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+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+Understanding and Creating Layers
+*********************************
+
+The OpenEmbedded build system supports organizing
+:term:`Metadata` into multiple layers.
+Layers allow you to isolate different types of customizations from each
+other. For introductory information on the Yocto Project Layer Model,
+see the
+":ref:`overview-manual/yp-intro:the yocto project layer model`"
+section in the Yocto Project Overview and Concepts Manual.
+
+Creating Your Own Layer
+=======================
+
+.. note::
+
+   It is very easy to create your own layers to use with the OpenEmbedded
+   build system, as the Yocto Project ships with tools that speed up creating
+   layers. This section describes the steps you perform by hand to create
+   layers so that you can better understand them. For information about the
+   layer-creation tools, see the
+   ":ref:`bsp-guide/bsp:creating a new bsp layer using the \`\`bitbake-layers\`\` script`"
+   section in the Yocto Project Board Support Package (BSP) Developer's
+   Guide and the ":ref:`dev-manual/layers:creating a general layer using the \`\`bitbake-layers\`\` script`"
+   section further down in this manual.
+
+Follow these general steps to create your layer without using tools:
+
+#. *Check Existing Layers:* Before creating a new layer, you should be
+   sure someone has not already created a layer containing the Metadata
+   you need. You can see the :oe_layerindex:`OpenEmbedded Metadata Index <>`
+   for a list of layers from the OpenEmbedded community that can be used in
+   the Yocto Project. You could find a layer that is identical or close
+   to what you need.
+
+#. *Create a Directory:* Create the directory for your layer. When you
+   create the layer, be sure to create the directory in an area not
+   associated with the Yocto Project :term:`Source Directory`
+   (e.g. the cloned ``poky`` repository).
+
+   While not strictly required, prepend the name of the directory with
+   the string "meta-". For example::
+
+      meta-mylayer
+      meta-GUI_xyz
+      meta-mymachine
+
+   With rare exceptions, a layer's name follows this form::
+
+      meta-root_name
+
+   Following this layer naming convention can save
+   you trouble later when tools, components, or variables "assume" your
+   layer name begins with "meta-". A notable example is in configuration
+   files as shown in the following step where layer names without the
+   "meta-" string are appended to several variables used in the
+   configuration.
+
+#. *Create a Layer Configuration File:* Inside your new layer folder,
+   you need to create a ``conf/layer.conf`` file. It is easiest to take
+   an existing layer configuration file and copy that to your layer's
+   ``conf`` directory and then modify the file as needed.
+
+   The ``meta-yocto-bsp/conf/layer.conf`` file in the Yocto Project
+   :yocto_git:`Source Repositories </poky/tree/meta-yocto-bsp/conf>`
+   demonstrates the required syntax. For your layer, you need to replace
+   "yoctobsp" with a unique identifier for your layer (e.g. "machinexyz"
+   for a layer named "meta-machinexyz")::
+
+      # We have a conf and classes directory, add to BBPATH
+      BBPATH .= ":${LAYERDIR}"
+
+      # We have recipes-* directories, add to BBFILES
+      BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
+                  ${LAYERDIR}/recipes-*/*/*.bbappend"
+
+      BBFILE_COLLECTIONS += "yoctobsp"
+      BBFILE_PATTERN_yoctobsp = "^${LAYERDIR}/"
+      BBFILE_PRIORITY_yoctobsp = "5"
+      LAYERVERSION_yoctobsp = "4"
+      LAYERSERIES_COMPAT_yoctobsp = "walnascar"
+
+   Here is an explanation of the layer configuration file:
+
+   -  :term:`BBPATH`: Adds the layer's
+      root directory to BitBake's search path. Through the use of the
+      :term:`BBPATH` variable, BitBake locates class files (``.bbclass``),
+      configuration files, and files that are included with ``include``
+      and ``require`` statements. For these cases, BitBake uses the
+      first file that matches the name found in :term:`BBPATH`. This is
+      similar to the way the ``PATH`` variable is used for binaries. It
+      is recommended, therefore, that you use unique class and
+      configuration filenames in your custom layer.
+
+   -  :term:`BBFILES`: Defines the
+      location for all recipes in the layer.
+
+   -  :term:`BBFILE_COLLECTIONS`:
+      Establishes the current layer through a unique identifier that is
+      used throughout the OpenEmbedded build system to refer to the
+      layer. In this example, the identifier "yoctobsp" is the
+      representation for the container layer named "meta-yocto-bsp".
+
+   -  :term:`BBFILE_PATTERN`:
+      Expands immediately during parsing to provide the directory of the
+      layer.
+
+   -  :term:`BBFILE_PRIORITY`:
+      Establishes a priority to use for recipes in the layer when the
+      OpenEmbedded build finds recipes of the same name in different
+      layers.
+
+   -  :term:`LAYERVERSION`:
+      Establishes a version number for the layer. You can use this
+      version number to specify this exact version of the layer as a
+      dependency when using the
+      :term:`LAYERDEPENDS`
+      variable.
+
+   -  :term:`LAYERDEPENDS`:
+      Lists all layers on which this layer depends (if any).
+
+   -  :term:`LAYERSERIES_COMPAT`:
+      Lists the :yocto_wiki:`Yocto Project </Releases>`
+      releases for which the current version is compatible. This
+      variable is a good way to indicate if your particular layer is
+      current.
+
+
+   .. note::
+
+      A layer does not have to contain only recipes ``.bb`` or append files
+      ``.bbappend``. Generally, developers create layers using
+      ``bitbake-layers create-layer``.
