documentation/dev-manual: New Layer section and metadata link anchor
The change to the dev-manual-newbie.xml file was minor. I added a link anchor for the term "metadata." The change to the dev-manual-common-tasks.xml was extensive. I have added an entirely new section on layers called "Understanding and Using Layers." The new material has several sub-sections. Information was based on emails from Paul Eggleton. (From yocto-docs rev: 4fb34abd60180fc2482ddb9f62e476763cee7679) 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
Richard Purdie
parent
eb829f2768
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@ -16,58 +16,87 @@
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<section id="understanding-and-creating-layers">
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<title>Understanding and Creating Layers</title>
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<para>
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Often, developers want to extend the Yocto Project either by adding packages
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or by overriding files contained within the Yocto Project to add their own
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functionality.
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BitBake has a powerful mechanism called
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"layers", which provides a way to handle this extension in a fully
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supported and non-invasive fashion.
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The Yocto Project build system supports organizing <link linkend='metadata'>metadata</link>
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into multiple layers.
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Layers allow you to isolate different types of customizations from each other.
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You might find it tempting to keep everything in one layer when working on a single project.
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However, the more modular you organize your metadata, the easier it is to cope with future changes.
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</para>
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<para>
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The Yocto Project files include several additional layers such as
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<filename>meta-rt</filename> and <filename>meta-yocto</filename>
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that demonstrate this functionality.
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The <filename>meta-rt</filename> layer is not enabled by default.
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However, the <filename>meta-yocto</filename> layer is.
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To illustrate how layers are used to keep things modular, consider machine customizations.
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These types of customizations typically reside in a BSP Layer.
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Furthermore, the machine customizations should be isolated from recipes and metadata that support
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a new GUI environment, for example.
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This situation gives you a couple a layers: one for the machine configurations, and one for the
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GUI environment.
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It is important to understand, however, that the BSP layer can still make machine-specific
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additions to recipes within the GUI 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 an appending recipe (a <filename>.bbappend</filename> file),
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which is described later in this section.
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</para>
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<para>
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To enable a layer, you simply add the layer's path to the
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBLAYERS'>BBLAYERS</ulink></filename>
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variable in your
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<filename>conf/bblayers.conf</filename> file, which is found in the
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<link linkend='yocto-project-build-directory'>Yocto Project Build Directory</link>.
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The following example shows how to enable the <filename>meta-rt</filename>:
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<literallayout class='monospaced'>
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LCONF_VERSION = "1"
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BBFILES ?= ""
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BBLAYERS = " \
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/path/to/poky/meta \
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/path/to/poky/meta-yocto \
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/path/to/poky/meta-rt \
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"
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</literallayout>
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</para>
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<para>
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BitBake parses each <filename>conf/layer.conf</filename> file for each layer in
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<filename>BBLAYERS</filename>
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and adds the recipes, classes and configurations contained within the layer to
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the Yocto Project.
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To create your own layer, independent of the Yocto Project files,
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simply create a directory with a <filename>conf/layer.conf</filename> file and
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add the directory to your <filename>bblayers.conf</filename> file.
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</para>
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<section id='yocto-project-layers'>
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<title>Yocto Project Layers</title>
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<para>
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The <filename>meta-yocto/conf/layer.conf</filename> file demonstrates the
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required syntax:
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<literallayout class='monospaced'>
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<para>
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The Yocto Project contains several layers right out of the box.
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You can easily identify a layer in the Yocto Project by the name of its folder.
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Folders that are layers begin with the string <filename>meta</filename>.
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For example, when you set up the <link linkend='yocto-project-files'>Yocto Project Files</link>
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structure, you will see several layers: <filename>meta</filename>, <filename>meta-demoapps</filename>,
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<filename>meta-skeleton</filename>, and <filename>meta-yocto</filename>.
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Each of these folders is a layer.
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</para>
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<para>
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Furthermore, if you set up a local copy of the <filename>meta-intel</filename> Git repository
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and then explore that folder, you will discover many BSP layers within the
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<filename>meta-intel</filename> layer.
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For more information on BSP layers, see the
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"<ulink url='http://www.yoctoproject.org/docs/latest/bsp-guide/bsp-guide.html#bsp-layers'>BSP Layers</ulink>"
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section in the Yocto Project Development Manual.
