# Extending Poky

This section gives information about how to extend the functionality already present in Poky, documenting standard tasks such as adding new software packages, extending or customising images or porting poky to new hardware (adding a new machine). It also contains advice about how to manage the process of making changes to Poky to achieve best results.

## Adding a Package

To add package into Poky you need to write a recipe for it. Writing a recipe means creating a .bb file which sets various variables. The variables useful for recipes are detailed in the recipe reference section along with more detailed information about issues such as recipe naming.

The simplest way to add a new package is to base it on a similar pre-existing recipe. There are some examples below of how to add standard types of packages:

### Single .c File Package (Hello World!)

To build an application from a single file stored locally requires a recipe which has the file listed in the SRC_URI variable. In addition the `do_compile` and `do_install` tasks need to be manually written. The S variable defines the directory containing the source code which in this case is set equal to WORKDIR, the directory BitBake uses for the build.

    DESCRIPTION = "Simple helloworld application"
    SECTION = "examples"
    LICENSE = "MIT"

    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}
    }

As a result of the build process "helloworld" and "helloworld-dbg" packages will be built.

### Autotooled Package

Applications which use autotools (autoconf, automake) require a recipe which has a source archive listed in SRC_URI and `inherit autotools` to instruct BitBake to use the `autotools.bbclass` which has definitions of all the steps needed to build an autotooled application. The result of the build will be automatically packaged and if the application uses NLS to localise then packages with locale information will be generated (one package per language).

    DESCRIPTION = "GNU Helloworld application"
    SECTION = "examples"
    LICENSE = "GPLv2"

    SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.bz2"

    inherit autotools

### Makefile-Based Package

Applications which use GNU make require a recipe which has the source archive listed in SRC_URI. Adding a `do_compile` step is not needed as by default BitBake will start the "make" command to compile the application. If there is a need for additional options to make then they should be stored in the EXTRA_OEMAKE variable - BitBake will pass them into the GNU make invocation. A `do_install` task is required - otherwise BitBake will run an empty `do_install` task by default.

Some applications may require extra parameters to be passed to the compiler, for example an additional header path. This can be done buy adding to the CFLAGS variable, as in the example below.

    DESCRIPTION = "Tools for managing memory technology devices."
    SECTION = "base"
    DEPENDS = "zlib"
    HOMEPAGE = "http://www.linux-mtd.infradead.org/"
    LICENSE = "GPLv2"

    SRC_URI = "ftp://ftp.infradead.org/pub/mtd-utils/mtd-utils-${PV}.tar.gz"

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

    do_install() {
    	oe_runmake install DESTDIR=${D}
    }

### Controlling packages content

The variables PACKAGES and FILES are used to split an application into multiple packages.

Below the "libXpm" recipe is used as an example. By default the "libXpm" recipe generates one package which contains the library and also a few binaries. The recipe can be adapted to split the binaries into separate packages.

    require xorg-lib-common.inc

    DESCRIPTION = "X11 Pixmap library"
    LICENSE = "X-BSD"
    DEPENDS += "libxext"
    PE = "1"

    XORG_PN = "libXpm"

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

In this example we want to ship the "sxpm" and "cxpm" binaries in separate packages. Since "bindir" would be packaged into the main PN package as standard we prepend the PACKAGES variable so additional package names are added to the start of list. The extra FILES_* variables then contain information to specify which files and directories goes into which package.

### Post Install Scripts

To add a post-installation script to a package, add a `pkg_postinst_PACKAGENAME()` function to the .bb file where PACKAGENAME is the name of the package to attach the postinst script to. 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 gets called when the rootfs is made. If the script succeeds, the package is marked as installed. If the script fails, the package is marked as unpacked and the script will be executed again on the first boot of the image.

Sometimes it is necessary that the execution of a post-installation script is delayed until the first boot, because the script needs to be executed the device itself. To delay script execution until boot time, the post-installation function should have the following structure:

    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 structure above delays execution until first boot because the D variable points to the 'image' directory when the rootfs is being made at build time but is unset when executed on the first boot.

## Customising Images

Poky images can be customised to satisfy particular requirements. Several methods are detailed below along with guidelines of when to use them.

### Customising Images through a custom image .bb files

One way to get additional software into an image is by creating a custom image. The recipe will contain two lines:

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

    inherit poky-image

By creating a custom image, a developer has total control over the contents of the image. It is important use the correct names of packages in the IMAGE_INSTALL variable. The names must be in the OpenEmbedded notation instead of Debian notation, for example "glibc-dev" instead of "libc6-dev" etc.

