poky/handbook/extendpoky.xml

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<chapter id='extendpoky'>

<title>Extending Poky</title>

    <para>
        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.
    </para>

    <section id='usingpoky-extend-addpkg'>
        <title>Adding a Package</title>

        <para>
            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 <link linkend='ref-varlocality-recipe-required'>
                recipe reference</link> section along with more detailed information 
            about issues such as recipe naming.
        </para>

        <para>
            Before writing a recipe from scratch it is often useful to check
            someone else hasn't written one already. OpenEmbedded is a good place 
            to look as it has a wider scope and hence a wider range of packages.
            Poky aims to be compatible with OpenEmbedded so most recipes should
            just work in Poky.
        </para>

        <para>
            For new packages, the simplest way to add a recipe is to base it on a similar
            pre-existing recipe. There are some examples below of how to add 
            standard types of packages:
        </para>

        <section id='usingpoky-extend-addpkg-singlec'>
            <title>Single .c File Package (Hello World!)</title>

            <para>
                To build an application from a single file stored locally requires a
                recipe which has the file listed in the <glossterm><link
                linkend='var-SRC_URI'>SRC_URI</link></glossterm> variable. In addition 
                the <function>do_compile</function> and <function>do_install</function> 
                tasks need to be manually written. The <glossterm><link linkend='var-S'>
                S</link></glossterm> variable defines the directory containing the source 
                code which in this case is set equal to <glossterm><link linkend='var-WORKDIR'>
                WORKDIR</link></glossterm>, the directory BitBake uses for the build.
            </para>
            <programlisting>
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}
}
            </programlisting>

            <para>
                As a result of the build process "helloworld" and "helloworld-dbg"
                packages will be built.
            </para>
        </section>

        <section id='usingpoky-extend-addpkg-autotools'>
            <title>Autotooled Package</title>

            <para>
                Applications which use autotools (autoconf, automake)
                require a recipe which has a source archive listed in 
                <glossterm><link
                        linkend='var-SRC_URI'>SRC_URI</link></glossterm> and 
                <command>inherit autotools</command> to instruct BitBake to use the
                <filename>autotools.bbclass</filename> 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).
            </para>

            <programlisting>
DESCRIPTION = "GNU Helloworld application"
SECTION = "examples"
LICENSE = "GPLv2"

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

inherit autotools
            </programlisting>

        </section>

        <section id='usingpoky-extend-addpkg-makefile'>
            <title>Makefile-Based Package</title>

            <para>
                Applications which use GNU make require a recipe which has
                the source archive listed in <glossterm><link
                        linkend='var-SRC_URI'>SRC_URI</link></glossterm>. 
                Adding a <function>do_compile</function> 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
                <glossterm><link
                        linkend='var-EXTRA_OEMAKE'>EXTRA_OEMAKE</link></glossterm> variable - BitBake 
                will pass them into the GNU
                make invocation. A <function>do_install</function> task is required
                - otherwise BitBake will run an empty <function>do_install</function> 
                task by default. 
            </para>

            <para>
                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 <glossterm><link
                        linkend='var-CFLAGS'>CFLAGS</link></glossterm> variable, as in the example below.
            </para>

            <programlisting>
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}
}
            </programlisting>

        </section>

        <section id='usingpoky-extend-addpkg-files'>
            <title>Controlling packages content</title>

            <para>                        
                The variables <glossterm><link
                        linkend='var-PACKAGES'>PACKAGES</link></glossterm> and 
                <glossterm><link linkend='var-FILES'>FILES</link></glossterm> are used to split an
                application into multiple packages.
            </para>

            <para>
                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.
            </para>

            <programlisting>
require xorg-lib-common.inc

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

XORG_PN = "libXpm"

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

            <para>
                In this example we want to ship the "sxpm" and "cxpm" binaries 
                in separate packages. Since "bindir" would be packaged into the 
                main <glossterm><link linkend='var-PN'>PN</link></glossterm> 
                package as standard we prepend the <glossterm><link
                linkend='var-PACKAGES'>PACKAGES</link></glossterm> variable so
                additional package names are added to the start of list. The 
                extra <glossterm><link linkend='var-PN'>FILES</link></glossterm>_*
                variables then contain information to specify which files and
                directories goes into which package. 
            </para>
        </section>

        <section id='usingpoky-extend-addpkg-postinstalls'>
            <title>Post Install Scripts</title>

