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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<chapter id='ref-features'>
    <title>Reference: Features</title>

    <para>
        Features provide a mechanism for working out which packages
        should be included in the generated images. 
        Distributions can select which features they want to support through the
        <filename><link linkend='var-DISTRO_FEATURES'>DISTRO_FEATURES</link></filename>
        variable, which is set in the <filename>poky.conf</filename> distribution configuration file.
        Machine features are set in the
        <filename><link linkend='var-MACHINE_FEATURES'>MACHINE_FEATURES</link></filename>
        variable, which is set in the machine configuration file and
        specifies the hardware features for a given machine.
    </para>

    <para>
        These two variables combine to work out which kernel modules,
        utilities, and other packages to include. 
        A given distribution can support a selected subset of features so some machine features might not
        be included if the distribution itself does not support them.
    </para>

    <section id='ref-features-distro'>
        <title>Distro</title>

        <para>
            The items below are features you can use with  
            <link linkend='var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></link>.
            This list only represents features as shipped with the Yocto Project metadata:
            <itemizedlist>
                <listitem><para><emphasis>alsa:</emphasis> ALSA support will be included (OSS compatibility 
                    kernel modules will be installed if available).</para></listitem>
                <listitem><para><emphasis>bluetooth:</emphasis> Include bluetooth support (integrated BT only)
                    </para></listitem>
                <listitem><para><emphasis>ext2:</emphasis> Include tools for supporting for devices with internal
                    HDD/Microdrive for storing files (instead of Flash only devices)
                    </para></listitem>
                <listitem><para><emphasis>irda:</emphasis> Include Irda support
                    </para></listitem>
                <listitem><para><emphasis>keyboard:</emphasis> Include keyboard support (e.g. keymaps will be 
                    loaded during boot).
                    </para></listitem>
                <listitem><para><emphasis>pci:</emphasis> Include PCI bus support
                    </para></listitem>
                <listitem><para><emphasis>pcmcia:</emphasis> Include PCMCIA/CompactFlash support
                    </para></listitem>
                <listitem><para><emphasis>usbgadget:</emphasis> USB Gadget Device support (for USB
                    networking/serial/storage)
                    </para></listitem>
                <listitem><para><emphasis>usbhost:</emphasis> USB Host support (allows to connect external
                    keyboard, mouse, storage, network etc)
                    </para></listitem>
                <listitem><para><emphasis>wifi:</emphasis> WiFi support (integrated only)
                    </para></listitem>
                <listitem><para><emphasis>cramfs:</emphasis> CramFS support
                    </para></listitem>
                <listitem><para><emphasis>ipsec:</emphasis> IPSec support
                    </para></listitem>
                <listitem><para><emphasis>ipv6:</emphasis> IPv6 support
                    </para></listitem>
                <listitem><para><emphasis>nfs:</emphasis> NFS client support (for mounting NFS exports on
                    device)</para></listitem>
                <listitem><para><emphasis>ppp:</emphasis> PPP dialup support</para></listitem>
                <listitem><para><emphasis>smbfs:</emphasis> SMB networks client support (for mounting
                    Samba/Microsoft Windows shares on device)</para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='ref-features-machine'>
        <title>Machine</title>

