ref-manual: document new toolchain classes and variables

Document the new classes under classes/toolchain as well as
PREFERRED_TOOLCHAIN* and TOOLCHAIN variables, which allow selecting the
toolchain. For now there's "gcc" and "clang" as available toolchain.

(From yocto-docs rev: 69a9568f8bd7ed57efddab507a4294ad3408f4dd)

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

2 files changed, 158 insertions, 0 deletions

diff --git a/documentation/ref-manual/classes.rst b/documentation/ref-manual/classes.rst
index 4705ca3f4d..ff41c84a73 100644
--- a/documentation/ref-manual/classes.rst
+++ b/documentation/ref-manual/classes.rst
@@ -3443,6 +3443,86 @@ This class is not intended to be used directly.
 The :ref:`ref-classes-toolchain-scripts` class provides the scripts used for setting up
 the environment for installed SDKs.
 
+.. _ref-classes-toolchain-clang:
+
+``toolchain/clang``
+===================
+
+The :ref:`ref-classes-toolchain-clang` class defines commands for building
+recipes with Clang/LLVM compiler and utilities.
+
+This class is not meant to be inherited directly. Instead, you should either:
+
+-  set the :term:`PREFERRED_TOOLCHAIN_TARGET`, :term:`PREFERRED_TOOLCHAIN_NATIVE`
+   or :term:`PREFERRED_TOOLCHAIN_SDK` variables to "clang" from a
+   :term:`Configuration File`. This will make the :ref:`ref-classes-base` class
+   use the :ref:`ref-classes-toolchain-clang` accordingly. All recipes will
+   be built with the Clang/LLVM toolchain, exception be made for recipes that
+   override the value of :term:`TOOLCHAIN` or :term:`TOOLCHAIN_NATIVE` to
+   another value.
+
+-  set :term:`TOOLCHAIN` or :term:`TOOLCHAIN_NATIVE` to "clang" from a recipe
+   when the recipe needs to override the default toolchain set by
+   :term:`PREFERRED_TOOLCHAIN_TARGET`, :term:`PREFERRED_TOOLCHAIN_NATIVE` or
+   :term:`PREFERRED_TOOLCHAIN_SDK`.
+
+.. _ref-classes-toolchain-clang-native:
+
+``toolchain/clang-native``
+==========================
+
+The :ref:`ref-classes-toolchain-clang-native` class defines commands for
+building :ref:`ref-classes-native` recipes with Clang/LLVM compiler and
+utilities independently of the build context.
+
+The :ref:`ref-classes-toolchain-gcc-native` class defines :term:`BUILD_CC`,
+:term:`BUILD_CXX` and other such variables which are rarely used in recipes.
+Exception be made for target recipes that need to use the compiler from the
+build host at some point during the build.
+
+This class should not be inherited directly. It is inherited by the
+:ref:`ref-classes-base` class if :term:`TOOLCHAIN_NATIVE` is set to "clang".
+
+.. _ref-classes-toolchain-gcc:
+
+``toolchain/gcc``
+=================
+
+The :ref:`ref-classes-toolchain-gcc` class defines commands for building
+recipes with GCC/Binutils compiler and utilities.
+
+This class is not meant to be inherited directly. Instead, you should either:
+
+-  set the :term:`PREFERRED_TOOLCHAIN_TARGET`, :term:`PREFERRED_TOOLCHAIN_NATIVE`
+   or :term:`PREFERRED_TOOLCHAIN_SDK` variables to "gcc" from a
+   :term:`Configuration File`. This will make the :ref:`ref-classes-base` class
+   use the :ref:`ref-classes-toolchain-gcc` accordingly. All recipes will
+   be built with the GCC/Binutils toolchain, exception be made for recipes that
+   override the value of :term:`TOOLCHAIN` or :term:`TOOLCHAIN_NATIVE` to
+   another value.
+
+-  set :term:`TOOLCHAIN` or :term:`TOOLCHAIN_NATIVE` to "gcc" from a recipe
+   when the recipe needs to override the default toolchain set by
+   :term:`PREFERRED_TOOLCHAIN_TARGET`, :term:`PREFERRED_TOOLCHAIN_NATIVE` or
+   :term:`PREFERRED_TOOLCHAIN_SDK`.
+
+.. _ref-classes-toolchain-gcc-native:
+
+``toolchain/gcc-native``
+========================
+
+The :ref:`ref-classes-toolchain-gcc-native` class defines commands for building
+:ref:`ref-classes-native` recipes with GCC/Binutils compiler and utilities
+independently of the build context.
+
+The :ref:`ref-classes-toolchain-gcc-native` class defines :term:`BUILD_CC`,
+:term:`BUILD_CXX` and other such variables which are rarely used in recipes.
+Exception be made for target recipes that need to use the compiler from the build
+host at some point during the build.
+
+This class should not be inherited directly. It is inherited by the
+:ref:`ref-classes-base` class if :term:`TOOLCHAIN_NATIVE` is set to "gcc".
+
 .. _ref-classes-typecheck:
 
 ``typecheck``
diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 7e4f7e58ad..038e5541ef 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -7325,6 +7325,57 @@ system and gives an overview of their function and contents.
       ":ref:`dev-manual/new-recipe:using virtual providers`" section in the
       Yocto Project Development Tasks Manual.
 
