dev-manual: document how to provide confs from layer.conf

Add a section on providing global level configuration from the
layer.conf file. Since this file is parsed at an earlier stage in the
parsing process, it's not possible to combine bb.utils.contains and
{DISTRO,MACHINE}_FEATURES to conditionally set some configurations.

This patch documents:

- First that this file can be used for providing such configuration.
- Then demonstrate how to conditionally provide them, using a technique
  that is currently used in meta-virtualization
  (https://git.yoctoproject.org/meta-virtualization/tree/conf/layer.conf#n50).

Fixes [YOCTO #12688].

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: cc3fa1b0e51377f4e03eaa1ca60c2f2ee0cd917e)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/layers.rst

@@ -644,6 +644,96 @@ variable and append the layer's root name::
    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
 ===============