The other method of creating a new image is by modifying an existing image. For example if a developer wants to add "strace" into "poky-image-sato" the following recipe can be used:

    require poky-image-sato.bb

    IMAGE_INSTALL += "strace"

### Customising Images through custom tasks

For for complex custom images, the best approach is to create a custom task package which is them used to build the image (or images). A good example of a tasks package is `meta/packages/tasks/task-poky.bb` . The PACKAGES variable lists the task packages to build (along with the complimentary -dbg and -dev packages). For each package added, RDEPENDS and RRECOMMENDS entries can then be added each containing a list of packages the parent task package should contain. An example would be:

    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 this example, two tasks packages are created, task-custom-apps and task-custom-tools with the dependencies and recommended package dependencies listed. To build an image using these task packages, you would then add "task-custom-apps" and/or "task-custom-tools" to IMAGE_INSTALL or other forms of image dependencies as described in other areas of this section.

### Customising Images through custom IMAGE_FEATURES

Ultimately users may want to add extra image "features" as used by Poky with the IMAGE_FEATURES variable. To create these, the best reference is `meta/classes/poky-image.bbclass` which illustrates how poky achieves this. In summary, the file looks at the contents of the IMAGE_FEATURES variable and based on this generates the IMAGE_INSTALL variable automatically. Extra features can be added by extending the class or creating a custom class for use with specialised image .bb files.

### Customising Images through local.conf

It is possible to customise image contents by abusing variables used by distribution maintainers in local.conf. This method only allows the addition of packages and is not recommended.

To add an "strace" package into the image the following is added to local.conf:

    DISTRO_EXTRA_RDEPENDS += "strace"

However, since the DISTRO_EXTRA_RDEPENDS variable is for distribution maintainers this method does not make adding packages as simple as a custom .bb file. Using this method, a few packages will need to be recreated and the the image built.

    bitbake -cclean task-boot task-base task-poky
    bitbake poky-image-sato

Cleaning task-* packages is required because they use the DISTRO_EXTRA_RDEPENDS variable. There is no need to build them by hand as Poky images depend on the packages they contain so dependencies will be built automatically. For this reason we don't use the "rebuild" task in this case since "rebuild" does not care about dependencies - it only rebuilds the specified package.

## Porting Poky to a new machine

Adding a new machine to Poky is a straightforward process and this section gives an idea of the changes that are needed. This guide is meant to cover adding machines similar to those Poky already supports. Adding a totally new architecture might require gcc/glibc changes as well as updates to the site information and, whilst well within Poky's capabilities, is outside the scope of this section.

### Adding the machine configuration file

A .conf file needs to be added to conf/machine/ with details of the device being added. The name of the file determines the name Poky will use to reference this machine.

The most important variables to set in this file are TARGET_ARCH (e.g. "arm"), PREFERRED_PROVIDER_virtual/kernel (see below) and MACHINE_FEATURES (e.g. "kernel26 apm screen wifi"). Other variables like SERIAL_CONSOLE (e.g. "115200 ttyS0"), KERNEL_IMAGETYPE (e.g. "zImage") and IMAGE_FSTYPES (e.g. "tar.gz jffs2") might also be needed. Full details on what these variables do and the meaning of their contents is available through the links.

### Adding a kernel for the machine

Poky 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. There are plenty of kernel examples in the packages/linux directory which can be used as references.

If creating a new recipe the "normal" recipe writing rules apply for setting up a SRC_URI including any patches and setting S to point at the source code. You will need to create a configure task which configures the unpacked kernel with a defconfig be that through a "make defconfig" command or more usually though copying in a suitable defconfig and running "make oldconfig". By making use of "inherit kernel" and also maybe some of the linux-*.inc files, most other functionality is centralised and the the defaults of the class normally work well.

If extending an existing kernel it is usually a case of adding a suitable defconfig file in a location similar to that used by other machine's defconfig files in a given kernel, possibly listing it in the SRC_URI and adding the machine to the expression in COMPATIBLE_MACHINE .

### Adding a formfactor configuration file

A formfactor configuration file provides information about the target hardware on which Poky is running, and that Poky 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 screen resolution.

Sane defaults should be used in most cases, but if customisation is necessary you need to create a `machconfig` file under `meta/packages/formfactor/files/MACHINENAME/` where `MACHINENAME` is the name for which this infomation applies. For information about the settings available and the defaults, please see `meta/packages/formfactor/files/config`.

## Making and Maintaining Changes

We recognise that people will want to extend/configure/optimise Poky for their specific uses, especially due to the extreme configurability and flexibility Poky offers. To ensure ease of keeping pace with future changes in Poky we recommend making changes to Poky in a controlled way.