+      See ":ref:`dev-manual/layers:creating a general layer using the \`\`bitbake-layers\`\` script`",
+      explaining how the ``layer.conf`` file is created from a template located in
+      ``meta/lib/bblayers/templates/layer.conf``.
+      In fact, none of the variables set in ``layer.conf`` are mandatory,
+      except when :term:`BBFILE_COLLECTIONS` is present. In this case
+      :term:`LAYERSERIES_COMPAT` and :term:`BBFILE_PATTERN` have to be
+      defined too.
+
+#. *Add Content:* Depending on the type of layer, add the content. If
+   the layer adds support for a machine, add the machine configuration
+   in a ``conf/machine/`` file within the layer. If the layer adds
+   distro policy, add the distro configuration in a ``conf/distro/``
+   file within the layer. If the layer introduces new recipes, put the
+   recipes you need in ``recipes-*`` subdirectories within the layer.
+
+   .. note::
+
+      For an explanation of layer hierarchy that is compliant with the
+      Yocto Project, see the ":ref:`bsp-guide/bsp:example filesystem layout`"
+      section in the Yocto Project Board Support Package (BSP) Developer's Guide.
+
+#. *Optionally Test for Compatibility:* If you want permission to use
+   the Yocto Project Compatibility logo with your layer or application
+   that uses your layer, perform the steps to apply for compatibility.
+   See the
+   ":ref:`dev-manual/layers:making sure your layer is compatible with yocto project`"
+   section for more information.
+
+Following Best Practices When Creating Layers
+=============================================
+
+To create layers that are easier to maintain and that will not impact
+builds for other machines, you should consider the information in the
+following list:
+
+-  *Avoid "Overlaying" Entire Recipes from Other Layers in Your
+   Configuration:* In other words, do not copy an entire recipe into
+   your layer and then modify it. Rather, use an append file
+   (``.bbappend``) to override only those parts of the original recipe
+   you need to modify.
+
+-  *Avoid Duplicating Include Files:* Use append files (``.bbappend``)
+   for each recipe that uses an include file. Or, if you are introducing
+   a new recipe that requires the included file, use the path relative
+   to the original layer directory to refer to the file. For example,
+   use ``require recipes-core/``\ `package`\ ``/``\ `file`\ ``.inc`` instead
+   of ``require`` `file`\ ``.inc``. If you're finding you have to overlay
+   the include file, it could indicate a deficiency in the include file
+   in the layer to which it originally belongs. If this is the case, you
+   should try to address that deficiency instead of overlaying the
+   include file. For example, you could address this by getting the
+   maintainer of the include file to add a variable or variables to make
+   it easy to override the parts needing to be overridden.
+
+-  *Structure Your Layers:* Proper use of overrides within append files
+   and placement of machine-specific files within your layer can ensure
+   that a build is not using the wrong Metadata and negatively impacting
+   a build for a different machine. Here are some examples:
+
+   -  *Modify Variables to Support a Different Machine:* Suppose you
+      have a layer named ``meta-one`` that adds support for building
+      machine "one". To do so, you use an append file named
+      ``base-files.bbappend`` and create a dependency on "foo" by
+      altering the :term:`DEPENDS`
+      variable::
+
+         DEPENDS = "foo"
+
+      The dependency is created during any
+      build that includes the layer ``meta-one``. However, you might not
+      want this dependency for all machines. For example, suppose you
+      are building for machine "two" but your ``bblayers.conf`` file has
+      the ``meta-one`` layer included. During the build, the
+      ``base-files`` for machine "two" will also have the dependency on
+      ``foo``.
+
+      To make sure your changes apply only when building machine "one",
+      use a machine override with the :term:`DEPENDS` statement::
+
+         DEPENDS:one = "foo"
+
+      You should follow the same strategy when using ``:append``
+      and ``:prepend`` operations::
+
+         DEPENDS:append:one = " foo"
+         DEPENDS:prepend:one = "foo "
+
+      As an actual example, here's a
+      snippet from the generic kernel include file ``linux-yocto.inc``,
+      wherein the kernel compile and link options are adjusted in the
+      case of a subset of the supported architectures::
+
+         DEPENDS:append:aarch64 = " libgcc"
+         KERNEL_CC:append:aarch64 = " ${TOOLCHAIN_OPTIONS}"
+         KERNEL_LD:append:aarch64 = " ${TOOLCHAIN_OPTIONS}"
+
+         DEPENDS:append:nios2 = " libgcc"
+         KERNEL_CC:append:nios2 = " ${TOOLCHAIN_OPTIONS}"
+         KERNEL_LD:append:nios2 = " ${TOOLCHAIN_OPTIONS}"
+
+         DEPENDS:append:arc = " libgcc"
+         KERNEL_CC:append:arc = " ${TOOLCHAIN_OPTIONS}"
+         KERNEL_LD:append:arc = " ${TOOLCHAIN_OPTIONS}"
+
+         KERNEL_FEATURES:append:qemuall=" features/debug/printk.scc"
+
+   -  *Place Machine-Specific Files in Machine-Specific Locations:* When
+      you have a base recipe, such as ``base-files.bb``, that contains a
+      :term:`SRC_URI` statement to a
+      file, you can use an append file to cause the build to use your
+      own version of the file. For example, an append file in your layer
+      at ``meta-one/recipes-core/base-files/base-files.bbappend`` could
+      extend :term:`FILESPATH` using :term:`FILESEXTRAPATHS` as follows::
+
+         FILESEXTRAPATHS:prepend := "${THISDIR}/${BPN}:"
+
+      The build for machine "one" will pick up your machine-specific file as
+      long as you have the file in
+      ``meta-one/recipes-core/base-files/base-files/``. However, if you
+      are building for a different machine and the ``bblayers.conf``
+      file includes the ``meta-one`` layer and the location of your
+      machine-specific file is the first location where that file is
+      found according to :term:`FILESPATH`, builds for all machines will
+      also use that machine-specific file.