            <para>
                To add a post-installation script to a package, add
                a <function>pkg_postinst_PACKAGENAME()</function> 
                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:
            </para>
            <programlisting>
pkg_postinst_PACKAGENAME () {
#!/bin/sh -e
# Commands to carry out
}
            </programlisting>
            <para>
                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.
            </para>

            <para>
                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 on the device itself.  To delay script execution
                until boot time, the post-installation function should have the
                following structure:
            </para>
            
            <programlisting>
pkg_postinst_PACKAGENAME () {
#!/bin/sh -e
if [ x"$D" = "x" ]; then
# Actions to carry out on the device go here
else
exit 1
fi
}
            </programlisting>
            
            <para>
                The structure above delays execution until first boot 
                because the <glossterm><link
                        linkend='var-D'>D</link></glossterm> variable points
                to the 'image'
                directory when the rootfs is being made at build time but
                is unset when executed on the first boot. 
            </para>
        </section>

    </section>

    <section id='usingpoky-extend-customimage'>
        <title>Customising Images</title>

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

        <section id='usingpoky-extend-customimage-custombb'>
            <title>Customising Images through a custom image .bb files</title>

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

            <programlisting>
IMAGE_INSTALL = "task-poky-x11-base package1 package2"

inherit poky-image
            </programlisting>

            <para>
                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 <glossterm><link
                linkend='var-IMAGE_INSTALL'>IMAGE_INSTALL</link></glossterm> variable. 
                The names must be in 
                the OpenEmbedded notation instead of Debian notation, for example
                "glibc-dev" instead of "libc6-dev" etc.
            </para>

            <para>
                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:
            </para>

            <programlisting>
require poky-image-sato.bb

IMAGE_INSTALL += "strace"
            </programlisting>

        </section>

        <section id='usingpoky-extend-customimage-customtasks'>
            <title>Customising Images through custom tasks</title>

            <para>
                For complex custom images, the best approach is to create a custom 
                task package which is then used to build the image (or images). A good
                example of a tasks package is <filename>meta/packages/tasks/task-poky.bb
                </filename>. The <glossterm><link linkend='var-PACKAGES'>PACKAGES</link></glossterm> 
                variable lists the task packages to build (along with the complementary
                -dbg and -dev packages). For each package added,
                <glossterm><link linkend='var-PACKAGES'>RDEPENDS</link></glossterm> and
                <glossterm><link linkend='var-PACKAGES'>RRECOMMENDS</link></glossterm> 
                entries can then be added each containing a list of packages the parent 
                task package should contain. An example would be:
            </para>

            <para>
            <programlisting>
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"
</programlisting>
            </para>

            <para>
                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 <glossterm><link 
                linkend='var-IMAGE_INSTALL'>IMAGE_INSTALL</link></glossterm> or other forms 
                of image dependencies as described in other areas of this section.
            </para>
        </section>

        <section id='usingpoky-extend-customimage-imagefeatures'>
            <title>Customising Images through custom <glossterm><link linkend='var-IMAGE_FEATURES'>IMAGE_FEATURES</link></glossterm></title>

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

        <section id='usingpoky-extend-customimage-localconf'>
            <title>Customising Images through local.conf</title>

            <para>
                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.
            </para>

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

            <programlisting>
DISTRO_EXTRA_RDEPENDS += "strace"
            </programlisting>

            <para>
                However, since the <glossterm><link linkend='var-DISTRO_EXTRA_RDEPENDS'>
                DISTRO_EXTRA_RDEPENDS</link></glossterm> 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.
            </para>
            <programlisting>
bitbake -cclean task-boot task-base task-poky
bitbake poky-image-sato
            </programlisting>

            <para>
                Cleaning task-* packages is required because they use the
                <glossterm><link linkend='var-DISTRO_EXTRA_RDEPENDS'>
                DISTRO_EXTRA_RDEPENDS</link></glossterm> 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.
            </para>

        </section>

    </section>

<section id="platdev-newmachine">
    <title>Porting Poky to a new machine</title>
    <para>
        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.
    </para>

    <section id="platdev-newmachine-conffile">
        <title>Adding the machine configuration file</title>
        <para>
            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.
        </para>

