ref-manual: Fix PACKAGECONFIG term and add an example

PACKAGECONFIG's first and second flag value will be added to PACKAGECONFIG_CONFARGS
and then it will be added to the appropriate variable (EXTRA_OECMAKE, or ...)
So we need to only mention PACKAGECONFIG_CONFARGS and it will lead to other variables.

I added a custom example that can help understanding very well PACKAGECONFIG.

(From yocto-docs rev: 7f26b0c0a08d6be9810128369265b0c494e7191b)

Signed-off-by: Talel BELHAJSALEM <bhstalel@gmail.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 36 insertions, 6 deletions

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index dbd21826a3..cf69d5a86c 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -6064,13 +6064,11 @@ system and gives an overview of their function and contents.
       omit any argument you like but must retain the separating commas. The
       order is important and specifies the following:
 
-      #. Extra arguments that should be added to the configure script
-         argument list (:term:`EXTRA_OECONF` or
-         :term:`PACKAGECONFIG_CONFARGS`) if
-         the feature is enabled.
+      #. Extra arguments that should be added to :term:`PACKAGECONFIG_CONFARGS`
+         if the feature is enabled.
 
-      #. Extra arguments that should be added to :term:`EXTRA_OECONF` or
-         :term:`PACKAGECONFIG_CONFARGS` if the feature is disabled.
+      #. Extra arguments that should be added to :term:`PACKAGECONFIG_CONFARGS`
+         if the feature is disabled.
 
       #. Additional build dependencies (:term:`DEPENDS`)
          that should be added if the feature is enabled.
@@ -6128,6 +6126,38 @@ system and gives an overview of their function and contents.
 
             PACKAGECONFIG:append:pn-recipename = " f4"
 
+      Consider the following example of a :ref:`ref-classes-cmake` recipe with a systemd service
+      in which :term:`PACKAGECONFIG` is used to transform the systemd service
+      into a feature that can be easily enabled or disabled via :term:`PACKAGECONFIG`::
+
+         example.c
+         example.service
+         CMakeLists.txt
+
+      The ``CMakeLists.txt`` file contains::
+
+         if(WITH_SYSTEMD)
+            install(FILES ${PROJECT_SOURCE_DIR}/example.service DESTINATION /etc/systemd/systemd)
+         endif(WITH_SYSTEMD)
+
+      In order to enable the installation of ``example.service`` we need to
+      ensure that ``-DWITH_SYSTEMD=ON`` is passed to the ``cmake`` command
+      execution.  Recipes that have ``CMakeLists.txt`` generally inherit the
+      :ref:`ref-classes-cmake` class, that runs ``cmake`` with
+      :term:`EXTRA_OECMAKE`, which :term:`PACKAGECONFIG_CONFARGS` will be
+      appended to.  Now, knowing that :term:`PACKAGECONFIG_CONFARGS` is
+      automatically filled with either the first or second element of
+      :term:`PACKAGECONFIG` flag value, the recipe would be like::
+
+         inherit cmake
+         PACKAGECONFIG = "systemd"
+         PACKAGECONFIG[systemd] = "-DWITH_SYSTEMD=ON,-DWITH_SYSTEMD=OFF"
+
+      A side note to this recipe is to check if ``systemd`` is in fact the used :term:`INIT_MANAGER`
+      or not::
+
+         PACKAGECONFIG = "${@'systemd' if d.getVar('INIT_MANAGER') == 'systemd' else ''}"
+
    :term:`PACKAGECONFIG_CONFARGS`
       A space-separated list of configuration options generated from the
       :term:`PACKAGECONFIG` setting.