+
+      You can make sure that a machine-specific file is used for a
+      particular machine by putting the file in a subdirectory specific
+      to the machine. For example, rather than placing the file in
+      ``meta-one/recipes-core/base-files/base-files/`` as shown above,
+      put it in ``meta-one/recipes-core/base-files/base-files/one/``.
+      Not only does this make sure the file is used only when building
+      for machine "one", but the build process locates the file more
+      quickly.
+
+      In summary, you need to place all files referenced from
+      :term:`SRC_URI` in a machine-specific subdirectory within the layer in
+      order to restrict those files to machine-specific builds.
+
+-  *Perform Steps to Apply for Yocto Project Compatibility:* If you want
+   permission to use the Yocto Project Compatibility logo with your
+   layer or application that uses your layer, perform the steps to apply
+   for compatibility. See the
+   ":ref:`dev-manual/layers:making sure your layer is compatible with yocto project`"
+   section for more information.
+
+-  *Follow the Layer Naming Convention:* Store custom layers in a Git
+   repository that use the ``meta-layer_name`` format.
+
+-  *Group Your Layers Locally:* Clone your repository alongside other
+   cloned ``meta`` directories from the :term:`Source Directory`.
+
+Making Sure Your Layer is Compatible With Yocto Project
+=======================================================
+
+When you create a layer used with the Yocto Project, it is advantageous
+to make sure that the layer interacts well with existing Yocto Project
+layers (i.e. the layer is compatible with the Yocto Project). Ensuring
+compatibility makes the layer easy to be consumed by others in the Yocto
+Project community and could allow you permission to use the Yocto
+Project Compatible Logo.
+
+.. note::
+
+   Only Yocto Project member organizations are permitted to use the
+   Yocto Project Compatible Logo. The logo is not available for general
+   use. For information on how to become a Yocto Project member
+   organization, see the :yocto_home:`Yocto Project Website <>`.
+
+The Yocto Project Compatibility Program consists of a layer application
+process that requests permission to use the Yocto Project Compatibility
+Logo for your layer and application. The process consists of two parts:
+
+#. Successfully passing a script (``yocto-check-layer``) that, when run
+   against your layer, tests it against constraints based on experiences
+   of how layers have worked in the real world and where pitfalls have
+   been found. Getting a "PASS" result from the script is required for
+   successful compatibility registration.
+
+#. Completion of an application acceptance form, which you can find at
+   :yocto_home:`/compatible-registration/`.
+
+To be granted permission to use the logo, you need to satisfy the
+following:
+
+-  Be able to check the box indicating that you got a "PASS" when
+   running the script against your layer.
+
+-  Answer "Yes" to the questions on the form or have an acceptable
+   explanation for any questions answered "No".
+
+-  Be a Yocto Project Member Organization.
+
+The remainder of this section presents information on the registration
+form and on the ``yocto-check-layer`` script.
+
+Yocto Project Compatible Program Application
+--------------------------------------------
+
+Use the form to apply for your layer's approval. Upon successful
+application, you can use the Yocto Project Compatibility Logo with your
+layer and the application that uses your layer.
+
+To access the form, use this link:
+:yocto_home:`/compatible-registration`.
+Follow the instructions on the form to complete your application.
+
+The application consists of the following sections:
+
+-  *Contact Information:* Provide your contact information as the fields
+   require. Along with your information, provide the released versions
+   of the Yocto Project for which your layer is compatible.
+
+-  *Acceptance Criteria:* Provide "Yes" or "No" answers for each of the
+   items in the checklist. There is space at the bottom of the form for
+   any explanations for items for which you answered "No".
+
+-  *Recommendations:* Provide answers for the questions regarding Linux
+   kernel use and build success.
+
+``yocto-check-layer`` Script
+----------------------------
+
+The ``yocto-check-layer`` script provides you a way to assess how
+compatible your layer is with the Yocto Project. You should run this
+script prior to using the form to apply for compatibility as described
+in the previous section. You need to achieve a "PASS" result in order to
+have your application form successfully processed.
+
+The script divides tests into three areas: COMMON, BSP, and DISTRO. For
+example, given a distribution layer (DISTRO), the layer must pass both
+the COMMON and DISTRO related tests. Furthermore, if your layer is a BSP
+layer, the layer must pass the COMMON and BSP set of tests.
+
+To execute the script, enter the following commands from your build
+directory::
+
+   $ source oe-init-build-env
+   $ yocto-check-layer your_layer_directory
+
+Be sure to provide the actual directory for your
+layer as part of the command.
+
+Entering the command causes the script to determine the type of layer
+and then to execute a set of specific tests against the layer. The
+following list overviews the test:
+
+-  ``common.test_readme``: Tests if a ``README`` file exists in the
+   layer and the file is not empty.
+
+-  ``common.test_parse``: Tests to make sure that BitBake can parse the
+   files without error (i.e. ``bitbake -p``).
