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+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+********
+Features
+********
+
+This chapter provides a reference of shipped machine and distro features
+you can include as part of your image, a reference on image features you
+can select, and a reference on feature backfilling.
+
+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 ``DISTRO_FEATURES`` variable,
+which is set or appended to in a distribution's configuration file such
+as ``poky.conf``, ``poky-tiny.conf``, ``poky-lsb.conf`` and so forth.
+Machine features are set in the ``MACHINE_FEATURES`` variable, which is
+set in the machine configuration file and specifies the hardware
+features for a given machine.
+
+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.
+
+One method you can use to determine which recipes are checking to see if
+a particular feature is contained or not is to ``grep`` through the
+:term:`Metadata` for the feature. Here is an example that
+discovers the recipes whose build is potentially changed based on a
+given feature:
+::
+
+   $ cd poky
+   $ git grep 'contains.*MACHINE_FEATURES.*feature'
+
+.. _ref-features-machine:
+
+Machine Features
+================
+
+The items below are features you can use with
+:term:`MACHINE_FEATURES`. 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 the :ref:`ref-tasks-configure` task
+for a particular recipe.
+
+This feature list only represents features as shipped with the Yocto
+Project metadata:
+
+-  *acpi:* Hardware has ACPI (x86/x86_64 only)
+
+-  *alsa:* Hardware has ALSA audio drivers
+
+-  *apm:* Hardware uses APM (or APM emulation)
+
+-  *bluetooth:* Hardware has integrated BT
+
+-  *efi:* Support for booting through EFI
+
+-  *ext2:* Hardware HDD or Microdrive
+
+-  *keyboard:* Hardware has a keyboard
+
+-  *pcbios:* Support for booting through BIOS
+
+-  *pci:* Hardware has a PCI bus
+
+-  *pcmcia:* Hardware has PCMCIA or CompactFlash sockets
+
+-  *phone:* Mobile phone (voice) support
+
+-  *qvga:* Machine has a QVGA (320x240) display
+
+-  *rtc:* Machine has a Real-Time Clock
+
+-  *screen:* Hardware has a screen
+
+-  *serial:* Hardware has serial support (usually RS232)
+
+-  *touchscreen:* Hardware has a touchscreen
+
+-  *usbgadget:* Hardware is USB gadget device capable
+
+-  *usbhost:* Hardware is USB Host capable
+
+-  *vfat:* FAT file system support
+
+-  *wifi:* Hardware has integrated WiFi
+
+.. _ref-features-distro:
+
+Distro Features
+===============
+
+The items below are features you can use with
+:term:`DISTRO_FEATURES` to enable features across
+your distribution. 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. In most cases, the presence or absence of a
+feature translates to the appropriate option supplied to the configure
+script during the :ref:`ref-tasks-configure` task for
+the recipes that optionally support the feature.
+
+Some distro features are also machine features. These select features
+make sense to be controlled both at the machine and distribution
+configuration level. See the
+:term:`COMBINED_FEATURES` variable for more
+information.
+
+This list only represents features as shipped with the Yocto Project
+metadata:
+
+-  *alsa:* Include ALSA support (OSS compatibility kernel modules
+   installed if available).
+
+-  *api-documentation:* Enables generation of API documentation during
+   recipe builds. The resulting documentation is added to SDK tarballs
+   when the ``bitbake -c populate_sdk`` command is used. See the
+   ":ref:`sdk-manual/appendix-customizing-standard:adding api documentation to the standard sdk`"
+   section in the Yocto Project Application Development and the
+   Extensible Software Development Kit (eSDK) manual.
+
+-  *bluetooth:* Include bluetooth support (integrated BT only).
+
+-  *cramfs:* Include CramFS support.
+
+-  *directfb:* Include DirectFB support.
+
+-  *ext2:* Include tools for supporting for devices with internal
+   HDD/Microdrive for storing files (instead of Flash only devices).
+
+-  *ipsec:* Include IPSec support.
+
+-  *ipv6:* Include IPv6 support.
+
+-  *keyboard:* Include keyboard support (e.g. keymaps will be loaded
+   during boot).
+
+-  *ldconfig:* Include support for ldconfig and ``ld.so.conf`` on the
+   target.
+
+-  *nfs:* Include NFS client support (for mounting NFS exports on
+   device).
+
+-  *opengl:* Include the Open Graphics Library, which is a
+   cross-language, multi-platform application programming interface used
+   for rendering two and three-dimensional graphics.
+
+-  *pci:* Include PCI bus support.
+
+-  *pcmcia:* Include PCMCIA/CompactFlash support.
+
+-  *ppp:* Include PPP dialup support.
+
+-  *ptest:* Enables building the package tests where supported by
+   individual recipes. For more information on package tests, see the
+   ":ref:`dev-manual/common-tasks:testing packages with ptest`" section
+   in the Yocto Project Development Tasks Manual.
+
+-  *smbfs:* Include SMB networks client support (for mounting
+   Samba/Microsoft Windows shares on device).
+
+-  *systemd:* Include support for this ``init`` manager, which is a full
+   replacement of for ``init`` with parallel starting of services,
+   reduced shell overhead, and other features. This ``init`` manager is
+   used by many distributions.
+
+-  *usbgadget:* Include USB Gadget Device support (for USB
+   networking/serial/storage).
+
+-  *usbhost:* Include USB Host support (allows to connect external
+   keyboard, mouse, storage, network etc).
+
+-  *usrmerge:* Merges the ``/bin``, ``/sbin``, ``/lib``, and ``/lib64``
+   directories into their respective counterparts in the ``/usr``
+   directory to provide better package and application compatibility.
+
+-  *wayland:* Include the Wayland display server protocol and the
+   library that supports it.