        <para>
            The items below are features you can use with  
            <link linkend='var-MACHINE_FEATURES'><filename>MACHINE_FEATURES</filename></link>.
            This list only represents features as shipped with the Yocto Project metadata:
            <itemizedlist>
                <listitem><para><emphasis>acpi:</emphasis> Hardware has ACPI (x86/x86_64 only)
                    </para></listitem>
                <listitem><para><emphasis>alsa:</emphasis> Hardware has ALSA audio drivers
                    </para></listitem>
                <listitem><para><emphasis>apm:</emphasis> Hardware uses APM (or APM emulation)
                    </para></listitem>
                <listitem><para><emphasis>bluetooth:</emphasis> Hardware has integrated BT
                    </para></listitem>
                <listitem><para><emphasis>ext2:</emphasis> Hardware HDD or Microdrive
                    </para></listitem>
                <listitem><para><emphasis>irda:</emphasis> Hardware has Irda support
                    </para></listitem>
                <listitem><para><emphasis>keyboard:</emphasis> Hardware has a keyboard
                    </para></listitem>
                <listitem><para><emphasis>pci:</emphasis> Hardware has a PCI bus
                    </para></listitem>
                <listitem><para><emphasis>pcmcia:</emphasis> Hardware has PCMCIA or CompactFlash sockets
                    </para></listitem>
                <listitem><para><emphasis>screen:</emphasis> Hardware has a screen
                    </para></listitem>
                <listitem><para><emphasis>serial:</emphasis> Hardware has serial support (usually RS232)
                    </para></listitem>
                <listitem><para><emphasis>touchscreen:</emphasis> Hardware has a touchscreen
                    </para></listitem>
                <listitem><para><emphasis>usbgadget:</emphasis> Hardware is USB gadget device capable
                    </para></listitem>
                <listitem><para><emphasis>usbhost:</emphasis> Hardware is USB Host capable
                    </para></listitem>
                <listitem><para><emphasis>wifi:</emphasis> Hardware has integrated WiFi
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='ref-features-image'>
        <title>Images</title>

        <para>
            The contents of images generated by the OpenEmbedded build system can be controlled by the 
            <filename><link linkend='var-IMAGE_FEATURES'>IMAGE_FEATURES</link></filename>
            and <filename><link linkend='var-EXTRA_IMAGE_FEATURES'>EXTRA_IMAGE_FEATURES</link></filename>
            variables that you typically configure in your image recipes.
            Through these variables you can add several different
            predefined packages such as development utilities or packages with debug
            information needed to investigate application problems or profile applications.
        </para>

        <para>
            Current list of 
            <filename>IMAGE_FEATURES</filename> contains the following:
            <itemizedlist>
                <listitem><para><emphasis>splash:</emphasis> Enables showing a splash screen during boot. 
                    By default, this screen is provided by <filename>psplash</filename>, which does 
                    allow customization. 
                    If you prefer to use an alternative splash screen package, you can do so by 
                    setting the <filename>SPLASH</filename> variable
                    to a different package name (or names) within the image recipe or at the distro
                    configuration level.</para></listitem>
                <listitem><para><emphasis>ssh-server-dropbear:</emphasis> Installs the Dropbear minimal 
                    SSH server.
                    </para></listitem>
                <listitem><para><emphasis>ssh-server-openssh:</emphasis> Installs the OpenSSH SSH server,
                    which is more full-featured than Dropbear. 
                    Note that if both the OpenSSH SSH server and the Dropbear minimal SSH server
                    are present in <filename>IMAGE_FEATURES</filename>, then OpenSSH will take
                    precedence and Dropbear will not be installed.</para></listitem>
                <listitem><para><emphasis>x11:</emphasis> Installs the X server</para></listitem>
                <listitem><para><emphasis>x11-base:</emphasis> Installs the X server with a 
                    minimal environment.</para></listitem>
                <listitem><para><emphasis>x11-sato:</emphasis> Installs the OpenedHand Sato environment.
                    </para></listitem>
                <listitem><para><emphasis>tools-sdk:</emphasis> Installs a full SDK that runs on the device.
                    </para></listitem>
                <listitem><para><emphasis>tools-debug:</emphasis> Installs debugging tools such as 
                    <filename>strace</filename> and <filename>gdb</filename>.
                    </para></listitem>
                <listitem><para><emphasis>tools-profile:</emphasis> Installs profiling tools such as 
                    <filename>oprofile</filename>, <filename>exmap</filename>, and 
                    <filename>LTTng</filename>.</para></listitem>
                <listitem><para><emphasis>tools-testapps:</emphasis> Installs device testing tools (e.g.
                    touchscreen debugging).</para></listitem>
                <listitem><para><emphasis>nfs-server:</emphasis> Installs an NFS server.</para></listitem>
                <listitem><para><emphasis>dev-pkgs:</emphasis> Installs development packages (headers and 
                    extra library links) for all packages installed in a given image.</para></listitem>
                <listitem><para><emphasis>staticdev-pkgs:</emphasis> Installs static development 
                    packages (i.e. static libraries containing <filename>*.a</filename> files) for all 
                    packages installed in a given image.</para></listitem>
                <listitem><para><emphasis>dbg-pkgs:</emphasis> Installs debug symbol packages for all packages 
                    installed in a given image.</para></listitem>
                <listitem><para><emphasis>doc-pkgs:</emphasis> Installs documentation packages for all packages 
                    installed in a given image.</para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='ref-features-backfill'>
        <title>Feature Backfilling</title>