+
+-  ``common.test_show_environment``: Tests that the global or per-recipe
+   environment is in order without errors (i.e. ``bitbake -e``).
+
+-  ``common.test_world``: Verifies that ``bitbake world`` works.
+
+-  ``common.test_signatures``: Tests to be sure that BSP and DISTRO
+   layers do not come with recipes that change signatures.
+
+-  ``common.test_layerseries_compat``: Verifies layer compatibility is
+   set properly.
+
+-  ``bsp.test_bsp_defines_machines``: Tests if a BSP layer has machine
+   configurations.
+
+-  ``bsp.test_bsp_no_set_machine``: Tests to ensure a BSP layer does not
+   set the machine when the layer is added.
+
+-  ``bsp.test_machine_world``: Verifies that ``bitbake world`` works
+   regardless of which machine is selected.
+
+-  ``bsp.test_machine_signatures``: Verifies that building for a
+   particular machine affects only the signature of tasks specific to
+   that machine.
+
+-  ``distro.test_distro_defines_distros``: Tests if a DISTRO layer has
+   distro configurations.
+
+-  ``distro.test_distro_no_set_distros``: Tests to ensure a DISTRO layer
+   does not set the distribution when the layer is added.
+
+Enabling Your Layer
+===================
+
+Before the OpenEmbedded build system can use your new layer, you need to
+enable it. To enable your layer, simply add your layer's path to the
+:term:`BBLAYERS` variable in your ``conf/bblayers.conf`` file, which is
+found in the :term:`Build Directory`. The following example shows how to
+enable your new ``meta-mylayer`` layer (note how your new layer exists
+outside of the official ``poky`` repository which you would have checked
+out earlier)::
+
+   # POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
+   # changes incompatibly
+   POKY_BBLAYERS_CONF_VERSION = "2"
+   BBPATH = "${TOPDIR}"
+   BBFILES ?= ""
+   BBLAYERS ?= " \
+       /home/user/poky/meta \
+       /home/user/poky/meta-poky \
+       /home/user/poky/meta-yocto-bsp \
+       /home/user/mystuff/meta-mylayer \
+       "
+
+BitBake parses each ``conf/layer.conf`` file from the top down as
+specified in the :term:`BBLAYERS` variable within the ``conf/bblayers.conf``
+file. During the processing of each ``conf/layer.conf`` file, BitBake
+adds the recipes, classes and configurations contained within the
+particular layer to the source directory.
+
+Appending Other Layers Metadata With Your Layer
+===============================================
+
+A recipe that appends Metadata to another recipe is called a BitBake
+append file. A BitBake append file uses the ``.bbappend`` file type
+suffix, while the corresponding recipe to which Metadata is being
+appended uses the ``.bb`` file type suffix.
+
+You can use a ``.bbappend`` file in your layer to make additions or
+changes to the content of another layer's recipe without having to copy
+the other layer's recipe into your layer. Your ``.bbappend`` file
+resides in your layer, while the main ``.bb`` recipe file to which you
+are appending Metadata resides in a different layer.
+
+Being able to append information to an existing recipe not only avoids
+duplication, but also automatically applies recipe changes from a
+different layer into your layer. If you were copying recipes, you would
+have to manually merge changes as they occur.
+
+When you create an append file, you must use the same root name as the
+corresponding recipe file. For example, the append file
+``someapp_3.1.bbappend`` must apply to ``someapp_3.1.bb``. This
+means the original recipe and append filenames are version
+number-specific. If the corresponding recipe is renamed to update to a
+newer version, you must also rename and possibly update the
+corresponding ``.bbappend`` as well.
+
+During the build process, BitBake displays an error on startup if it detects a
+``.bbappend`` file that does not have a corresponding recipe with a matching
+name. To handle these errors, the best practice is to rename the ``.bbappend``
+to match the original recipe version. This also gives you the opportunity to see
+if the ``.bbappend`` is still relevant for the new version of the recipe.
+
+Another method is to use the character ``%`` in the ``.bbappend`` filename. For
+example, to append information to every ``6.*`` minor versions of the recipe
+``someapp``, the ``someapp_6.%.bbappend`` file can be created. This way, an
+error will only be triggered if the ``someapp`` recipe has a major version
+update.
+
+Finally, another method to deal with these errors is to use the variable
+:term:`BBMASK`, especially in cases where modifying the ``.bbappend`` is not
+possible.
+
+Overlaying a File Using Your Layer
+----------------------------------
+
+As an example, consider the main formfactor recipe and a corresponding
+formfactor append file both from the :term:`Source Directory`.
+Here is the main
+formfactor recipe, which is named ``formfactor_0.0.bb`` and located in
+the "meta" layer at ``meta/recipes-bsp/formfactor``::
+
+   SUMMARY = "Device formfactor information"
+   DESCRIPTION = "A formfactor configuration file provides information about the \
+   target hardware for which the image is being built and information that the \
+   build system cannot obtain from other sources such as the kernel."
+   SECTION = "base"
+   LICENSE = "MIT"
+   LIC_FILES_CHKSUM = "file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
+
+   SRC_URI = "file://config file://machconfig"
+   S = "${UNPACKDIR}"
+
+   PACKAGE_ARCH = "${MACHINE_ARCH}"
+   INHIBIT_DEFAULT_DEPS = "1"
+
+   do_install() {
+           # Install file only if it has contents
+           install -d ${D}${sysconfdir}/formfactor/
+           install -m 0644 ${S}/config ${D}${sysconfdir}/formfactor/
+           if [ -s "${S}/machconfig" ]; then
+                   install -m 0644 ${S}/machconfig ${D}${sysconfdir}/formfactor/
+           fi
+   }
+
+In the main recipe, note the :term:`SRC_URI`
+variable, which tells the OpenEmbedded build system where to find files
+during the build.