+   :term:`PREFERRED_TOOLCHAIN`
+      The :term:`PREFERRED_TOOLCHAIN` variable selects the toolchain to use for
+      compiling recipes. This variable is not meant to be overridden globally.
+      Instead, the values of :term:`PREFERRED_TOOLCHAIN_TARGET`,
+      :term:`PREFERRED_TOOLCHAIN_NATIVE` and :term:`PREFERRED_TOOLCHAIN_SDK`
+      should be overridden.
+
+   :term:`PREFERRED_TOOLCHAIN_NATIVE`
+      This variable controls the toolchain used for compiling
+      :ref:`ref-classes-native` recipes.
+
+      This variable should be set globally from a :term:`configuration file`.
+
+      See :term:`PREFERRED_TOOLCHAIN_TARGET` for more details on the possible
+      values for this variable.
+
+      A recipe that does not support the toolchain specified by
+      :term:`PREFERRED_TOOLCHAIN_NATIVE` can override it locally with
+      :term:`TOOLCHAIN_NATIVE`.
+
+   :term:`PREFERRED_TOOLCHAIN_SDK`
+      This variable controls the toolchain used for compiling
+      :ref:`ref-classes-nativesdk` recipes.
+
+      This variable should be set globally from a :term:`configuration file`.
+
+      See :term:`PREFERRED_TOOLCHAIN_TARGET` for more details on the possible
+      values for this variable.
+
+   :term:`PREFERRED_TOOLCHAIN_TARGET`
+      This variable controls the toolchain used for compiling recipes in the
+      architecture of the target :term:`MACHINE`.
+
+      There are two possible values for this variable at the moment:
+
+      -  :ref:`gcc <ref-classes-toolchain-gcc>` (default): the GCC/Binutils toolchain.
+      -  :ref:`clang <ref-classes-toolchain-clang>`: the Clang/LLVM toolchain.
+
+      :term:`PREFERRED_TOOLCHAIN_TARGET` will make the :ref:`ref-classes-base`
+      class inherit one of the toolchain classes defined in
+      :oe_git:`meta/classes/toolchain
+      </openembedded-core/tree/meta/classes/toolchain>`. As a consequence, this
+      variable should be set globally from a :term:`configuration file`.
+
+      These classes define commands used for cross-compiling such as :term:`CC`,
+      :term:`CXX`, :term:`LD` and so on.
+
+      A recipe that does not support the toolchain specified by
+      :term:`PREFERRED_TOOLCHAIN_TARGET` can override it locally with
+      :term:`TOOLCHAIN`.
+
    :term:`PREFERRED_VERSION`
       If there are multiple versions of a recipe available, this variable
       determines which version should be given preference. You must always
@@ -10125,6 +10176,21 @@ system and gives an overview of their function and contents.
       implementations, NFS does not meet this minimum requirement.
       Consequently, :term:`TMPDIR` cannot be on NFS.
 
+   :term:`TOOLCHAIN`
+      The :term:`TOOLCHAIN` variable can be used to override the toolchain used
+      by a recipe.
+
+      The default value for this variable is the value of
+      :term:`PREFERRED_TOOLCHAIN`. See the description of
+      :term:`PREFERRED_TOOLCHAIN` to know the list of possible values for
+      :term:`TOOLCHAIN`.
+
+      It is possible to override the value of this variable from a recipe if
+      this recipe is known to support only a specific toolchain. For example,
+      the :oe_git:`Pseudo </openembedded-core/tree/meta/recipes-devtools/pseudo/pseudo_git.bb>`
+      recipe overrides this variable to "gcc", because Pseudo uses GCC compiler
+      built-ins options that the Clang/LLVM compiler does not provide.
+
    :term:`TOOLCHAIN_HOST_TASK`
       This variable lists packages the OpenEmbedded build system uses when
       building an SDK, which contains a cross-development environment. The
@@ -10180,6 +10246,18 @@ system and gives an overview of their function and contents.
       :term:`SDK_VERSION` variables for additional
       information.
 
+   :term:`TOOLCHAIN_NATIVE`
+      The :term:`TOOLCHAIN_NATIVE` variable can be used to override the
+      toolchain used by a :ref:`ref-classes-native` recipe.
+
+      The default value for this variable is the value of
+      :term:`PREFERRED_TOOLCHAIN` (in :ref:`ref-classes-native` contexts). See
+      the description of :term:`PREFERRED_TOOLCHAIN` to know the list of
+      possible values for :term:`TOOLCHAIN_NATIVE`.
+
+      It is possible to override the value of this variable from a recipe if
+      this recipe is known to support only a specific toolchain.
+
    :term:`TOOLCHAIN_TARGET_TASK`
       This variable lists packages the OpenEmbedded build system uses when
       it creates the target part of an SDK (i.e. the part built for the