        <para>
            Sometimes it is necessary to control functionality enabled by features 
            that are listed with  
            <link linkend='var-MACHINE_FEATURES'><filename>MACHINE_FEATURES</filename></link>
            and <link linkend='var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></link>. 
            For example, some functionality exists that is enabled
            by default for all configurations.
            For these cases, the metadata, as shipped with the Yocto Project, ensures 
            the feature is "backfilled" into all the specific distro
            and machine configurations.
            You can see how this is done by finding the 
            <link linkend='var-DISTRO_FEATURES_BACKFILL'><filename>DISTRO_FEATURES_BACKFILL</filename></link>
            and <link linkend='var-MACHINE_FEATURES_BACKFILL'><filename>MACHINE_FEATURES_BACKFILL</filename></link>
            variables in the <filename>meta/conf/bitbake.conf</filename> file.
        </para>

        <para>
            Because certain functionality is enabled across all configurations as 
            described in the previous paragraph, it also becomes necessary 
            to give the developer the ability to disable (remove) a feature
            from a particular configuration. 
            For example, suppose you have a machine feature that needs to be 
            disabled but the metadata has backfilled it across all configurations as enabled.
            You need to be able to remove the feature from your configuration's
            <filename>MACHINE_FEATURES</filename> 
            without disturbing existing configurations that still 
            might need the functionality?
        </para>

        <para>
            Feature backfilling allows you to selectively prevent a feature from 
            being backfilled to <filename>MACHINE_FEATURES</filename>, 
            <filename>DISTRO_FEATURES</filename>, or both.
        </para>

        <para>
            Here are two examples to help illustrate feature backfilling:
            <itemizedlist>
                <listitem><para><emphasis>The "pulseaudio" distro feature option</emphasis>: 
                    Previously, PulseAudio support was enabled within the Qt and 
                    GStreamer frameworks.
                    Because of this, the feature is backfilled and thus 
                    enabled for all distros through the 
                    <filename>DISTRO_FEATURES_BACKFILL</filename>
                    variable in the <filename>meta/conf/bitbake.conf</filename> file.
                    However, your distro needs to disable the feature.
                    You can disable the feature without affecting 
                    other existing distro configurations that need PulseAudio support
                    by adding "pulseaudio" to
                    <link linkend='var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></link>
                    in your distro's <filename>.conf</filename> file.
                    Adding the feature to this variable when it also
                    exists in the <filename>DISTRO_FEATURES_BACKFILL</filename>
                    variable prevents the build system from adding the feature to 
                    your configuration's <filename>DISTRO_FEATURES</filename>, effectively disabling
                    the feature for that particular distro.</para></listitem>
                <listitem><para><emphasis>The "rtc" machine feature option</emphasis>: 
                    Previously, real time clock (RTC) support was enabled for all 
                    target devices.
                    Because of this, the feature is backfilled and thus enabled
                    for all machines through the <filename>MACHINE_FEATURES_BACKFILL</filename>
                    variable in the <filename>meta/conf/bitbake.conf</filename> file.
                    However, your target device does not have this capability.
                    You can disable RTC support for your device without 
                    affecting other machines that need RTC support 
                    by adding the feature to your machine's 
                    <link linkend='var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></link>
                    list in the machine's <filename>.conf</filename> file.
                    Adding the feature to this variable when it also
                    exists in the <filename>MACHINE_FEATURES_BACKFILL</filename>
                    variable prevents the build system from adding the feature to 
                    your configuration's <filename>MACHINE_FEATURES</filename>, effectively 
                    disabling RTC support for that particular machine.</para></listitem>
            </itemizedlist>
        </para>
    </section>
</chapter>

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