+
+Here is the append file, which is named ``formfactor_0.0.bbappend``
+and is from the Raspberry Pi BSP Layer named ``meta-raspberrypi``. The
+file is in the layer at ``recipes-bsp/formfactor``::
+
+   FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
+
+By default, the build system uses the
+:term:`FILESPATH` variable to
+locate files. This append file extends the locations by setting the
+:term:`FILESEXTRAPATHS`
+variable. Setting this variable in the ``.bbappend`` file is the most
+reliable and recommended method for adding directories to the search
+path used by the build system to find files.
+
+The statement in this example extends the directories to include
+``${``\ :term:`THISDIR`\ ``}/${``\ :term:`PN`\ ``}``,
+which resolves to a directory named ``formfactor`` in the same directory
+in which the append file resides (i.e.
+``meta-raspberrypi/recipes-bsp/formfactor``. This implies that you must
+have the supporting directory structure set up that will contain any
+files or patches you will be including from the layer.
+
+Using the immediate expansion assignment operator ``:=`` is important
+because of the reference to :term:`THISDIR`. The trailing colon character is
+important as it ensures that items in the list remain colon-separated.
+
+.. note::
+
+   BitBake automatically defines the :term:`THISDIR` variable. You should
+   never set this variable yourself. Using ":prepend" as part of the
+   :term:`FILESEXTRAPATHS` ensures your path will be searched prior to other
+   paths in the final list.
+
+   Also, not all append files add extra files. Many append files simply
+   allow to add build options (e.g. ``systemd``). For these cases, your
+   append file would not even use the :term:`FILESEXTRAPATHS` statement.
+
+The end result of this ``.bbappend`` file is that on a Raspberry Pi, where
+``rpi`` will exist in the list of :term:`OVERRIDES`, the file
+``meta-raspberrypi/recipes-bsp/formfactor/formfactor/rpi/machconfig`` will be
+used during :ref:`ref-tasks-fetch` and the test for a non-zero file size in
+:ref:`ref-tasks-install` will return true, and the file will be installed.
+
+Installing Additional Files Using Your Layer
+--------------------------------------------
+
+As another example, consider the main ``xserver-xf86-config`` recipe and a
+corresponding ``xserver-xf86-config`` append file both from the :term:`Source
+Directory`.  Here is the main ``xserver-xf86-config`` recipe, which is named
+``xserver-xf86-config_0.1.bb`` and located in the "meta" layer at
+``meta/recipes-graphics/xorg-xserver``::
+
+   SUMMARY = "X.Org X server configuration file"
+   HOMEPAGE = "http://www.x.org"
+   SECTION = "x11/base"
+   LICENSE = "MIT"
+   LIC_FILES_CHKSUM = "file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"
+
+   SRC_URI = "file://xorg.conf"
+
+   S = "${UNPACKDIR}"
+
+   CONFFILES:${PN} = "${sysconfdir}/X11/xorg.conf"
+
+   PACKAGE_ARCH = "${MACHINE_ARCH}"
+   ALLOW_EMPTY:${PN} = "1"
+
+   do_install () {
+        if test -s ${UNPACKDIR}/xorg.conf; then
+                install -d ${D}/${sysconfdir}/X11
+                install -m 0644 ${UNPACKDIR}/xorg.conf ${D}/${sysconfdir}/X11/
+        fi
+   }
+
+Here is the append file, which is named ``xserver-xf86-config_%.bbappend``
+and is from the Raspberry Pi BSP Layer named ``meta-raspberrypi``. The
+file is in the layer at ``recipes-graphics/xorg-xserver``::
+
+   FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
+
+   SRC_URI:append:rpi = " \
+       file://xorg.conf.d/98-pitft.conf \
+       file://xorg.conf.d/99-calibration.conf \
+   "
+   do_install:append:rpi () {
+       PITFT="${@bb.utils.contains("MACHINE_FEATURES", "pitft", "1", "0", d)}"
+       if [ "${PITFT}" = "1" ]; then
+           install -d ${D}/${sysconfdir}/X11/xorg.conf.d/
+           install -m 0644 ${UNPACKDIR}/xorg.conf.d/98-pitft.conf ${D}/${sysconfdir}/X11/xorg.conf.d/
+           install -m 0644 ${UNPACKDIR}/xorg.conf.d/99-calibration.conf ${D}/${sysconfdir}/X11/xorg.conf.d/
+       fi
+   }
+
+   FILES:${PN}:append:rpi = " ${sysconfdir}/X11/xorg.conf.d/*"
+
+Building off of the previous example, we once again are setting the
+:term:`FILESEXTRAPATHS` variable.  In this case we are also using
+:term:`SRC_URI` to list additional source files to use when ``rpi`` is found in
+the list of :term:`OVERRIDES`.  The :ref:`ref-tasks-install` task will then perform a
+check for an additional :term:`MACHINE_FEATURES` that if set will cause these
+additional files to be installed.  These additional files are listed in
+:term:`FILES` so that they will be packaged.
+
+Prioritizing Your Layer
+=======================
+
+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 takes precedence. Priority values also affect the
+order in which multiple ``.bbappend`` 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.