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</para>
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</section>
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<section id='creating-your-own-layer'>
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<title>Creating Your Own Layer</title>
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<para>
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It is very easy to create your own layer to use with the Yocto Project.
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Follow these general steps to create your layer:
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<orderedlist>
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<listitem><para><emphasis>Check Existing Layers:</emphasis> Before creating a new layer,
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you should be sure someone has not already created a layer containing the metadata
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you need.
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You can see the
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<ulink url='http://www.openembedded.org/wiki/LayerIndex'><filename>LayerIndex</filename></ulink>
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to determine what types of layers already exist in the Yocto Project.</para></listitem>
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<listitem><para><emphasis>Create a Directory:</emphasis> Create the directory
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for your layer.
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Traditionally, prepend the name of the folder with the string
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<filename>meta</filename>.
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For example:
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<literallayout class='monospaced'>
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meta-mylayer
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meta-GUI_xyz
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meta-mymachine
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</literallayout></para></listitem>
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<listitem><para><emphasis>Create a Layer Configuration File:</emphasis> Inside your new
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layer folder, you need to create a <filename>conf/layer.conf</filename> file.
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It is easiest to take an existing layer configuration file and copy that to your
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layer's <filename>conf</filename> directory and then modify the file as needed.</para>
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<para>The <filename>meta-yocto/conf/layer.conf</filename> file demonstrates the
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required syntax:
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<literallayout class='monospaced'>
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# We have a conf and classes directory, add to BBPATH
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BBPATH := "${BBPATH}:${LAYERDIR}"
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BBPATH := "${LAYERDIR}:${BBPATH}"
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# We have a packages directory, add to BBFILES
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BBFILES := "${BBFILES} ${LAYERDIR}/recipes-*/*/*.bb \
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@ -76,58 +105,243 @@
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BBFILE_COLLECTIONS += "yocto"
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BBFILE_PATTERN_yocto := "^${LAYERDIR}/"
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BBFILE_PRIORITY_yocto = "5"
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</literallayout>
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</para>
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</literallayout></para>
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<para>In the previous example, the recipes for the layers are added to
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILES'>BBFILES</ulink></filename>.
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The
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILE_COLLECTIONS'>BBFILE_COLLECTIONS</ulink></filename>
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variable is then appended with the layer name.
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The
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILE_PATTERN'>BBFILE_PATTERN</ulink></filename>
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variable immediately expands with a regular expression used to match files from
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<filename>BBFILES</filename> into a particular layer, in this case by using
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the base pathname.
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The
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILE_PRIORITY'>BBFILE_PRIORITY</ulink></filename>
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variable then assigns different priorities to the files in different layers.
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Applying priorities is useful in situations where the same package might appear in multiple
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layers and allows you to choose what layer should take precedence.</para>
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<para>Note the use of the
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-LAYERDIR'>LAYERDIR</ulink></filename>
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variable with the immediate expansion operator.
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The <filename>LAYERDIR</filename> variable expands to the directory of the current layer and
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requires the immediate expansion operator so that BitBake does not wait to expand the variable
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when it's parsing a different directory.</para>
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<para>BitBake can locate where other <filename>.bbclass</filename> and configuration files
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are applied through the <filename>BBPATH</filename> environment variable.
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For these cases, BitBake uses the first file with the matching name found in
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<filename>BBPATH</filename>.
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This is similar to the way the <filename>PATH</filename> variable is used for binaries.
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We recommend, therefore, that you use unique <filename>.bbclass</filename>
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and configuration file names in your custom layer.</para></listitem>
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<listitem><para><emphasis>Add Content:</emphasis> Depending on the type of layer,
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add the content.
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If the layer adds support for a machine, add the machine configuration in
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a <filename>conf/machine/</filename> file within the layer.
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If the layer adds distro policy, add the distro configuration in a
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<filename>conf/distro/</filename> file with the layer.
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If the layer introduces new recipes, put the recipes you need in
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<filename>recipes-*</filename> subdirectories within the layer.</para></listitem>
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</orderedlist>
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</para>
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<para>
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In the previous example, the recipes for the layers are added to
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILES'>BBFILES</ulink></filename>.