Poky supports the idea of "collections" which when used properly can massively ease future upgrades and allow segregation between the Poky core and a given developer's changes. Some other advice on managing changes to Poky is also given in the following section.

### Bitbake Collections

Often, people want to extend Poky either through adding packages or overriding files contained within Poky to add their own functionality. Bitbake has a powerful mechanism called collections which provide a way to handle this which is fully supported and actively encouraged within Poky.

In the standard tree, meta-extras is an example of how you can do this. As standard the data in meta-extras is not used on a Poky build but local.conf.sample shows how to enable it:

    BBFILES := "${OEROOT}/meta/packages/*/*.bb ${OEROOT}/meta-extras/packages/*/*.bb"
    BBFILE_COLLECTIONS = "normal extras"
    BBFILE_PATTERN_normal = "^${OEROOT}/meta/"
    BBFILE_PATTERN_extras = "^${OEROOT}/meta-extras/"
    BBFILE_PRIORITY_normal = "5"
    BBFILE_PRIORITY_extras = "5"

As can be seen, the extra recipes are added to BBFILES. The BBFILE_COLLECTIONS variable is then set to contain a list of collection names. The BBFILE_PATTERN variables are regular expressions used to match files from BBFILES into a particular collection in this case by using the base pathname. The BBFILE_PRIORITY variable then assigns the different priorities to the files in different collections. This is useful in situations where the same package might appear in both repositories and allows you to choose which collection should 'win'.

This works well for recipes. For bbclasses and configuration files, you can use the BBPATH environment variable. In this case, the first file with the matching name found in BBPATH is the one that is used, just like the PATH variable for binaries.

### Committing Changes

Modifications to Poky are often managed under some kind of source revision control system. The policy for committing to such systems is important as some simple policy can significantly improve usability. The tips below are based on the policy that OpenedHand uses for commits to Poky.

It helps to use a consistent style for commit messages when committing changes. We've found a style where the first line of a commit message summarises the change and starts with the name of any package affected work well. Not all changes are to specific packages so the prefix could also be a machine name or class name instead. If a change needs a longer description this should follow the summary.

Any commit should be self contained in that it should leave the metadata in a consistent state, buildable before and after the commit. This helps ensure the autobuilder test results are valid but is good practice regardless.

### Package Revision Incrementing

If a committed change will result in changing the package output then the value of the PR variable needs to be increased (commonly referred to as 'bumped') as part of that commit. Only integer values are used and PR = "r0" should not be added into new recipes as this is default value. When upgrading the version of a package (PV), the PR variable should be removed.

The aim is that the package version will only ever increase. If for some reason PV will change and but not increase, the PE (Package Epoch) can be increased (it defaults to '0'). The version numbers aim to follow the Debian Version Field Policy Guidelines which define how versions are compared and hence what "increasing" means.

There are two reasons for doing this, the first is to ensure that when a developer updates and rebuilds, they get all the changes to the repository and don't have to remember to rebuild any sections. The second is to ensure that target users are able to upgrade their devices via their package manager such as with the `opkg update;opkg upgrade` commands (or similar for dpkg/apt or rpm based systems). The aim is to ensure Poky has upgradable packages in all cases.

## Modifying Package Source Code

Poky is usually used to build software rather than modifying it. However, there are ways Poky can be used to modify software.

During building, the sources are available in WORKDIR directory. Where exactly this is depends on the type of package and the architecture of target device. For a standard recipe not related to MACHINE it will be `tmp/work/PACKAGE_ARCH-poky-TARGET_OS/PN-PV-PR/`. Target device dependent packages use MACHINE instead of PACKAGE_ARCH in the directory name.

Check the package recipe sets the S variable to something other than standard `WORKDIR/PN-PV/` value.

After building a package, a user can modify the package source code without problem. The easiest way to test changes is by calling the "compile" task:

    bitbake --cmd compile --force NAME_OF_PACKAGE

Other tasks may also be called this way.

### Modifying Package Source Code with quilt

By default Poky uses quilt to manage patches in `do_patch` task. It is a powerful tool which can be used to track all modifications done to package sources.

Before modifying source code it is important to notify quilt so it will track changes into new patch file:

    quilt new NAME-OF-PATCH.patch

Then add all files which will be modified into that patch:

    quilt add file1 file2 file3

Now start editing. At the end quilt needs to be used to generate final patch which will contain all modifications:

    quilt refresh

The resulting patch file can be found in the `patches/` subdirectory of the source (S) directory. For future builds it should be copied into Poky metadata and added into SRC_URI of a recipe:

    SRC_URI += "file://NAME-OF-PATCH.patch;patch=1"

This also requires a bump of PR value in the same recipe as we changed resulting packages.