+
+To specify the layer's priority manually, use the
+:term:`BBFILE_PRIORITY`
+variable and append the layer's root name::
+
+   BBFILE_PRIORITY_mylayer = "1"
+
+.. note::
+
+   It is possible for a recipe with a lower version number
+   :term:`PV` in a layer that has a higher
+   priority to take precedence.
+
+   Also, the layer priority does not currently affect the precedence
+   order of ``.conf`` or ``.bbclass`` files. Future versions of BitBake
+   might address this.
+
+Providing Global-level Configurations With Your Layer
+-----------------------------------------------------
+
+When creating a layer, you may need to define configurations that should take
+effect globally in your build environment when the layer is part of the build.
+The ``layer.conf`` file is a :term:`configuration file` that affects the build
+system globally, so it is a candidate for this use-case.
+
+.. warning::
+
+   Providing unconditional global level configuration from the ``layer.conf``
+   file is *not* a good practice, and should be avoided. For this reason, the
+   section :ref:`ref-conditional-layer-confs` below shows how the ``layer.conf``
+   file can be used to provide configurations only if a certain condition is
+   met.
+
+For example, if your layer provides a Linux kernel recipe named
+``linux-custom``, you may want to make :term:`PREFERRED_PROVIDER_virtual/kernel
+<PREFERRED_PROVIDER>` point to ``linux-custom``::
+
+   PREFERRED_PROVIDER_virtual/kernel = "linux-custom"
+
+This can be defined in the ``layer.conf`` file. If your layer is at the last
+position in the :term:`BBLAYERS` list, it will take precedence over previous
+``PREFERRED_PROVIDER_virtual/kernel`` assignments (unless one is set from a
+:term:`configuration file` that is parsed later, such as machine or distro
+configuration files).
+
+.. _ref-conditional-layer-confs:
+
+Conditionally Provide Global-level Configurations With Your Layer
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In some cases, your layer may provide global configurations only if some
+features it provides are enabled. Since the ``layer.conf`` file is parsed at an
+earlier stage in the parsing process, the :term:`DISTRO_FEATURES` and
+:term:`MACHINE_FEATURES` variables are not yet available to ``layer.conf``, and
+declaring conditional assignments based on these variables is not possible. The
+following technique shows a way to bypass this limitation by using the
+:term:`USER_CLASSES` variable and a conditional ``require`` command.
+
+In the following steps, let's assume our layer is named ``meta-mylayer`` and
+that this layer defines a custom :ref:`distro feature <ref-features-distro>`
+named ``mylayer-kernel``. We will set the :term:`PREFERRED_PROVIDER` variable
+for the kernel only if our feature ``mylayer-kernel`` is part of the
+:term:`DISTRO_FEATURES`:
+
+#. Create an include file in the directory
+   ``meta-mylayer/conf/distro/include/``, for example a file named
+   ``mylayer-kernel-provider.inc`` that sets the kernel provider to
+   ``linux-custom``::
+
+      PREFERRED_PROVIDER_virtual/kernel = "linux-custom"
+
+#. Provide a path to this include file in your ``layer.conf``::
+
+      META_MYLAYER_KERNEL_PROVIDER_PATH = "${LAYERDIR}/conf/distro/include/mylayer-kernel-provider.inc"
+
+#. Create a new class in ``meta-mylayer/classes-global/``, for example a class
+   ``meta-mylayer-cfg.bbclass``. Make it conditionally require the file
+   ``mylayer-kernel-provider.inc`` defined above, using the variable
+   ``META_MYLAYER_KERNEL_PROVIDER_PATH`` defined in ``layer.conf``::
+
+      require ${@bb.utils.contains('DISTRO_FEATURES', 'mylayer-kernel', '${META_MYLAYER_KERNEL_PROVIDER_PATH}', '', d)}
+
+   For details on the ``bb.utils.contains`` function, see its definition in
+   :bitbake_git:`lib/bb/utils.py </tree/lib/bb/utils.py>`.
+
+   .. note::
+
+      The ``require`` command is designed to not fail if the function
+      ``bb.utils.contains`` returns an empty string.
+
+#. Back to your ``layer.conf`` file, add the class ``meta-mylayer-cfg`` class to
+   the :term:`USER_CLASSES` variable::
+
+      USER_CLASSES:append = " meta-mylayer-cfg"
+
+   This will add the class ``meta-mylayer-cfg`` to the list of classes to
+   globally inherit. Since the ``require`` command is conditional in
+   ``meta-mylayer-cfg.bbclass``, even though inherited the class will have no
+   effect unless the feature ``mylayer-kernel`` is enabled through
+   :term:`DISTRO_FEATURES`.
+
+This technique can also be used for :ref:`Machine features
+<ref-features-machine>` by following the same steps. Though not mandatory, it is
+recommended to put include files for :term:`DISTRO_FEATURES` in your layer's
+``conf/distro/include`` and the ones for :term:`MACHINE_FEATURES` in your
+layer's ``conf/machine/include``.
+
+Managing Layers
+===============
+
+You can use the BitBake layer management tool ``bitbake-layers`` to
+provide a view into the structure of recipes across a multi-layer
+project. Being able to generate output that reports on configured layers
+with their paths and priorities and on ``.bbappend`` files and their
+applicable recipes can help to reveal potential problems.
+
+For help on the BitBake layer management tool, use the following
+command::
+
+   $ bitbake-layers --help
+
+The following list describes the available commands:
+
+-  ``help:`` Displays general help or help on a specified command.
