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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>
This commit is contained in:
Scott Rifenbark 2012-10-19 06:28:12 -07:00 committed by Richard Purdie
parent c6ce06b172
commit 014f91cb3f
1 changed files with 24 additions and 27 deletions
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51
documentation/poky-ref-manual/ref-features.xml
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@ -178,38 +178,35 @@
<title>Feature Backfilling</title>
<para>
Sometimes it is necessary to control functionality enabled by features
that are listed with
Sometimes it is necessary in the OpenEmbedded build system to extend
<link linkend='var-MACHINE_FEATURES'><filename>MACHINE_FEATURES</filename></link>
and <link linkend='var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></link>.
For example, some functionality exists that is enabled
by default for all configurations.
For these cases, the metadata, as shipped with the Yocto Project, ensures
the feature is "backfilled" into all the specific distro
and machine configurations.
You can see how this is done by finding the
or <link linkend='var-DISTRO_FEATURES'><filename>DISTRO_FEATURES</filename></link>
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
<link linkend='var-DISTRO_FEATURES_BACKFILL'><filename>DISTRO_FEATURES_BACKFILL</filename></link>
and <link linkend='var-MACHINE_FEATURES_BACKFILL'><filename>MACHINE_FEATURES_BACKFILL</filename></link>
variables in the <filename>meta/conf/bitbake.conf</filename> file.
</para>
<para>
Because certain functionality is enabled across all configurations as
described in the previous paragraph, it also becomes necessary
to give the developer the ability to disable (remove) a feature
from a particular configuration.
For example, suppose you have a machine feature that needs to be
disabled but the metadata has backfilled it across all configurations as enabled.
You need to be able to remove the feature from your configuration's
<filename>MACHINE_FEATURES</filename>
without disturbing existing configurations that still
might need the functionality?
</para>
<para>
Feature backfilling allows you to selectively prevent a feature from
being backfilled to <filename>MACHINE_FEATURES</filename>,
<filename>DISTRO_FEATURES</filename>, or both.
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
<link linkend='var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></link>
or <link linkend='var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></link>
variables for distro features and machine features respectively.
</para>
<para>
@ -226,7 +223,7 @@
You can disable the feature without affecting
other existing distro configurations that need PulseAudio support
by adding "pulseaudio" to
<link linkend='var-DISTRO_FEATURES_BACKFILL_CONSIDERED'><filename>DISTRO_FEATURES_BACKFILL_CONSIDERED</filename></link>
DISTRO_FEATURES_BACKFILL_CONSIDERED
in your distro's <filename>.conf</filename> file.
Adding the feature to this variable when it also
exists in the <filename>DISTRO_FEATURES_BACKFILL</filename>
@ -243,7 +240,7 @@
You can disable RTC support for your device without
affecting other machines that need RTC support
by adding the feature to your machine's
<link linkend='var-MACHINE_FEATURES_BACKFILL_CONSIDERED'><filename>MACHINE_FEATURES_BACKFILL_CONSIDERED</filename></link>
MACHINE_FEATURES_BACKFILL_CONSIDERED
list in the machine's <filename>.conf</filename> file.
Adding the feature to this variable when it also
exists in the <filename>MACHINE_FEATURES_BACKFILL</filename>