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1<html>
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3<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
4<title>9.4. Feature Backfilling</title>
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8<link rel="up" href="ref-features.html" title="Chapter 9. Reference: Features">
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12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="section" title="9.4. Feature Backfilling">
13<div class="titlepage"><div><div><h2 class="title" style="clear: both">
14<a name="ref-features-backfill"></a>9.4. Feature Backfilling</h2></div></div></div>
15<p>
16 Sometimes it is necessary in the OpenEmbedded build system to extend
17 <a class="link" href="ref-variables-glos.html#var-MACHINE_FEATURES" title="MACHINE_FEATURES"><code class="filename">MACHINE_FEATURES</code></a>
18 or <a class="link" href="ref-variables-glos.html#var-DISTRO_FEATURES" title="DISTRO_FEATURES"><code class="filename">DISTRO_FEATURES</code></a>
19 to control functionality that was previously enabled and not able
20 to be disabled.
21 For these cases, we need to add an
22 additional feature item to appear in one of these variables,
23 but we do not want to force developers who have existing values
24 of the variables in their configuration to add the new feature
25 in order to retain the same overall level of functionality.
26 Thus, the OpenEmbedded build system has a mechanism to
27 automatically "backfill" these added features into existing
28 distro or machine configurations.
29 You can see the list of features for which this is done by
30 finding the
31 <a class="link" href="ref-variables-glos.html#var-DISTRO_FEATURES_BACKFILL" title="DISTRO_FEATURES_BACKFILL"><code class="filename">DISTRO_FEATURES_BACKFILL</code></a>
32 and <a class="link" href="ref-variables-glos.html#var-MACHINE_FEATURES_BACKFILL" title="MACHINE_FEATURES_BACKFILL"><code class="filename">MACHINE_FEATURES_BACKFILL</code></a>
33 variables in the <code class="filename">meta/conf/bitbake.conf</code> file.
34 </p>
35<p>
36 Because such features are backfilled by default into all
37 configurations as described in the previous paragraph, developers
38 who wish to disable the new features need to be able to selectively
39 prevent the backfilling from occurring.
40 They can do this by adding the undesired feature or features to the
41 <a class="link" href="ref-variables-glos.html#var-DISTRO_FEATURES_BACKFILL_CONSIDERED" title="DISTRO_FEATURES_BACKFILL_CONSIDERED"><code class="filename">DISTRO_FEATURES_BACKFILL_CONSIDERED</code></a>
42 or <a class="link" href="ref-variables-glos.html#var-MACHINE_FEATURES_BACKFILL_CONSIDERED" title="MACHINE_FEATURES_BACKFILL_CONSIDERED"><code class="filename">MACHINE_FEATURES_BACKFILL_CONSIDERED</code></a>
43 variables for distro features and machine features respectively.
44 </p>
45<p>
46 Here are two examples to help illustrate feature backfilling:
47 </p>
48<div class="itemizedlist"><ul class="itemizedlist" type="disc">
49<li class="listitem"><p><span class="emphasis"><em>The "pulseaudio" distro feature option</em></span>:
50 Previously, PulseAudio support was enabled within the Qt and
51 GStreamer frameworks.
52 Because of this, the feature is backfilled and thus
53 enabled for all distros through the
54 <code class="filename">DISTRO_FEATURES_BACKFILL</code>
55 variable in the <code class="filename">meta/conf/bitbake.conf</code> file.
56 However, your distro needs to disable the feature.
57 You can disable the feature without affecting
58 other existing distro configurations that need PulseAudio support
59 by adding "pulseaudio" to
60 <code class="filename">DISTRO_FEATURES_BACKFILL_CONSIDERED</code>
61 in your distro's <code class="filename">.conf</code> file.
62 Adding the feature to this variable when it also
63 exists in the <code class="filename">DISTRO_FEATURES_BACKFILL</code>
64 variable prevents the build system from adding the feature to
65 your configuration's <code class="filename">DISTRO_FEATURES</code>, effectively disabling
66 the feature for that particular distro.</p></li>
67<li class="listitem"><p><span class="emphasis"><em>The "rtc" machine feature option</em></span>:
68 Previously, real time clock (RTC) support was enabled for all
69 target devices.
70 Because of this, the feature is backfilled and thus enabled
71 for all machines through the <code class="filename">MACHINE_FEATURES_BACKFILL</code>
72 variable in the <code class="filename">meta/conf/bitbake.conf</code> file.
73 However, your target device does not have this capability.
74 You can disable RTC support for your device without
75 affecting other machines that need RTC support
76 by adding the feature to your machine's
77 <code class="filename">MACHINE_FEATURES_BACKFILL_CONSIDERED</code>
78 list in the machine's <code class="filename">.conf</code> file.
79 Adding the feature to this variable when it also
80 exists in the <code class="filename">MACHINE_FEATURES_BACKFILL</code>
81 variable prevents the build system from adding the feature to
82 your configuration's <code class="filename">MACHINE_FEATURES</code>, effectively
83 disabling RTC support for that particular machine.</p></li>
84</ul></div>
85<p>
86 </p>
87</div></body>
88</html>
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