9.4. Feature Backfilling

-9.4. 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.