# 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 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'

## Machine Features

The items below are features you can use with `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 `do_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

## Distro Features

The items below are features you can use with `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 `do_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 `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 "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 "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).

- 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.

## Image Features

The contents of images generated by the OpenEmbedded build system can be controlled by the `IMAGE_FEATURES` and `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.

  To make the `/var/log` directory on the target persistent, use the `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 "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 `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 Yocto Project Application Development and the Extensible Software Development Kit (eSDK) 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 "Debugging With the GNU Project Debugger (GDB) Remotely" section in the Yocto Project Development Tasks Manual. For information on tracing and profiling, see the Yocto Project Profiling and Tracing Manual.

- 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.

## Feature Backfilling

Sometimes it is necessary in the OpenEmbedded build system to extend `MACHINE_FEATURES` or `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 `DISTRO_FEATURES_BACKFILL` and `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 `DISTRO_FEATURES_BACKFILL_CONSIDERED` or `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.