ref-manual: document "reproducible_build" class and SOURCE_DATE_EPOCH

(From yocto-docs rev: ab6d3dbf57cac560b9b142cf5becf11d3edf09b7)

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

documentation/migration-guides/migration-3.1.rst

@@ -55,8 +55,9 @@ Reproducible builds now enabled by default
 
 In order to avoid unnecessary differences in output files (aiding binary
 reproducibility), the Poky distribution configuration
-(``DISTRO = "poky"``) now inherits the ``reproducible_build`` class by
+(``DISTRO = "poky"``) now inherits the
-default.
+:ref:`reproducible-build <ref-classes-reproducible-build>` class
+by default.
 
 .. _migration-3.1-ptest-feature-impact:
 

documentation/ref-manual/classes.rst

@@ -2209,6 +2209,18 @@ commit, and log. From the information, report files using a JSON format
 are created and stored in
 ``${``\ :term:`LOG_DIR`\ ``}/error-report``.
 
+.. _ref-classes-reproducible-build:
+
+``reproducible_build.bbclass``
+==============================
+
+The ``reproducible_build.bbclass`` class enables
+:ref:`test-manual/reproducible-builds:reproducible builds` by computing
+a :term:`SOURCE_DATE_EPOCH` value in each component's build environment, so
+that the build is independent from the time when the component was built.
+
+Poky inherits this class by default since version 3.1.
+
 .. _ref-classes-rm-work:
 
 ``rm_work.bbclass``

documentation/ref-manual/variables.rst

@@ -6938,6 +6938,23 @@ system and gives an overview of their function and contents.
       You will see this variable referenced in the default values of
       ``FILES:${PN}-dev``.
 
+   :term:`SOURCE_DATE_EPOCH`
+      This defines a date expressed in number of seconds since
+      the UNIX EPOCH (01 Jan 1970 00:00:00 UTC), which is used by
+      multiple build systems to force a timestamp in built binaries.
+      Many upstream projects already support this variable.
+
+      You will find more details in the `official specifications
+      <https://reproducible-builds.org/specs/source-date-epoch/>`__.
+
+      A value for each recipe is computed from the sources by
+      the :ref:`reproducible-build <ref-classes-reproducible-build>` class.
+
+      If a recipe wishes to override the default behavior, it should set its
+      own :term:`SOURCE_DATE_EPOCH` value::
+
+          SOURCE_DATE_EPOCH = "1613559011"
+
    :term:`SOURCE_MIRROR_FETCH`
       When you are fetching files to create a mirror of sources (i.e.
       creating a source mirror), setting :term:`SOURCE_MIRROR_FETCH` to "1" in