. The example in Section B.1, “Modifying the Kernel Source Code” provides a detailed example.

What happens during the build? 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.

The following figure shows the temporary file structure created on your host system when the build occurs. This build directory contains all the source files used during the build.

Again, 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. Also, you can reference Section B.1, “Modifying the Kernel Source Code” for a detailed example that modifies the kernel.

5.1.2.2. Kernel Modification Workflow

This illustration and the following list summarizes the kernel modification general workflow.

1. Set up your host development system to support development using the Yocto Project: See "The Linux Distributions" and "The Packages" sections both in the Yocto Project Quick Start for requirements.

2. Establish a local copy of the Yocto Project files on your system: Having the Yocto Project files on your system gives you access to the build process and tools you need. For information on how to get these files, see the bulleted item "Yocto Project Release" earlier in this manual.

3. Set up the poky-extras Git repository: This repository is the area for your configuration fragments, new kernel recipes, and the kernel .bbappend file used during the build. It is good practice to set this repository up inside the local Yocto Project files Git repository. For information on how to get these files, see the bulleted item "The poky-extras Git Repository" earlier in this manual.

   Note

   While it is certainly possible to modify the kernel without involving a local Git repository, the suggested workflow for kernel modification using the Yocto Project does use a Git repository.

4. Establish a local copy of the Linux Yocto kernel files on your system: In order to make modifications to the kernel you need two things: a bare clone of the Linux Yocto kernel you are modifying and a copy of that bare clone. The bare clone is required by the build process and is the area to which you push your kernel source changes (pulling does not work with bare clones). The copy of the bare clone is a local Git repository that contains all the kernel's source files. You make your changes to the files in this copy of the bare clone. For information on how to set these two items up, see the bulleted item "Linux Yocto Kernel" earlier in this manual.

5. Make changes to the kernel source code if applicable: Modifying the kernel does not always mean directly changing source files. However, if you have to do this, you make the changes in the local Git repository you set up to hold the source files (i.e. the copy of the bare clone). Once the changes are made, you need to use Git commands to commit the changes and then push them to the bare clone.

6. Make kernel configuration changes if applicable: If your situation calls for changing the kernel's configuration, you can use menuconfig to enable and disable kernel configurations. Using menuconfig allows you to interactively develop and test the configuration changes you are making to the kernel. When saved, changes using menuconfig update the kernel's .config. Try to resist the temptation of directly editing the .config file found in the Yocto Project Build Directory at tmp/sysroots/<machine-name>/kernel. Doing so, can produce unexpected results when the Yocto Project build system regenerates the configuration file.

   Once you are satisfied with the configuration changes made using menuconfig, you can directly examine the .config file against a saved original and gather those changes into a config fragment to be referenced from within the kernel's .bbappend file.

7. Add or extend kernel recipes if applicable: The standard layer structure organizes recipe files inside the meta-kernel-dev layer that is within the poky-extras Git repository. If you need to add new kernel recipes, you add them within this layer. Also within this area, you will find the .bbappend file that appends information to the kernel's recipe file used during the build.

8. Prepare for the build: Once you have made all the changes to your kernel (configurations, source code changes, recipe additions, or recipe changes), there remains a few things you need to do in order for the Yocto Project build system to create your image. If you have not done so, you need to get the build environment ready by sourcing the environment setup script described earlier. You also need to be sure two key configuration files (local.conf and bblayers.conf) are configured appropriately.

   The entire process for building an image is overviewed in the "Building an Image" section of the Yocto Project Quick Start. You might want to reference this information. Also, you should look at the detailed examples found in the appendices at at the end of this manual.

9. Build the image: The Yocto Project build system Poky uses the BitBake tool to build images based on the type of image you want to create. You can find more information on BitBake here.

   The build process supports several types of images to satisfy different needs. See the appendix "Reference: Images" in The Yocto Project Reference Manual for information on supported images.

10. Make your configuration changes available in the kernel layer: Up to this point, all the configuration changes to the kernel have been done and tested iteratively. Once they are tested and ready to go, you can move them into the kernel layer, which allows you to distribute the layer.

11. If applicable, share your in-tree changes: If the changes you made are suited for all Linux Yocto users, you might want to send them on for inclusion into the Linux Yocto Git repository. If the changes are accepted, the Yocto Project Maintainer pulls them into the master branch of the kernel tree. Doing so makes them available to everyone using the kernel.

To help you understand how application development works in the Yocto Project ADT environment, this section provides an overview of the general development process. If you want to see a detailed example of the process as it is used from within the Eclipse IDE, see The Application Development Toolkit (ADT) User's Manual.

This illustration and the following list summarizes the application development general workflow.

If you want to develop an application outside of the Yocto Project ADT environment, you can still employ the cross-development toolchain, the QEMU emulator, and a number of supported target image files. You just need to follow these general steps: