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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>
<para>
One method you can use to determine which recipes are checking to see if a
particular feature is contained or not is to <filename>grep</filename> through
the metadata for the feature.
Here is an example that discovers the recipes whose build is potentially
changed based on a given feature:
<literallayout class='monospaced'>
$ cd $HOME/poky
$ git grep 'contains.*MACHINE_FEATURES.*<feature>'
</literallayout>
</para>
<para>
This chapter provides a reference of shipped machine and distro features
you can include as part of the image, a reference on image types you can
build, and a reference on feature backfilling.
</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>.
Features do not have a one-to-one correspondence to packages, and they can
go beyond simply controlling the installation of a package or packages.
Sometimes a feature can influence how certain recipes are built.
For example, a feature might determine whether a particular configure option
is specified within <filename>do_configure</filename> for a particular
recipe.
</para>
<para>
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>.
Features do not have a one-to-one correspondence to packages, and they can
go beyond simply controlling the installation of a package or packages.
Sometimes a feature can influence how certain recipes are built.
For example, a feature might determine whether a particular configure option
is specified within <filename>do_configure</filename> for a particular
recipe.
</para>
<para>
This feature 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 in the OpenEmbedded build system to extend
<link linkend='var-MACHINE_FEATURES'><filename>MACHINE_FEATURES</filename></link>
or <link linkend='var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></link>
to control functionality that was previously enabled and not able
to be disabled.
For these cases, we need to add an
additional feature item to appear in one of these variables,
but we do not want to force developers who have existing values
of the variables in their configuration to add the new feature
in order to retain the same overall level of functionality.
Thus, the OpenEmbedded build system has a mechanism to
automatically "backfill" these added features into existing
distro or machine configurations.
You can see the list of features for which 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 such features are backfilled by default into all
configurations as described in the previous paragraph, developers
who wish to disable the new features need to be able to selectively
prevent the backfilling from occurring.
They can do this by adding the undesired feature or features to the
<link linkend='var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></link>
or <link linkend='var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></link>
variables for distro features and machine features respectively.
</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
<filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename>
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
<filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename>
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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