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The
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILE_COLLECTIONS'>BBFILE_COLLECTIONS</ulink></filename>
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variable is then appended with the layer name.
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The
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILE_PATTERN'>BBFILE_PATTERN</ulink></filename>
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variable immediately expands with a regular expression used to match files from
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<filename>BBFILES</filename> into
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a particular layer, in this case by using the base pathname.
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The
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILE_PRIORITY'>BBFILE_PRIORITY</ulink></filename>
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variable
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then assigns different priorities to the files in different layers.
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Applying priorities is useful in situations where the same package might appear in multiple
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layers and allows you to choose what layer should take precedence.
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</para>
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<para>
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To create layers that are easier to maintain, you should consider the following:
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<itemizedlist>
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<listitem><para>Avoid "overlaying" entire recipes from other layers in your
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configuration.
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In other words, don't copy an entire recipe into your layer and then modify it.
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Use <filename>.bbappend</filename> files to override the parts of the
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recipe you need to modify.</para></listitem>
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<listitem><para>Avoid duplicating include files.
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Use <filename>.bbappend</filename> files for each recipe that uses an include
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file.
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Or, if you are introducing a new recipe that requires the inc file, use the
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full path to refer to the original.
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For example, use <filename>require recipes-core/somepackage/somefile.inc</filename>
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instead of <filename>require somefile.inc</filename>.
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If you're finding you have to overlay the include file, it could indicate a
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deficiency in the include file in the layer to which it originally belongs.
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If this is the case, you need to address that deficiency instead of overlaying
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the include file.
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For example, consider how Qt 4 database support plugins are configured.
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The Yocto Project does not have MySQL or PostgreSQL, however OpenEmbedded's
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layer <filename>meta-oe</filename> does.
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Consequently, <filename>meta-oe</filename> uses <filename>.bbappend</filename>
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files to modify the <filename>QT_SQL_DRIVER_FLAGS</filename> variable to enable
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the appropriate plugins.
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This variable was added to the <filename>qt4.inc</filename> include file in
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The Yocto Project specifically to allow the <filename>meta-oe</filename> layer
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to be able to control which plugins are built.</para></listitem>
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</itemizedlist>
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</para>
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<para>
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Note the use of the
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-LAYERDIR'>LAYERDIR</ulink></filename>
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variable with the immediate expansion operator.
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The <filename>LAYERDIR</filename> variable expands to the directory of the current layer and
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requires the immediate expansion operator so that BitBake does not wait to expand the variable
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when it's parsing a different directory.
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</para>
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<para>
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We also recommend the following:
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<itemizedlist>
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<listitem><para>Store custom layers in a Git repository that uses the
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<filename>meta-prvt-XXXX</filename> format.</para></listitem>
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<listitem><para>Clone the repository alongside other <filename>meta</filename>
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directories in the Yocto Project source files area.</para></listitem>
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</itemizedlist>
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Following these recommendations keeps your Yocto Project files area and
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its configuration entirely inside the Yocto Project's core base.
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</para>
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</section>
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<para>
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BitBake can locate where other <filename>.bbclass</filename> and configuration files
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are applied through the <filename>BBPATH</filename> environment variable.
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For these cases, BitBake uses the first file with the matching name found in
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<filename>BBPATH</filename>.
|
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This is similar to the way the <filename>PATH</filename> variable is used for binaries.
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We recommend, therefore, that you use unique <filename>.bbclass</filename>
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and configuration file names in your custom layer.
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</para>
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<section id='enabling-your-layer'>
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<title>Enabling Your Layer</title>
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<para>
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We also recommend the following:
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<itemizedlist>
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<listitem><para>Store custom layers in a Git repository that uses the
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<filename>meta-prvt-XXXX</filename> format.</para></listitem>
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<listitem><para>Clone the repository alongside other <filename>meta</filename>
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directories in the Yocto Project source files area.</para></listitem>
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</itemizedlist>
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Following these recommendations keeps your Yocto Project files area and
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its configuration entirely inside the Yocto Project's core base.
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</para>
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<para>
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Before the Yocto Project build system can use your new layer, you need to enable it.
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To enable your layer, simply add your layer's path to the
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<filename><ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBLAYERS'>BBLAYERS</ulink></filename>
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variable in your <filename>conf/bblayers.conf</filename> file, which is found in the
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<link linkend='yocto-project-build-directory'>Yocto Project Build Directory</link>.
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The following example shows how to enable a layer named <filename>meta-mylayer</filename>:
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<literallayout class='monospaced'>
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LCONF_VERSION = "1"
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BBFILES ?= ""
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BBLAYERS = " \
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/path/to/poky/meta \
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/path/to/poky/meta-yocto \
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/path/to/poky/meta-mylayer \
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"
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</literallayout>
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</para>
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<para>
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BitBake parses each <filename>conf/layer.conf</filename> file as specified in the
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<filename>BBLAYERS</filename> variable within the <filename>conf/bblayers.conf</filename>
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file.
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During the processing of each <filename>conf/layer.conf</filename> file, BitBake adds the
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recipes, classes and configurations contained within the particular layer to the Yocto
|
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Project.
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To create your own layer, independent of the Yocto Project files,
|
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simply create a directory with a <filename>conf/layer.conf</filename> file and
|
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add the directory to your <filename>bblayers.conf</filename> file.
|
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</para>
|
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</section>
|
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<section id='using-bbappend-files'>
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<title>Using .bbappend Files</title>
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<para>
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Recipe append files (<filename>.bbappend</filename> type) allow your layer to make additions or
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changes to the content of another layer's recipe without having to copy the other recipe into
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your layer.
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Your <filename>.bbappend</filename> file resides in your layer, while the underlying recipe
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to which you are appending resides in a different layer.
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</para>
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<para>
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Append files files must have the same name as the underlying recipe.
|
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For example, the append file <filename>someapp_1.1.bbappend</filename> must
|
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apply to <filename>someapp_1.1.bb</filename>.
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This means the original recipe and append file names are version number specific.
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If the underlying recipe is renamed to update to a newer version, the
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corresponding <filename>.bbappend</filename> file must be renamed as well.
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During the build process, BitBake displays warnings on starting if it detects a
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<filename>.bbappend</filename> file that does not have an underlying recipe
|
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with the proper name.
|
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</para>
|
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|
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<para>
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Being able to append information to an existing recipe not only avoids duplication,
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but also automatically applies recipe changes in a different layer to your layer.
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If you were copying recipes, you would have to manually merge changes as they occur.
|
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</para>
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<para>
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As an example, consider the main formfactor recipe and a corresponding formfactor
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append file both from the Yocto Project Files.
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Here is the main formfactor recipe, which is named <filename>formfactor_0.0.bb</filename> and
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located in the meta layer at <filename>meta/bsp-recipes/formfactor</filename>:
|
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<literallayout class='monospaced'>
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DESCRIPTION = "Device formfactor information"
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SECTION = "base"
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LICENSE = "MIT"
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LIC_FILES_CHKSUM = "file://${COREBASE}/LICENSE;md5=3f40d7994397109285ec7b81fdeb3b58 \
|
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file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
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PR = "r19"
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SRC_URI = "file://config file://machconfig"
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S = "${WORKDIR}"
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|
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PACKAGE_ARCH = "${MACHINE_ARCH}"
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INHIBIT_DEFAULT_DEPS = "1"
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|
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do_install() {
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# Only install file if it has a contents
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install -d ${D}${sysconfdir}/formfactor/
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install -m 0644 ${S}/config ${D}${sysconfdir}/formfactor/
|
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if [ -s "${S}/machconfig" ]; then
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install -m 0644 ${S}/machconfig ${D}${sysconfdir}/formfactor/
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fi
|
||||
}
|
||||
</literallayout>
|
||||
Here is the append file, which is named <filename>formfactor_0.0.bbappend</filename> and is from the
|
||||
Crown Bay BSP Layer named <filename>meta-intel/meta-crownbay</filename>:
|
||||
<literallayout class='monospaced'>
|
||||
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
|
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|
||||
PRINC = "1"
|
||||
</literallayout>
|
||||
This example adds or overrides files in
|
||||
<ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-SRC_URI'><filename>SRC_URI</filename></ulink>
|
||||
within a bbappend by extending the path BitBake uses to search for files.
|
||||
The most reliable way to do this is by prepending the
|
||||
<filename>FILESEXTRAPATHS</filename> variable.
|
||||
For example, if you have your files in a directory that is named the same as your package
|
||||
(<ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-PN'><filename>PN</filename></ulink>),
|
||||
you can add this directory by adding the following to your bbappend file:
|
||||
<literallayout class='monospaced'>
|
||||
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
|
||||
</literallayout>
|
||||
Using the immediate expansion assignment operator <filename>:=</filename> and the trailing colon
|
||||
are important so that the resulting list of pathnames is syntactically correct.
|
||||
<note>BitBake automatically defines the <filename>THISDIR</filename> variable.
|
||||
You should never set this variable yourself.
|
||||
Using <filename>_prepend</filename> ensures your path will be searched prior to other
|
||||
paths in the final list.
|
||||
</note>
|
||||
</para>
|
||||
</section>
|
||||
|
||||
<section id='prioritizing-your-layer'>
|
||||
<title>Prioritizing Your Layer</title>
|
||||
|
||||
<para>
|
||||
Each layer is assigned a priority value.
|
||||
Priority values control which layer takes precedence if there are recipe files with
|
||||
the same name in multiple layers.
|
||||
For these cases, the recipe file from the layer with a higher priority number taking precedence.
|
||||
Priority values also affect the order in which multiple <filename>.bbappend</filename> files
|
||||
for the same recipe are applied.
|
||||
You can either specify the priority manually, or allow the build system to calculate it
|
||||
based on the layer's dependencies.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
To specify the layer's priority manually, use the
|
||||
<ulink url='http://yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-BBFILE_PRIORITY'><filename>BBFILE_PRIORITY</filename></ulink>
|
||||
variable.
|
||||
For example:
|
||||
<literallayout class='monospaced'>
|
||||
BBFILE_PRIORITY := "1"
|
||||
</literallayout>
|
||||
</para>
|
||||
|
||||
<note>
|
||||
<para>It is possible for a recipe with a lower version number
|
||||
<ulink url='http://yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#var-PV'><filename>PV</filename></ulink>
|
||||
in a layer that has a higher priority to take precedence.</para>
|
||||
<para>Also, the layer priority does not currently affect the precedence order of
|
||||
<filename>.conf</filename> or <filename>.bbclass</filename> files.
|
||||
Future versions of BitBake might address this.</para>
|
||||
</note>
|
||||
</section>
|
||||
</section>
|
||||
|
||||
<section id='usingpoky-extend-addpkg'>
|
||||
|
|
|
|||
|
|
@ -177,12 +177,13 @@
|
|||
For a list of the supported image types that the Yocto Project provides, see the
|
||||
"<ulink url='http://www.yoctoproject.org/docs/latest/poky-ref-manual/poky-ref-manual.html#ref-images'>Reference: Images</ulink>"
|
||||
appendix in The Yocto Project Reference Manual.</para></listitem>
|
||||
<listitem><para><emphasis>Layer:</emphasis> A collection of recipes representing the core,
|
||||
<listitem><para id='layer'><emphasis>Layer:</emphasis> A collection of recipes representing the core,
|
||||
a BSP, or an application stack.
|
||||
For a discussion on BSP Layers, see the
|
||||
"<ulink url='http://www.yoctoproject.org/docs/latest/bsp-guide/bsp-guide.html#bsp-layers'>BSP Layers</ulink>"
|
||||
section in the Yocto Project Board Support Packages (BSP) Developer's Guide.</para></listitem>
|
||||
<listitem><para><emphasis>Metadata:</emphasis> The files that BitBake parses when building an image.
|
||||
<listitem><para id='metadata'><emphasis>Metadata:</emphasis> The files that BitBake parses when
|
||||
building an image.
|
||||
Metadata includes recipes, classes, and configuration files.</para></listitem>
|
||||
<listitem><para><emphasis>OE-Core:</emphasis> A core set of metadata originating
|
||||
with OpenEmbedded (OE) that is shared between OE and the Yocto Project.
|
||||
|
|
|
|||
Loading…
Reference in New Issue