dev-manual/multiconfig: add suggested best practices and baremetal sections

After the suggestions from Mark Hatle on the list
(https://lists.yoctoproject.org/g/docs/topic/110487932), add two
sections to the multiconfig doc:

- Suggested best practices: suggestion for better design of multiconfig
  builds.

- Common use case: baremetal build.

  This section applies the guidelines from the first sections and apply
  it to a real-life example of how to use multiconfig. This one to build
  some baremetal firmware alongside a regular Linux build.

Suggested-by: Mark Hatle <mark.hatle@kernel.crashing.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: 36fb1e9e5099aa0d858d5478530143e9bac39588)

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

documentation/dev-manual/multiconfig.rst

@@ -174,3 +174,139 @@ and have separate configuration files, BitBake places the artifacts for
 each build in the respective temporary build directories (i.e.
 :term:`TMPDIR`).
 
+Suggested best practices
+========================
+
+-  :term:`TMPDIR` (other than the default set in bitbake.conf) is only set in
+   ``local.conf`` by the user. This means that we should **not** manipulate
+   :term:`TMPDIR` in any way within the Machine or Distro :term:`configuration
+   file`.
+
+-  A multiconfig should specify a :term:`TMPDIR`, and should specify it by
+   appending the multiconfig name with :term:`BB_CURRENT_MC`.
+
+-  Recipes that are used to transfer the output from a multiconfig build to
+   another should use ``do_task[mcdepends]`` to trigger the build of the
+   component, and then transfer the item to the current configuration in
+   :ref:`ref-tasks-install` and :ref:`ref-tasks-deploy`, assuming the value of
+   the deployed item based on :term:`TMPDIR`.
+
+   The :ref:`ref-tasks-install` and :ref:`ref-tasks-deploy` tasks should look
+   like this::
+
+      do_install() {
+          install -m 0644 ${TMPDIR}-<multiconfig>/tmp/deploy/images/<machine>/somefile ${D}/some/path
+      }
+
+      do_deploy() {
+          install -m 0644 ${TMPDIR}-<multiconfig>/tmp/deploy/images/<machine>/somefile ${DEPLOYDIR}/somefile
+      }
+
+   In the example above:
+
+   -  ``<multiconfig>`` is the multiconfig name as set by the multiconfig
+      :term:`configuration file` (see the :ref:`dev-manual/multiconfig:Setting
+      Up and Running a Multiple Configuration Build` section above).
+
+   -  ``<machine>`` must be the :term:`MACHINE` for which ``somefile`` was built
+      and deployed. This value may differ from the current :term:`MACHINE` if
+      the multiconfig :term:`configuration file` overrides it.
+
+-  Firmware recipes can set the :term:`INHIBIT_DEFAULT_DEPS` variable to ``1``
+   if they don't rely on default dependencies such as the standard C library.
+
+Common use case: building baremetal firmware alongside a Linux build
+====================================================================
+
+A common use case for multiconfig is to use the default configuration as the
+regular Linux build, while one or more multiconfigs can be used to build special
+components, such as baremetal firmware. It would also apply to a scenario where
+a microcontroller, for example, is companion to a main processor where Linux is
+running. This section details how one can achieve these kinds of scenarios with
+a multiconfig build.
+
+Adding a multiconfig configuration file and recipe for a baremetal firmware
+---------------------------------------------------------------------------
+
+As described in :ref:`dev-manual/multiconfig:Setting Up and Running a Multiple
+Configuration Build`, each multiconfig will require a separate
+:term:`Configuration File`. In addition, we will define a separate
+:term:`TMPDIR` for our baremetal firmware build configuration.
+
+For example, we will define a new ``conf/multiconfig/baremetal-firmware.conf``
+as follows::
+
+   TMPDIR .= "-${BB_CURRENT_MC}"
+   TCLIBC = "newlib"
+
+The ``baremetal-firmware.conf`` file configures a separate :term:`TMPDIR` for
+holding binaries compiled with the `newlib <https://sourceware.org/newlib/>`__
+toolchain (see :term:`TCLIBC`).
+
+.. note::
+
+   Here, the default :term:`MACHINE` is not overridden by the multiconfig
+   configuration file. As a consequence, the architecture of the built baremetal
+   binaries will be the same. In other cases, where the firmware runs on a
+   completely different architecture, the :term:`MACHINE` must be overridden.
+
+We then create a recipe ``my-firmware.bb`` that defines how the baremetal
+firmware is built. The recipe should contain enough information for the
+:term:`OpenEmbedded build system` to properly compile the firmware with our
+toolchain. The building tasks may vary depending on the nature of the firmware.
+However, the recipe should define a :ref:`ref-classes-deploy` task that deploys
+the output into the :term:`DEPLOYDIR` directory. We will consider in the
+following that the file is named ``my-firmware.elf``.
+
+Building the firmware
+---------------------
+
+The firmware can be built with BitBake with the following command::
+
+   $ bitbake mc:baremetal-firmware:my-firmware
+
+However, we would prefer for ``my-firmware`` to be automatically built when
+triggering a normal Linux build.
+
+Using a ``mcdepend``, a recipe belonging to the Linux build can trigger the
+build of ``my-firmware``. For example, let's consider that our Linux build needs
+to assemble a "special" firmware that uses the output of our ``my-firmware``
+recipe - let's call it ``my-parent-firmware.bb``. Then, we should specify this
+dependency in ``my-parent-firmware.bb`` with::
+
+   do_compile[mcdepends] = "mc::baremetal-firmware:my-firmware:do_deploy"
+
+The above will ensure that when the :ref:`ref-tasks-compile` task of
+``my-parent-firmware`` is triggered, the :ref:`ref-tasks-deploy` task of
+``my-firmware`` will already have run successfully.
+
+Using the output of ``my-firmware``
+-----------------------------------
+
+After ``my-firmware`` recipe has deployed ``my-firmware.elf``, we need to use
+the output in some way. We can make a series of assumptions, based on the
+default Yocto Project variables in order to get the binary for packaging.
+
+First, we can set the following in ``my-parent-firmware.bb``::
+
+   FIRMWARE_FILE ??= "${TMPDIR}-baremetal-firmware/deploy/images/<machine>/my-firmware.elf"
+   FIRMWARE_FILE[vardepsexclude] += "TMPDIR"
+
+The first assignment stores the value of the path to the firmware built and
+deployed by the ``my-firmware.bb`` recipe. The second assignment excludes the
+:term:`TMPDIR` variable from being part of ``FIRMWARE_FILE``'s dependencies ---
+meaning that changing the value of :term:`TMPDIR` (for example, changing the
+host on which the firmware is built) will not invalidate the :ref:`shared state
+cache <overview-manual/concepts:shared state cache>`.
+
+Additionally, ``<machine>`` should be replaced by the :term:`MACHINE` for which
+we are building in the baremetal-firmware context.
+
+We can then add a :ref:`ref-tasks-install` task to ``my-parent-firmware``::
+
+   do_install() {
+       install -Dm 0644 ${FIRMWARE_FILE} ${D}/lib/firmware/my-firmware.elf
+   }
+
+Doing the above will allow the firmware binary to be transferred and packaged
+into the Linux context and rootfs.