+
+-  *wifi:* Include WiFi support (integrated only).
+
+-  *x11:* Include the X server and libraries.
+
+.. _ref-features-image:
+
+Image Features
+==============
+
+The contents of images generated by the OpenEmbedded build system can be
+controlled by the :term:`IMAGE_FEATURES` and
+:term:`EXTRA_IMAGE_FEATURES` 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.
+
+The following image features are available for all images:
+
+-  *allow-empty-password:* Allows Dropbear and OpenSSH to accept root
+   logins and logins from accounts having an empty password string.
+
+-  *dbg-pkgs:* Installs debug symbol packages for all packages installed
+   in a given image.
+
+-  *debug-tweaks:* Makes an image suitable for development (e.g. allows
+   root logins without passwords and enables post-installation logging).
+   See the 'allow-empty-password', 'empty-root-password', and
+   'post-install-logging' features in this list for additional
+   information.
+
+-  *dev-pkgs:* Installs development packages (headers and extra library
+   links) for all packages installed in a given image.
+
+-  *doc-pkgs:* Installs documentation packages for all packages
+   installed in a given image.
+
+-  *empty-root-password:* Sets the root password to an empty string,
+   which allows logins with a blank password.
+
+-  *package-management:* Installs package management tools and preserves
+   the package manager database.
+
+-  *post-install-logging:* Enables logging postinstall script runs to
+   the ``/var/log/postinstall.log`` file on first boot of the image on
+   the target system.
+
+   .. note::
+
+      To make the ``/var/log`` directory on the target persistent, use the
+      :term:`VOLATILE_LOG_DIR` variable by setting it to "no".
+
+-  *ptest-pkgs:* Installs ptest packages for all ptest-enabled recipes.
+
+-  *read-only-rootfs:* Creates an image whose root filesystem is
+   read-only. See the
+   ":ref:`dev-manual/common-tasks:creating a read-only root filesystem`"
+   section in the Yocto Project Development Tasks Manual for more
+   information.
+
+-  *splash:* Enables showing a splash screen during boot. By default,
+   this screen is provided by ``psplash``, which does allow
+   customization. If you prefer to use an alternative splash screen
+   package, you can do so by setting the ``SPLASH`` variable to a
+   different package name (or names) within the image recipe or at the
+   distro configuration level.
+
+-  *staticdev-pkgs:* Installs static development packages, which are
+   static libraries (i.e. ``*.a`` files), for all packages installed in
+   a given image.
+
+Some image features are available only when you inherit the
+:ref:`core-image <ref-classes-core-image>` class. The current list of
+these valid features is as follows:
+
+-  *hwcodecs:* Installs hardware acceleration codecs.
+
+-  *nfs-server:* Installs an NFS server.
+
+-  *perf:* Installs profiling tools such as ``perf``, ``systemtap``, and
+   ``LTTng``. For general information on user-space tools, see the
+   :doc:`/sdk-manual/index` manual.
+
+-  *ssh-server-dropbear:* Installs the Dropbear minimal SSH server.
+
+-  *ssh-server-openssh:* 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
+   ``IMAGE_FEATURES``, then OpenSSH will take precedence and Dropbear
+   will not be installed.
+
+-  *tools-debug:* Installs debugging tools such as ``strace`` and
+   ``gdb``. For information on GDB, see the
+   ":ref:`dev-manual/common-tasks:debugging with the gnu project debugger (gdb) remotely`" section
+   in the Yocto Project Development Tasks Manual. For information on
+   tracing and profiling, see the :doc:`/profile-manual/index`.
+
+-  *tools-sdk:* Installs a full SDK that runs on the device.
+
+-  *tools-testapps:* Installs device testing tools (e.g. touchscreen
+   debugging).
+
+-  *x11:* Installs the X server.
+
+-  *x11-base:* Installs the X server with a minimal environment.
+
+-  *x11-sato:* Installs the OpenedHand Sato environment.
+
+.. _ref-features-backfill:
+
+Feature Backfilling
+===================
+
+Sometimes it is necessary in the OpenEmbedded build system to extend
+:term:`MACHINE_FEATURES` or
+:term:`DISTRO_FEATURES` 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
+:term:`DISTRO_FEATURES_BACKFILL` and
+:term:`MACHINE_FEATURES_BACKFILL`
+variables in the ``meta/conf/bitbake.conf`` file.
+
+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
+:term:`DISTRO_FEATURES_BACKFILL_CONSIDERED`
+or
+:term:`MACHINE_FEATURES_BACKFILL_CONSIDERED`
+variables for distro features and machine features respectively.
+
+Here are two examples to help illustrate feature backfilling:
+
+-  *The "pulseaudio" distro feature option*: 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 ``DISTRO_FEATURES_BACKFILL`` variable in the
+   ``meta/conf/bitbake.conf`` 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 ``DISTRO_FEATURES_BACKFILL_CONSIDERED`` in
+   your distro's ``.conf`` file. Adding the feature to this variable
+   when it also exists in the ``DISTRO_FEATURES_BACKFILL`` variable
+   prevents the build system from adding the feature to your
+   configuration's ``DISTRO_FEATURES``, effectively disabling the
+   feature for that particular distro.
+
+-  *The "rtc" machine feature option*: 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
+   ``MACHINE_FEATURES_BACKFILL`` variable in the
+   ``meta/conf/bitbake.conf`` 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 ``MACHINE_FEATURES_BACKFILL_CONSIDERED``
+   list in the machine's ``.conf`` file. Adding the feature to this
+   variable when it also exists in the ``MACHINE_FEATURES_BACKFILL``
+   variable prevents the build system from adding the feature to your
+   configuration's ``MACHINE_FEATURES``, effectively disabling RTC
+   support for that particular machine.