+
+-  ``show-layers:`` Shows the current configured layers.
+
+-  ``show-overlayed:`` Lists overlayed recipes. A recipe is overlayed
+   when a recipe with the same name exists in another layer that has a
+   higher layer priority.
+
+-  ``show-recipes:`` Lists available recipes and the layers that
+   provide them.
+
+-  ``show-appends:`` Lists ``.bbappend`` files and the recipe files to
+   which they apply.
+
+-  ``show-cross-depends:`` Lists dependency relationships between
+   recipes that cross layer boundaries.
+
+-  ``add-layer:`` Adds a layer to ``bblayers.conf``.
+
+-  ``remove-layer:`` Removes a layer from ``bblayers.conf``
+
+-  ``flatten:`` Flattens the layer configuration into a separate
+   output directory. Flattening your layer configuration builds a
+   "flattened" directory that contains the contents of all layers, with
+   any overlayed recipes removed and any ``.bbappend`` files appended to
+   the corresponding recipes. You might have to perform some manual
+   cleanup of the flattened layer as follows:
+
+   -  Non-recipe files (such as patches) are overwritten. The flatten
+      command shows a warning for these files.
+
+   -  Anything beyond the normal layer setup has been added to the
+      ``layer.conf`` file. Only the lowest priority layer's
+      ``layer.conf`` is used.
+
+   -  Overridden and appended items from ``.bbappend`` files need to be
+      cleaned up. The contents of each ``.bbappend`` end up in the
+      flattened recipe. However, if there are appended or changed
+      variable values, you need to tidy these up yourself. Consider the
+      following example. Here, the ``bitbake-layers`` command adds the
+      line ``#### bbappended ...`` so that you know where the following
+      lines originate::
+
+         ...
+         DESCRIPTION = "A useful utility"
+         ...
+         EXTRA_OECONF = "--enable-something"
+         ...
+
+         #### bbappended from meta-anotherlayer ####
+
+         DESCRIPTION = "Customized utility"
+         EXTRA_OECONF += "--enable-somethingelse"
+
+
+      Ideally, you would tidy up these utilities as follows::
+
+         ...
+         DESCRIPTION = "Customized utility"
+         ...
+         EXTRA_OECONF = "--enable-something --enable-somethingelse"
+         ...
+
+-  ``layerindex-fetch``: Fetches a layer from a layer index, along
+   with its dependent layers, and adds the layers to the
+   ``conf/bblayers.conf`` file.
+
+-  ``layerindex-show-depends``: Finds layer dependencies from the
+   layer index.
+
+-  ``save-build-conf``: Saves the currently active build configuration
+   (``conf/local.conf``, ``conf/bblayers.conf``) as a template into a layer.
+   This template can later be used for setting up builds via :term:`TEMPLATECONF`.
+   For information about saving and using configuration templates, see
+   ":ref:`dev-manual/custom-template-configuration-directory:creating a custom template configuration directory`".
+
+-  ``create-layer``: Creates a basic layer.
+
+-  ``create-layers-setup``: Writes out a configuration file and/or a script that
+   can replicate the directory structure and revisions of the layers in a current build.
+   For more information, see ":ref:`dev-manual/layers:saving and restoring the layers setup`".
+
+Creating a General Layer Using the ``bitbake-layers`` Script
+============================================================
+
+The ``bitbake-layers`` script with the ``create-layer`` subcommand
+simplifies creating a new general layer.
+
+.. note::
+
+   -  For information on BSP layers, see the ":ref:`bsp-guide/bsp:bsp layers`"
+      section in the Yocto
+      Project Board Specific (BSP) Developer's Guide.
+
+   -  In order to use a layer with the OpenEmbedded build system, you
+      need to add the layer to your ``bblayers.conf`` configuration
+      file. See the ":ref:`dev-manual/layers:adding a layer using the \`\`bitbake-layers\`\` script`"
+      section for more information.
+
+The default mode of the script's operation with this subcommand is to
+create a layer with the following:
+
+-  A layer priority of 6.
+
+-  A ``conf`` subdirectory that contains a ``layer.conf`` file.
+
+-  A ``recipes-example`` subdirectory that contains a further
+   subdirectory named ``example``, which contains an ``example.bb``
+   recipe file.
+
+-  A ``COPYING.MIT``, which is the license statement for the layer. The
+   script assumes you want to use the MIT license, which is typical for
+   most layers, for the contents of the layer itself.
+
+-  A ``README`` file, which is a file describing the contents of your
+   new layer.
+
+In its simplest form, you can use the following command form to create a
+layer. The command creates a layer whose name corresponds to
+"your_layer_name" in the current directory::
+
+   $ bitbake-layers create-layer your_layer_name
+
+As an example, the following command creates a layer named ``meta-scottrif``
+in your home directory::
+
+   $ cd /usr/home
+   $ bitbake-layers create-layer meta-scottrif
+   NOTE: Starting bitbake server...
+   Add your new layer with 'bitbake-layers add-layer meta-scottrif'
+
+If you want to set the priority of the layer to other than the default
+value of "6", you can either use the ``--priority`` option or you
+can edit the
+:term:`BBFILE_PRIORITY` value
+in the ``conf/layer.conf`` after the script creates it. Furthermore, if
+you want to give the example recipe file some name other than the
+default, you can use the ``--example-recipe-name`` option.
+
+The easiest way to see how the ``bitbake-layers create-layer`` command
+works is to experiment with the script. You can also read the usage
+information by entering the following::
+
+   $ bitbake-layers create-layer --help
+   NOTE: Starting bitbake server...
