11 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'

11.1 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

  • bluetooth: Hardware has integrated BT

  • efi: Support for booting through EFI

  • ext2: Hardware HDD or Microdrive

  • keyboard: Hardware has a keyboard

  • numa: Hardware has non-uniform memory access

  • pcbios: Support for booting through BIOS

  • pci: Hardware has a PCI bus

  • pcmcia: Hardware has PCMCIA or CompactFlash sockets

  • phone: Mobile phone (voice) support

  • qemu-usermode: QEMU can support user-mode emulation for this machine

  • 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

11.2 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. Appropriate options must be supplied, and enabling/disabling PACKAGECONFIG for the concerned packages is one way of supplying such options.

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.

Note

DISTRO_FEATURES is normally independent of kernel configuration, so if a feature specified in DISTRO_FEATURES also relies on support in the kernel, you will also need to ensure that support is enabled in the kernel configuration.

This list only represents features as shipped with the Yocto Project metadata, as extra layers can define their own:

  • 3g: Include support for cellular data.

  • acl: Include Access Control List support.

  • alsa: Include Advanced Linux Sound Architecture 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.

  • debuginfod: Include support for getting ELF debugging information through a debuginfod server.

  • directfb: Include DirectFB support.

  • ext2: Include tools for supporting for devices with internal HDD/Microdrive for storing files (instead of Flash only devices).

  • gobject-introspection-data: Include data to support GObject Introspection.

  • ipsec: Include IPSec support.

  • ipv4: Include IPv4 support.

  • ipv6: Include IPv6 support.

  • keyboard: Include keyboard support (e.g. keymaps will be loaded during boot).

  • multiarch: Enable building applications with multiple architecture support.

  • ld-is-gold: Use the gold linker instead of the standard GCC linker (bfd).

  • ldconfig: Include support for ldconfig and ld.so.conf on the target.

  • lto: Enable Link-Time Optimisation.

  • nfc: Include support for Near Field Communication.

  • nfs: Include NFS client support (for mounting NFS exports on device).

  • nls: Include National Language Support (NLS).

  • opengl: Include the Open Graphics Library, which is a cross-language, multi-platform application programming interface used for rendering two and three-dimensional graphics.

  • overlayfs: Include OverlayFS support.

  • pam: Include Pluggable Authentication Module (PAM) support.

  • pci: Include PCI bus support.

  • pcmcia: Include PCMCIA/CompactFlash support.

  • polkit: Include Polkit 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.

  • pulseaudio: Include support for PulseAudio.

  • selinux: Include support for Security-Enhanced Linux (SELinux) (requires meta-selinux).

  • seccomp: Enables building applications with seccomp support, to allow them to strictly restrict the system calls that they are allowed to invoke.

  • 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.

  • systemd-resolved: Include support and use systemd-resolved as the main DNS name resolver in glibc Name Service Switch. This is a DNS resolver daemon from systemd.

  • 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.

  • vfat: Include FAT filesystem support.

  • vulkan: Include support for the Vulkan API.

  • 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.

  • xattr: Include support for extended file attributes.

  • zeroconf: Include support for zero configuration networking.

11.3 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.

Here are the image features available for all images:

  • allow-empty-password: Allows Dropbear and OpenSSH to accept logins from accounts having an empty password string.

  • allow-root-login: Allows Dropbear and OpenSSH to accept root logins.

  • 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, logins without passwords —including root ones, and enables post-installation logging). See the allow-empty-password, allow-root-login, 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: This feature or debug-tweaks is required if you want to allow root login with an empty password. If these features are not present in IMAGE_FEATURES, a non-empty password is forced in /etc/passwd and /etc/shadow if such files exist.

    Note

    empty-root-password doesn’t set an empty root password by itself. You get an initial empty root password thanks to the base-passwd and shadow recipes, and the presence of empty-root-password or debug-tweaks just disables the mechanism which forces an non-empty password for the root user.

  • lic-pkgs: Installs license packages for all packages installed in a given image.

  • overlayfs-etc: Configures the /etc directory to be in overlayfs. This allows to store device specific information elsewhere, especially if the root filesystem is configured to be read-only.

  • 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 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.

  • read-only-rootfs-delayed-postinsts: when specified in conjunction with read-only-rootfs, specifies that post-install scripts are still permitted (this assumes that the root filesystem will be made writeable for the first boot; this feature does not do anything to ensure that - it just disables the check for post-install scripts.)

  • serial-autologin-root: when specified in conjunction with empty-root-password will automatically login as root on the serial console. This of course opens up a security hole if the serial console is potentially accessible to an attacker, so use with caution.

  • 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.

  • stateless-rootfs:: specifies that the image should be created as stateless - when using systemd, systemctl-native will not be run on the image, leaving the image for population at runtime by systemd.

  • 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.

    Note

    As of the 4.1 release, the ssh-server-dropbear feature also recommends the openssh-sftp-server package, which by default will be pulled into the image. This is because recent versions of the OpenSSH scp client now use the SFTP protocol, and thus require an SFTP server to be present to connect to. However, if you wish to use the Dropbear ssh server without the SFTP server installed, you can either remove ssh-server-dropbear from IMAGE_FEATURES and add dropbear to IMAGE_INSTALL instead, or alternatively still use the feature but set BAD_RECOMMENDATIONS as follows:

    BAD_RECOMMENDATIONS += "openssh-sftp-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).

  • weston: Installs Weston (reference Wayland environment).

  • x11: Installs the X server.

  • x11-base: Installs the X server with a minimal environment.

  • x11-sato: Installs the OpenedHand Sato environment.

11.4 Feature Backfilling

Sometimes it is necessary in the OpenEmbedded build system to add new functionality to MACHINE_FEATURES or DISTRO_FEATURES, but at the same time, allow existing distributions or machine definitions to opt out of such new features, to retain the same overall level of functionality.

To make this possible, the OpenEmbedded build system has a mechanism to automatically “backfill” features into existing distro or machine configurations. You can see the list of features for which this is done by checking the DISTRO_FEATURES_BACKFILL and MACHINE_FEATURES_BACKFILL variables in the meta/conf/bitbake.conf file.

These two variables are paired with the DISTRO_FEATURES_BACKFILL_CONSIDERED and MACHINE_FEATURES_BACKFILL_CONSIDERED variables which allow distro or machine configuration maintainers to consider any added feature, and decide when they wish to keep or exclude such feature, thus preventing the backfilling from happening.

Here are two examples to 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 now backfilled and thus enabled for all distros through the DISTRO_FEATURES_BACKFILL variable in the meta/conf/bitbake.conf file. However, if your distro needs to disable the feature, you can do so without affecting other existing distro configurations that need PulseAudio support. You do this by adding “pulseaudio” to DISTRO_FEATURES_BACKFILL_CONSIDERED in your distro’s .conf file. So, 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, if your target device does not have this capability, you can disable RTC support for your device without affecting other machines that need RTC support. You do this by adding the “rtc” feature to the MACHINE_FEATURES_BACKFILL_CONSIDERED list in your machine’s .conf file. So, 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.