bitbake: doc: bitbake-user-manual: AUTOREV and SRCREV_FORMAT roles in fetching

Describe the roles of AUTOREV and SRCREV_FORMAT in the fetching step.
Add their missing descriptions to the variable reference section.

Fixes [YOCTO #14498]

(Bitbake rev: fbf0a8c5711f56bd64ae4c9d2c1d5ba884508c9f)

Signed-off-by: Adam Blank <adam.blank.g@gmail.com>
Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-fetching.rst

@@ -173,6 +173,11 @@ govern the behavior of the unpack stage:
 -  *subdir:* Unpacks the specific URL to the specified subdirectory
    within the root directory.
 
+-  *name:* Assigns a name to a given component of the :term:`SRC_URI`.
+   This component is later referenced by this name when specifying its
+   :term:`SRCREV` or :term:`SRC_URI` checksum, or to correctly place its
+   revision in the package version string with aid of :term:`SRCREV_FORMAT`.
+
 The unpack call automatically decompresses and extracts files with ".Z",
 ".z", ".gz", ".xz", ".zip", ".jar", ".ipk", ".rpm". ".srpm", ".deb" and
 ".bz2" extensions as well as various combinations of tarball extensions.
@@ -453,7 +458,7 @@ This fetcher supports the following parameters:
    By default, the path is ``git/``.
 
 -  *"usehead":* Enables local ``git://`` URLs to use the current branch
-   HEAD as the revision for use with ``AUTOREV``. The "usehead"
+   HEAD as the revision for use with :term:`AUTOREV`. The "usehead"
    parameter implies no branch and only works when the transfer protocol
    is ``file://``.
 
@@ -843,4 +848,42 @@ submodules. However, you might find the code helpful and readable.
 Auto Revisions
 ==============
 
-We need to document ``AUTOREV`` and :term:`SRCREV_FORMAT` here.
+For recipes which need to use the latest revision of their source code,
+the way to achieve it is to use :term:`AUTOREV` as the value of the
+source code repository's :term:`SRCREV`::
+
+   SRCREV = "${AUTOREV}"
+
+.. note::
+
+   With :term:`AUTOREV`, BitBake will always need to take the additional step of
+   querying the remote repository to retrieve the latest available revision.
+
+   Also, recipes using it are not part of the parsing-time cache,
+   and hence are parsed every time.
+
+Multiple Source Control Repositories
+====================================
+
+For some recipes it is necessary to make use of more than one
+version controlled source code repository. In such case, the recipe
+must provide BitBake with information about how it should include
+the different SCM revisions in its package version string, instead of its
+usual approach with a single :term:`SRCREV`.
+
+For this purpose, the recipe must set the :term:`SRCREV_FORMAT`
+variable. Consider the following example::
+
+   SRC_URI = " \
+       git://git.some.example.com/source-tree.git;name=machine \
+       git://git.some.example.com/metadata.git;name=meta \
+       "
+   SRCREV_machine = "3f9db490a81eeb0077be3c5a5aa1388a2372232f"
+   SRCREV_meta = "1ac1d0ff730fe1dd1371823d562db8126750a98c"
+   SRCREV_FORMAT ?= "meta_machine"
+
+The value given to :term:`SRCREV_FORMAT` references names, which were
+assigned using the ``name`` parameter in the :term:`SRC_URI` definition,
+and which represent the version controlled source code repositories.
+In the above example, the :term:`SRC_URI` contained two URLs named
+"meta" and "machine".

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

@@ -36,6 +36,14 @@ overview of their function and contents.
       when specified allows for the Git binary from the host to be used
       rather than building ``git-native``.
 
+   :term:`AUTOREV`
+      This is a special variable used during fetching. When :term:`SRCREV` is
+      set to the value of this variable, the latest revision from the version
+      controlled source code repository is used.
+      It should be set as follows::
+
+         SRCREV = "${AUTOREV}"
+
    :term:`AZ_SAS`
       Azure Storage Shared Access Signature, when using the
       :ref:`Azure Storage fetcher <bitbake-user-manual/bitbake-user-manual-fetching:fetchers>`
@@ -1622,7 +1630,8 @@ overview of their function and contents.
 
       -  ``name``: Specifies a name to be used for association with
          :term:`SRC_URI` checksums or :term:`SRCREV` when you have more than one
-         file or git repository specified in :term:`SRC_URI`. For example::
+         file or source control repository specified in :term:`SRC_URI`.
+         For example::
 
             SRC_URI = "git://example.com/foo.git;branch=main;name=first \
                        git://example.com/bar.git;branch=main;name=second \
@@ -1657,11 +1666,11 @@ overview of their function and contents.
       identifier and not just a tag.
 
    :term:`SRCREV_FORMAT`
-      Helps construct valid :term:`SRCREV` values when
+      Helps construct a valid package version string when
       multiple source controlled URLs are used in
       :term:`SRC_URI`.
 
-      The system needs help constructing these values under these
+      The system needs help constructing this value under these
       circumstances. Each component in the :term:`SRC_URI` is assigned a name
       and these are referenced in the :term:`SRCREV_FORMAT` variable. Consider
       an example with URLs named "machine" and "meta". In this case,