+   usage: bitbake-layers create-layer [-h] [--priority PRIORITY]
+                                      [--example-recipe-name EXAMPLERECIPE]
+                                      layerdir
+
+   Create a basic layer
+
+   positional arguments:
+     layerdir              Layer directory to create
+
+   optional arguments:
+     -h, --help            show this help message and exit
+     --priority PRIORITY, -p PRIORITY
+                           Layer directory to create
+     --example-recipe-name EXAMPLERECIPE, -e EXAMPLERECIPE
+                           Filename of the example recipe
+
+Adding a Layer Using the ``bitbake-layers`` Script
+==================================================
+
+Once you create your general layer, you must add it to your
+``bblayers.conf`` file. Adding the layer to this configuration file
+makes the OpenEmbedded build system aware of your layer so that it can
+search it for metadata.
+
+Add your layer by using the ``bitbake-layers add-layer`` command::
+
+   $ bitbake-layers add-layer your_layer_name
+
+Here is an example that adds a
+layer named ``meta-scottrif`` to the configuration file. Following the
+command that adds the layer is another ``bitbake-layers`` command that
+shows the layers that are in your ``bblayers.conf`` file::
+
+   $ bitbake-layers add-layer meta-scottrif
+   NOTE: Starting bitbake server...
+   Parsing recipes: 100% |##########################################################| Time: 0:00:49
+   Parsing of 1441 .bb files complete (0 cached, 1441 parsed). 2055 targets, 56 skipped, 0 masked, 0 errors.
+   $ bitbake-layers show-layers
+   NOTE: Starting bitbake server...
+   layer                 path                                      priority
+   ==========================================================================
+   meta                  /home/scottrif/poky/meta                  5
+   meta-poky             /home/scottrif/poky/meta-poky             5
+   meta-yocto-bsp        /home/scottrif/poky/meta-yocto-bsp        5
+   workspace             /home/scottrif/poky/build/workspace       99
+   meta-scottrif         /home/scottrif/poky/build/meta-scottrif   6
+
+
+Adding the layer to this file
+enables the build system to locate the layer during the build.
+
+.. note::
+
+   During a build, the OpenEmbedded build system looks in the layers
+   from the top of the list down to the bottom in that order.
+
+Saving and restoring the layers setup
+=====================================
+
+Once you have a working build with the correct set of layers, it is beneficial
+to capture the layer setup --- what they are, which repositories they come from
+and which SCM revisions they're at --- into a configuration file, so that this
+setup can be easily replicated later, perhaps on a different machine. Here's
+how to do this::
+
+   $ bitbake-layers create-layers-setup /srv/work/alex/meta-alex/
+   NOTE: Starting bitbake server...
+   NOTE: Created /srv/work/alex/meta-alex/setup-layers.json
+   NOTE: Created /srv/work/alex/meta-alex/setup-layers
+
+The tool needs a single argument which tells where to place the output, consisting
+of a json formatted layer configuration, and a ``setup-layers`` script that can use that configuration
+to restore the layers in a different location, or on a different host machine. The argument
+can point to a custom layer (which is then deemed a "bootstrap" layer that needs to be
+checked out first), or into a completely independent location.
+
+The replication of the layers is performed by running the ``setup-layers`` script provided
+above:
+
+#. Clone the bootstrap layer or some other repository to obtain
+   the json config and the setup script that can use it.
+
+#. Run the script directly with no options::
+
+      alex@Zen2:/srv/work/alex/my-build$ meta-alex/setup-layers
+      Note: not checking out source meta-alex, use --force-bootstraplayer-checkout to override.
+
+      Setting up source meta-intel, revision 15.0-hardknott-3.3-310-g0a96edae, branch master
+      Running 'git init -q /srv/work/alex/my-build/meta-intel'
+      Running 'git remote remove origin > /dev/null 2>&1; git remote add origin git://git.yoctoproject.org/meta-intel' in /srv/work/alex/my-build/meta-intel
+      Running 'git fetch -q origin || true' in /srv/work/alex/my-build/meta-intel
+      Running 'git checkout -q 0a96edae609a3f48befac36af82cf1eed6786b4a' in /srv/work/alex/my-build/meta-intel
+
+      Setting up source poky, revision 4.1_M1-372-g55483d28f2, branch akanavin/setup-layers
+      Running 'git init -q /srv/work/alex/my-build/poky'
+      Running 'git remote remove origin > /dev/null 2>&1; git remote add origin git://git.yoctoproject.org/poky' in /srv/work/alex/my-build/poky
+      Running 'git fetch -q origin || true' in /srv/work/alex/my-build/poky
+      Running 'git remote remove poky-contrib > /dev/null 2>&1; git remote add poky-contrib ssh://git@push.yoctoproject.org/poky-contrib' in /srv/work/alex/my-build/poky
+      Running 'git fetch -q poky-contrib || true' in /srv/work/alex/my-build/poky
+      Running 'git checkout -q 11db0390b02acac1324e0f827beb0e2e3d0d1d63' in /srv/work/alex/my-build/poky
+
+.. note::
+   This will work to update an existing checkout as well.
+
+.. note::
+   The script is self-sufficient and requires only python3
+   and git on the build machine.
+
+.. note::
+   Both the ``create-layers-setup`` and the ``setup-layers`` provided several additional options
+   that customize their behavior - you are welcome to study them via ``--help`` command line parameter.
+