        <para>
            The most important variables to set in this file are <glossterm>
            <link linkend='var-TARGET_ARCH'>TARGET_ARCH</link></glossterm> 
            (e.g. "arm"), <glossterm><link linkend='var-PREFERRED_PROVIDER'>
            PREFERRED_PROVIDER</link></glossterm>_virtual/kernel (see below) and 
            <glossterm><link linkend='var-MACHINE_FEATURES'>MACHINE_FEATURES
            </link></glossterm> (e.g. "kernel26 apm screen wifi"). Other variables
            like <glossterm><link linkend='var-SERIAL_CONSOLE'>SERIAL_CONSOLE
            </link></glossterm> (e.g. "115200 ttyS0"), <glossterm> 
            <link linkend='var-KERNEL_IMAGETYPE'>KERNEL_IMAGETYPE</link>
            </glossterm> (e.g. "zImage") and <glossterm><link linkend='var-IMAGE_FSTYPES'>
            IMAGE_FSTYPES</link></glossterm> (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.
        </para>
    </section>

    <section id="platdev-newmachine-kernel">
        <title>Adding a kernel for the machine</title>
        <para>
             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.
        </para>
        <para>
             If creating a new recipe  the "normal" recipe writing rules apply 
             for setting up a <glossterm><link linkend='var-SRC_URI'>SRC_URI
             </link></glossterm> including any patches and setting <glossterm>
             <link linkend='var-S'>S</link></glossterm> 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.
        </para>
        <para>
            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 <glossterm>
            <link linkend='var-COMPATIBLE_MACHINE'>COMPATIBLE_MACHINE</link>
            </glossterm>.
        </para>
    </section>

    <section id="platdev-newmachine-formfactor">
        <title>Adding a formfactor configuration file</title>
        <para>
            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.
        </para>
        <para>
            Sane defaults should be used in most cases, but if customisation is 
            necessary you need to create a <filename>machconfig</filename> file 
            under <filename>meta/packages/formfactor/files/MACHINENAME/</filename>
            where <literal>MACHINENAME</literal> is the name for which this infomation
            applies. For information about the settings available and the defaults, please see 
            <filename>meta/packages/formfactor/files/config</filename>.
        </para>
    </section>
</section>

<section id='usingpoky-changes'>
        <title>Making and Maintaining Changes</title>

        <para>
            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.
        </para>
        <para>
            Poky supports the idea of <link
                linkend='usingpoky-changes-collections'>"collections"</link> 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.
        </para>

        <section id="usingpoky-changes-collections">
            <title>Bitbake Collections</title>

            <para>
                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.
            </para>
            <para>
                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:
            </para>
            <para>
                <literallayout class='monospaced'>
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"</literallayout>
            </para>
            <para>
                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'.
            </para>
            <para>
                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.
            </para>
        </section>

        <section id="usingpoky-changes-supplement">
            <title>Supplementry Metadata Repositories</title>

            <para>
                Often when developing a project based on Poky there will be components 
                that are not ready for public consumption for whatever reason. By making
                use of the collections mechanism and other functionality within Poky, it
                is possible to have a public repository which is supplemented by a private 
                one just containing the pieces that need to be kept private. 
            </para>
            <para>
                The usual approach with these is to create a separate git repository called 
                "meta-prvt-XXX" which is checked out alongside the other meta-* 
                directories included in Poky. Under this directory there can be several 
                different directories such as classes, conf and packages which all 
                function as per the usual Poky directory structure. 
            </para>
            <para>
                If extra meta-* directories are found, Poky will automatically add them 
                into the BBPATH variable so the conf and class files contained there 
                are found. If a file called poky-extra-environment is found within the 
                meta-* directory, this will be sourced as the environment is setup, 
                allowing certain configuration to be overridden such as the location of the 
                local.conf.sample file that is used.
            </para>
            <para>
                Note that at present, BBFILES is not automatically changed and this needs 
                to be adjusted to find files in the packages/ directory. Usually a custom 
                local.conf.sample file will be used to handle this instead.
            </para>
        </section>

        <section id='usingpoky-changes-commits'>
            <title>Committing Changes</title>

            <para>
                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 followed for the 
                Poky core.
            </para>

            <para>
                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.
            </para>

            <para>
                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.
            </para>
        </section>

        <section id='usingpoky-changes-prbump'>
            <title>Package Revision Incrementing</title>

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

            <para>
                The aim is that the package version will only ever increase. If 
                for some reason <glossterm><link linkend='var-PV'>PV</link></glossterm> 
                will change and but not increase, the <glossterm><link 
                linkend='var-PE'>PE</link></glossterm> (Package Epoch) can 
                be increased (it defaults to '0'). The version numbers aim to 
                follow the <ulink url='http://www.debian.org/doc/debian-policy/ch-controlfields.html'>
                Debian Version Field Policy Guidelines</ulink> which define how 
                versions are compared and hence what "increasing" means.
            </para>

