bitbake: doc: Add section for variable context

This is inspired by the same section in the yocto-docs. It aims to provide
information in what contexts(recipes, .conf, bbclass,...) a variable is usually
used. For that I tried to group similar variables, so that a short overview is
given. This was inspired by [YOCTO #14072], but of course does not implement a
warning if a variable is used in an unintended context.

(Bitbake rev: 5ced476685376b1a32b7fdd9546f9b61c5962aa0)

Signed-off-by: Simone Weiß <simone.p.weiss@posteo.com>
Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables-context.rst

@@ -0,0 +1,91 @@
+.. SPDX-License-Identifier: CC-BY-2.5
+
+================
+Variable Context
+================
+
+|
+
+Variables might only have an impact or can be used in certain contexts. Some
+should only be used in global files like ``.conf``, while others are intended only
+for local files like ``.bb``. This chapter aims to describe some important variable
+contexts.
+
+.. _ref-varcontext-configuration:
+
+BitBake's own configuration
+===========================
+
+Variables starting with ``BB_`` usually configure the behaviour of BitBake itself.
+For example, one could configure:
+
+- System resources, like disk space to be used (:term:`BB_DISKMON_DIRS`),
+  or the number of tasks to be run in parallel by BitBake (:term:`BB_NUMBER_THREADS`).
+
+- How the fetchers shall behave, e.g., :term:`BB_FETCH_PREMIRRORONLY` is used
+  by BitBake to determine if BitBake's fetcher shall search only
+  :term:`PREMIRRORS` for files.
+
+Those variables are usually configured globally.
+
+BitBake configuration
+=====================
+
+There are variables:
+
+- Like :term:`B` or :term:`T`, that are used to specify directories used by
+  BitBake during the build of a particular recipe. Those variables are
+  specified in ``bitbake.conf``. Some, like :term:`B`, are quite often
+  overwritten in recipes.
+
+- Starting with ``FAKEROOT``, to configure how the ``fakeroot`` command is
+  handled. Those are usually set by ``bitbake.conf`` and might get adapted in a
+  ``bbclass``.
+
+- Detailing where BitBake will store and fetch information from, for
+  data reuse between build runs like :term:`CACHE`, :term:`DL_DIR` or
+  :term:`PERSISTENT_DIR`. Those are usually global.
+
+
+Layers and files
+================
+
+Variables starting with ``LAYER`` configure how BitBake handles layers.
+Additionally, variables starting with ``BB`` configure how layers and files are
+handled. For example:
+
+- :term:`LAYERDEPENDS` is used to configure on which layers a given layer
+  depends.
+
+- The configured layers are contained in :term:`BBLAYERS` and files in
+  :term:`BBFILES`.
+
+Those variables are often used in the files ``layer.conf`` and ``bblayers.conf``.
+
+Recipes and packages
+====================
+
+Variables handling recipes and packages can be split into:
+
+- :term:`PN`, :term:`PV` or :term:`PF` for example, contain information about
+  the name or revision of a recipe or package. Usually, the default set in
+  ``bitbake.conf`` is used, but those are from time to time overwritten in
+  recipes.
+
+- :term:`SUMMARY`, :term:`DESCRIPTION`, :term:`LICENSE` or :term:`HOMEPAGE`
+  contain the expected information and should be set specifically for every
+  recipe.
+
+- In recipes, variables are also used to control build and runtime
+  dependencies between recipes/packages with other recipes/packages. The
+  most common should be: :term:`PROVIDES`, :term:`RPROVIDES`, :term:`DEPENDS`,
+  and :term:`RDEPENDS`.
+
+- There are further variables starting with ``SRC`` that specify the sources in
+  a recipe like :term:`SRC_URI` or :term:`SRCDATE`. Those are also usually set
+  in recipes.
+
+- Which version or provider of a recipe should be given preference when
+  multiple recipes would provide the same item, is controlled by variables
+  starting with ``PREFERRED_``. Those are normally set in the configuration
+  files of a ``MACHINE`` or ``DISTRO``.

bitbake/doc/index.rst

@@ -13,6 +13,7 @@ BitBake User Manual
    bitbake-user-manual/bitbake-user-manual-intro
    bitbake-user-manual/bitbake-user-manual-execution
    bitbake-user-manual/bitbake-user-manual-metadata
+   bitbake-user-manual/bitbake-user-manual-ref-variables-context
    bitbake-user-manual/bitbake-user-manual-fetching
    bitbake-user-manual/bitbake-user-manual-ref-variables
    bitbake-user-manual/bitbake-user-manual-hello