migration-guides: Add start of 3.4 guide with override migration notes

(From yocto-docs rev: e60019c81e63250feaee4937873a90042074030b)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Reviewed-by: Quentin Schulz <foss@0leil.net>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 76 insertions, 0 deletions

diff --git a/documentation/migration-guides/migration-3.4.rst b/documentation/migration-guides/migration-3.4.rst
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+Release 3.4 (honister)
+======================
+
+This section provides migration information for moving to the Yocto
+Project 3.4 Release (codename "honister") from the prior release.
+
+Override syntax changes
+-----------------------
+
+This release requires changes to the metadata to indicate where overrides are
+being used in variable key names. This is done with the ``:`` character replacing
+the use of ``_`` previously. This means that an entry like::
+
+   SRC_URI_qemux86 = "file://somefile"
+
+becomes::
+
+   SRC_URI:qemux86 = "file://somefile"
+
+since ``qemux86`` is an override. This applies to any use of override syntax so::
+
+   SRC_URI_append = " file://somefile"
+   SRC_URI_append_qemux86 = " file://somefile2"
+   SRC_URI_remove_qemux86-64 = " file://somefile3"
+   SRC_URI_prepend_qemuarm = "file://somefile4 "
+   FILES_${PN}-ptest = "${bindir}/xyz"
+   IMAGE_CMD_tar = "tar"
+   BASE_LIB_tune-cortexa76 = "lib"
+   SRCREV_pn-bash = "abc"
+   BB_TASK_NICE_LEVEL_task-testimage = '0'
+
+becomes::
+
+   SRC_URI:append = " file://somefile"
+   SRC_URI:append:qemux86 = " file://somefile2"
+   SRC_URI:remove:qemux86-64 = " file://somefile3"
+   SRC_URI:prepend:qemuarm = "file://somefile4 "
+   FILES:${PN}-ptest = "${bindir}/xyz"
+   IMAGE_CMD:tar = "tar"
+   BASE_LIB:tune-cortexa76 = "lib"
+   SRCREV:pn-bash = "abc"
+   BB_TASK_NICE_LEVEL:task-testimage = '0'
+
+This also applies to
+:ref:`variable queries to the datastore <bitbake:bitbake-user-manual/bitbake-user-manual-metadata:functions for accessing datastore variables>`,
+for example using ``getVar`` and similar so ``d.getVar("RDEPENDS_${PN}")``
+becomes ``d.getVar("RDEPENDS:${PN}")``.
+
+Whilst some of these are fairly obvious such as :term:`MACHINE` and :term:`DISTRO`
+overrides, some are less obvious, for example the packaging variables such as
+:term:`RDEPENDS`, :term:`FILES` and so on taking package names (e.g. ``${PN}``,
+``${PN}-ptest``) as overrides. These overrides are not always in
+:term:`OVERRIDES` but applied conditionally in specific contexts
+such as packaging. ``task-<taskname>`` is another context specific override, the
+context being specific tasks in that case. Tune overrides are another special
+case where some code does use them as overrides but some does not. We plan to try
+and make the tune code use overrides more consistently in the future.
+
+To help with migration of layers there is a script in OE-Core. Once configured
+with the overrides used by a layer, this can be run as::
+
+   <oe-core>/scripts/contrib/convert-overrides.py <layerdir>
+
+.. note::
+
+   Please read the notes in the script as it isn't entirely automatic and it isn't
+   expected to handle every case. In particular, it needs to be told which overrides
+   the layer uses (usually machine and distro names/overrides) and the result should
+   be carefully checked since it can be a little enthusiastic and will convert
+   references to ``_append``, ``_remove`` and ``_prepend`` in function and variables names.
+
+For reference, this conversion is important as it allows BitBake to know what is
+an override and what is not. This should allow us to proceed with other syntax
+improvements and simplifications for usability. It also means bitbake no longer
+has to guess and maintain large lookup lists just in case ``functionname`` in
+``my_functionname`` is an override and this should improve efficiency.