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4<title>2.8.5.8. Setscene Tasks and Shared State</title>
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12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="section" title="2.8.5.8. Setscene Tasks and Shared State">
13<div class="titlepage"><div><div><h4 class="title">
14<a name="setscene-tasks-and-shared-state"></a>2.8.5.8. Setscene Tasks and Shared State</h4></div></div></div>
15<p>
16 The description of tasks so far assumes that BitBake needs to
17 build everything and there are no prebuilt objects available.
18 BitBake does support skipping tasks if prebuilt objects are
19 available.
20 These objects are usually made available in the form of a
21 shared state (sstate) cache.
22 </p>
23<div class="note" title="Note" style="margin-left: 0.5in; margin-right: 0.5in;">
24<h3 class="title">Note</h3>
25 For information on variables affecting sstate, see the
26 <a class="link" href="../ref-manual/var-SSTATE_DIR.html" target="_self"><code class="filename">SSTATE_DIR</code></a>
27 and
28 <a class="link" href="../ref-manual/var-SSTATE_MIRRORS.html" target="_self"><code class="filename">SSTATE_MIRRORS</code></a>
29 variables.
30 </div>
31<p>
32 </p>
33<p>
34 The idea of a setscene task (i.e
35 <code class="filename">do_</code><em class="replaceable"><code>taskname</code></em><code class="filename">_setscene</code>)
36 is a version of the task where
37 instead of building something, BitBake can skip to the end
38 result and simply place a set of files into specific locations
39 as needed.
40 In some cases, it makes sense to have a setscene task variant
41 (e.g. generating package files in the
42 <code class="filename">do_package_write_*</code> task).
43 In other cases, it does not make sense, (e.g. a
44 <a class="link" href="../ref-manual/ref-tasks-patch.html" target="_self"><code class="filename">do_patch</code></a>
45 task or
46 <a class="link" href="../ref-manual/ref-tasks-unpack.html" target="_self"><code class="filename">do_unpack</code></a>
47 task) since the work involved would be equal to or greater than
48 the underlying task.
49 </p>
50<p>
51 In the OpenEmbedded build system, the common tasks that have
52 setscene variants are
53 <a class="link" href="../ref-manual/ref-tasks-package.html" target="_self"><code class="filename">do_package</code></a>,
54 <code class="filename">do_package_write_*</code>,
55 <a class="link" href="../ref-manual/ref-tasks-deploy.html" target="_self"><code class="filename">do_deploy</code></a>,
56 <a class="link" href="../ref-manual/ref-tasks-packagedata.html" target="_self"><code class="filename">do_packagedata</code></a>,
57 and
58 <a class="link" href="../ref-manual/ref-tasks-populate_sysroot.html" target="_self"><code class="filename">do_populate_sysroot</code></a>.
59 Notice that these are most of the tasks whose output is an
60 end result.
61 </p>
62<p>
63 The OpenEmbedded build system has knowledge of the relationship
64 between these tasks and other tasks that precede them.
65 For example, if BitBake runs
66 <code class="filename">do_populate_sysroot_setscene</code> for
67 something, there is little point in running any of the
68 <code class="filename">do_fetch</code>, <code class="filename">do_unpack</code>,
69 <code class="filename">do_patch</code>,
70 <code class="filename">do_configure</code>,
71 <code class="filename">do_compile</code>, and
72 <code class="filename">do_install</code> tasks.
73 However, if <code class="filename">do_package</code> needs to be run,
74 BitBake would need to run those other tasks.
75 </p>
76<p>
77 It becomes more complicated if everything can come from an
78 sstate cache because some objects are simply not required at
79 all.
80 For example, you do not need a compiler or native tools, such
81 as quilt, if there is nothing to compile or patch.
82 If the <code class="filename">do_package_write_*</code> packages are
83 available from sstate, BitBake does not need the
84 <code class="filename">do_package</code> task data.
85 </p>
86<p>
87 To handle all these complexities, BitBake runs in two phases.
88 The first is the "setscene" stage.
89 During this stage, BitBake first checks the sstate cache for
90 any targets it is planning to build.
91 BitBake does a fast check to see if the object exists rather
92 than a complete download.
93 If nothing exists, the second phase, which is the setscene
94 stage, completes and the main build proceeds.
95 </p>
96<p>
97 If objects are found in the sstate cache, the OpenEmbedded
98 build system works backwards from the end targets specified
99 by the user.
100 For example, if an image is being built, the OpenEmbedded build
101 system first looks for the packages needed for that image and
102 the tools needed to construct an image.
103 If those are available, the compiler is not needed.
104 Thus, the compiler is not even downloaded.
105 If something was found to be unavailable, or the download or
106 setscene task fails, the OpenEmbedded build system then tries
107 to install dependencies, such as the compiler, from the cache.
108 </p>
109<p>
110 The availability of objects in the sstate cache is handled by
111 the function specified by the
112 <a class="link" href="../bitbake-user-manual/var-BB_HASHCHECK_FUNCTION.html" target="_self"><code class="filename">BB_HASHCHECK_FUNCTION</code></a>
113 variable and returns a list of the objects that are available.
114 The function specified by the
115 <a class="link" href="../bitbake-user-manual/var-BB_SETSCENE_DEPVALID.html" target="_self"><code class="filename">BB_SETSCENE_DEPVALID</code></a>
116 variable is the function that determines whether a given
117 dependency needs to be followed, and whether for any given
118 relationship the function needs to be passed.
119 The function returns a True or False value.
120 </p>
121</div></body>
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