documentation: poky-ref-manual - Edits to feature backfill
Final edits (I think) to this section from Paul Eggleton. (From yocto-docs rev: 95fd830ffb668109631205df4538454ccf023b20) Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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@ -178,38 +178,35 @@
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<title>Feature Backfilling</title>
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<para>
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Sometimes it is necessary to control functionality enabled by features
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that are listed with
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Sometimes it is necessary in the OpenEmbedded build system to extend
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<link linkend='var-MACHINE_FEATURES'><filename>MACHINE_FEATURES</filename></link>
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and <link linkend='var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></link>.
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For example, some functionality exists that is enabled
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by default for all configurations.
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For these cases, the metadata, as shipped with the Yocto Project, ensures
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the feature is "backfilled" into all the specific distro
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and machine configurations.
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You can see how this is done by finding the
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or <link linkend='var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></link>
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to control functionality that was previously enabled and not able
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to be disabled.
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For these cases, we need to add an
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additional feature item to appear in one of these variables,
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but we do not want to force developers who have existing values
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of the variables in their configuration to add the new feature
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in order to retain the same overall level of functionality.
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Thus, the OpenEmbedded build system has a mechanism to
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automatically "backfill" these added features into existing
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distro or machine configurations.
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You can see the list of features for which this is done by
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finding the
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<link linkend='var-DISTRO_FEATURES_BACKFILL'><filename>DISTRO_FEATURES_BACKFILL</filename></link>
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and <link linkend='var-MACHINE_FEATURES_BACKFILL'><filename>MACHINE_FEATURES_BACKFILL</filename></link>
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variables in the <filename>meta/conf/bitbake.conf</filename> file.
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</para>
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<para>
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Because certain functionality is enabled across all configurations as
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described in the previous paragraph, it also becomes necessary
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to give the developer the ability to disable (remove) a feature
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from a particular configuration.
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For example, suppose you have a machine feature that needs to be
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disabled but the metadata has backfilled it across all configurations as enabled.
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You need to be able to remove the feature from your configuration's
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<filename>MACHINE_FEATURES</filename>
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without disturbing existing configurations that still
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might need the functionality?
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</para>
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<para>
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Feature backfilling allows you to selectively prevent a feature from
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being backfilled to <filename>MACHINE_FEATURES</filename>,
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<filename>DISTRO_FEATURES</filename>, or both.
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Because such features are backfilled by default into all
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configurations as described in the previous paragraph, developers
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who wish to disable the new features need to be able to selectively
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prevent the backfilling from occurring.
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They can do this by adding the undesired feature or features to the
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<link linkend='var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></link>
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or <link linkend='var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></link>
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variables for distro features and machine features respectively.
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</para>
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<para>
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@ -226,7 +223,7 @@
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You can disable the feature without affecting
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other existing distro configurations that need PulseAudio support
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by adding "pulseaudio" to
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<link linkend='var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></link>
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DISTRO_FEATURES_BACKFILL_CONSIDERED
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in your distro's <filename>.conf</filename> file.
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Adding the feature to this variable when it also
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exists in the <filename>DISTRO_FEATURES_BACKFILL</filename>
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You can disable RTC support for your device without
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affecting other machines that need RTC support
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by adding the feature to your machine's
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<link linkend='var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></link>
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MACHINE_FEATURES_BACKFILL_CONSIDERED
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list in the machine's <filename>.conf</filename> file.
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Adding the feature to this variable when it also
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exists in the <filename>MACHINE_FEATURES_BACKFILL</filename>
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