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@@ -265,9 +265,123 @@
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</section>
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</section>
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- <section id='what-are-layers'>
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- <title>What are Layers?</title>
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+ <section id='the-yocto-project-layer-model'>
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+ <title>The Yocto Project Layer Model</title>
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+ <para>
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+ The Yocto Project's "Layer Model" is a development model for
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+ embedded and IoT Linux creation that distinguishes the
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+ Yocto Project from other simple build systems.
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+ The Layer Model simultaneously supports collaboration and
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+ customization.
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+ Layers are repositories that contain related sets of instructions
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+ that tell the OpenEmbedded build system what to do.
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+ You can collaborate, share, and reuse layers.
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+ </para>
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+
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+ <para>
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+ Layers can contain changes to previous instructions or settings
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+ at any time.
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+ This powerful override capability is what allows you to customize
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+ previously supplied collaborative or community layers to suit your
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+ product requirements.
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+ </para>
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+
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+ <para>
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+ You use different layers to logically separate information in your
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+ build.
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+ As an example, you could have BSP, GUI, distro configuration,
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+ middleware, or application layers.
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+ Putting your entire build into one layer limits and complicates
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+ future customization and reuse.
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+ Isolating information into layers, on the other hand, helps
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+ simplify future customizations and reuse.
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+ You might find it tempting to keep everything in one layer when
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+ working on a single project.
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+ However, the more modular your Metadata, the easier
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+ it is to cope with future changes.
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+ <note><title>Notes</title>
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+ <itemizedlist>
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+ <listitem><para>
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+ Use Board Support Package (BSP) layers from silicon
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+ vendors when possible.
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+ </para></listitem>
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+ <listitem><para>
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+ Familiarize yourself with the
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+ <ulink url='https://caffelli-staging.yoctoproject.org/software-overview/layers/'>Yocto Project curated layer index</ulink>
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+ or the
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+ <ulink url='http://layers.openembedded.org/layerindex/branch/master/layers/'>OpenEmbedded layer index</ulink>.
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+ The latter contains more layers but they are less
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+ universally validated.
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+ </para></listitem>
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+ <listitem><para>
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+ Layers support the inclusion of technologies, hardware
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+ components, and software components.
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+ The Yocto Project Compatible designation provides a
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+ minimum level of standardization that contributes to a
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+ strong ecosystem.
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+ "YP Compatible" is applied to appropriate products and
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+ software components such as BSPs, other OE-compatible
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+ layers, and related open-source projects, allowing the
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+ producer to use Yocto Project badges and branding
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+ assets.
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+ </para></listitem>
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+ </itemizedlist>
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+ </note>
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+ </para>
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+
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+ <para>
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+ To illustrate how layers are used to keep things modular, consider
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+ machine customizations.
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+ These types of customizations typically reside in a special layer,
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+ rather than a general layer, called a BSP Layer.
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+ Furthermore, the machine customizations should be isolated from
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+ recipes and Metadata that support a new GUI environment,
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+ for example.
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+ This situation gives you a couple of layers: one for the machine
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+ configurations, and one for the GUI environment.
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+ It is important to understand, however, that the BSP layer can
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+ still make machine-specific additions to recipes within the GUI
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+ environment layer without polluting the GUI layer itself
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+ with those machine-specific changes.
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+ You can accomplish this through a recipe that is a BitBake append
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+ (<filename>.bbappend</filename>) file, which is described later
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+ in this section.
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+ <note>
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+ For general information on BSP layer structure, see the
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+ <ulink url='&YOCTO_DOCS_BSP_URL;#bsp'>Board Support Packages (BSP) - Developer's Guide</ulink>.
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+ </note>
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+ </para>
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+
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+ <para>
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+ The
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+ <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
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+ contains both general layers and BSP layers right out of the box.
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+ You can easily identify layers that ship with a Yocto Project
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+ release in the Source Directory by their names.
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+ Layers typically have names that begin with the string
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+ <filename>meta-</filename>.
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+ <note>
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+ It is not a requirement that a layer name begin with the
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+ prefix <filename>meta-</filename>, but it is a commonly
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+ accepted standard in the Yocto Project community.
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+ </note>
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+ For example, if you were to examine the
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+ <ulink url='https://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/'>tree view</ulink>
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+ of the <filename>poky</filename> repository, you will see several
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+ layers: <filename>meta</filename>,
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+ <filename>meta-skeleton</filename>,
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+ <filename>meta-selftest</filename>,
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+ <filename>meta-poky</filename>, and
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+ <filename>meta-yocto-bsp</filename>.
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+ Each of these repositories represents a distinct layer.
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+ </para>
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+
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+ <para>
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+ For procedures on how to create layers, see the
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+ "<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>"
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+ section in the Yocto Project Development Tasks Manual.
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+ </para>
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</section>
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<section id='components-and-tools'>
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Scott Rifenbark