            <para>
                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 <command>
                opkg update;opkg upgrade</command> commands (or similar for 
                dpkg/apt or rpm based systems). The aim is to ensure Poky has 
                upgradable packages in all cases.
            </para>
        </section>
        <section id='usingpoky-changes-collaborate'>
            <title>Using Poky in a Team Environment</title>

            <para>
                It may not be immediately clear how Poky can work in a team environment, 
                or scale to a large team of developers. The specifics of any situation
                will determine the best solution and poky offers immense flexibility in 
                that aspect but there are some practises that experience has shown to work
                well.
            </para>

            <para>
                The core component of any development effort with Poky is often an 
                automated build testing framework and image generation process. This 
                can be used to check that the metadata is buildable, highlight when 
                commits break the builds and provide up to date images allowing people 
                to test the end result and use them as a base platform for further 
                development. Experience shows that buildbot is a good fit for this role 
                and that it works well to configure it to make two types of build - 
                incremental builds and 'from scratch'/full builds. The incremental builds 
                can be tied to a commit hook which triggers them each time a commit is 
                made to the metadata and are a useful acid test of whether a given commit 
                breaks the build in some serious way. They catch lots of simple errors 
                and whilst they won't catch 100% of failures, the tests are fast so 
                developers can get feedback on their changes quickly. The full builds
                are builds that build everything from the ground up and test everything. 
                They usually happen at preset times such as at night when the machine 
                load isn't high from the incremental builds.
            </para>

            <para>
                Most teams have pieces of software undergoing active development. It is of
                significant benefit to put these under control of a source control system 
                compatible with Poky such as git or svn. The autobuilder can then be set to 
                pull the latest revisions of these packages so the latest commits get tested 
                by the builds allowing any issues to be highlighted quickly. Poky easily
                supports configurations where there is both a stable known good revision 
                and a floating revision to test. Poky can also only take changes from specific
                source control branches giving another way it can be used to track/test only
                specified changes.
            </para>
            <para>
                Perhaps the hardest part of setting this up is the policy that surrounds 
                the different source control systems, be them software projects or the Poky 
                metadata itself. The circumstances will be different in each case but this is
                one of Poky's advantages - the system itself doesn't force any particular policy
                unlike a lot of build systems, allowing the best policy to be chosen for the 
                circumstances.
            </para>
        </section>
    </section>

    <section id='usingpoky-modifing-packages'>
        <title>Modifying Package Source Code</title>

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

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

        <tip>
            <para>
                Check the package recipe sets the <glossterm><link
                        linkend='var-S'>S</link></glossterm> variable to something
            other than standard <filename>WORKDIR/PN-PV/</filename> value.
            </para>
        </tip>
        <para>
            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:
        </para>

        <programlisting>
bitbake --cmd compile --force NAME_OF_PACKAGE
        </programlisting>

        <para>
            Other tasks may also be called this way.
        </para>

        <section id='usingpoky-modifying-packages-quilt'>
            <title>Modifying Package Source Code with quilt</title>

            <para>
                By default Poky uses <ulink
                    url='http://savannah.nongnu.org/projects/quilt'>quilt</ulink>
                to manage patches in <function>do_patch</function> task. 
                It is a powerful tool which can be used to track all
                modifications done to package sources.
            </para>

            <para>
                Before modifying source code it is important to
                notify quilt so it will track changes into new patch
                file:
                <programlisting>
quilt new NAME-OF-PATCH.patch
                </programlisting>

                Then add all files which will be modified into that
                patch:
                <programlisting>
quilt add file1 file2 file3
                </programlisting>

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

                The resulting patch file can be found in the
                <filename class="directory">patches/</filename> subdirectory of the source 
                (<glossterm><link linkend='var-S'>S</link></glossterm>) directory. For future builds it
                should be copied into
                Poky metadata and added into <glossterm><link
                        linkend='var-SRC_URI'>SRC_URI</link></glossterm> of a recipe:
                <programlisting>
SRC_URI += "file://NAME-OF-PATCH.patch;patch=1"
                </programlisting>

                This also requires a bump of <glossterm><link
                        linkend='var-PR'>PR</link></glossterm> value in the same recipe as we changed resulting packages.
            </para>

        </section>

</section>

</chapter>
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