New versions of OpenEmbedded will be based on OpenEmbedded-Core (OE-core). OE-core has evolved from collaboration efforts with the Yocto project as well as a recognition that the model previously being used in OpenEmbedded was unsustainable.
The original OpenEmbedded repository (also known as "OE-classic" or "oe-dev") has grown organically over the years to more than 7500 recipes, covering approximately 300 machines and 20 distros. Trying to maintain this amount of metadata with machine and distro overrides scattered throughout is very difficult, and near to impossible to support commercially. Like many other OE forks that have been created to solve the same kinds of issues, Poky was forked as a cleaner and more supportable version of OE in 2006. Fast forward to 2011, Poky is now maintained by the Yocto Project with the support of the Linux Foundation. OE-core was split out from Poky in 2011 to allow collaboration around a relatively small and easily supportable base, with real machines, distros and other items removed - more on this below.
OE-classic will stop being maintained in the near future - there are no further stable releases planned and new efforts should be built on top of OE-core. The new OE layer ecosystem doesn't yet contain the full wealth of machine and application support that OE classic currently has, however recipes can be easily brought over, tidied up and placed into the appropriate layer over time.
Differences to OE-classic
The most significant change is that metadata is now split into multiple layers rather than being in one repository. OE-core sits at the bottom, and machine / application / distro layers are added on top. Users are free to select support for whatever applications or platforms they wish in their configuration simply by including the appropriate layer in their bblayers.conf file.
Machine and distro-specific overrides should no longer be spread over the entire set of recipes - instead they should be concentrated in appropriate machine support layers (often referred to as BSP (Board Support Package) layers) and distro layers respectively. BitBake's .bbappend feature allows overriding almost any element of a recipe without too much duplication across multiple layers.
Additionally, OE-core moves changes to a pull model rather than the push model of OE-classic - instead of all developers having direct commit access, patches are sent to the mailing list for review and if/when satisfactory they are merged by the maintainer. Other layers may choose to use either the push or pull model for contributions.
The scope of OE-core is currently not as well defined as it could be, however the following initial rules have been established:
- Support for five architectures: ARM, x86, x86-64, PowerPC and MIPS
- Only QEMU emulated machines
- Distro-less (DISTRO = "" and default values are used)
- Only one X-based GUI environment (Sato) intended for testing purposes. (This may be substituted out in future if a suitable replacement testing framework can be created.)
- Attempt to keep only one version of each recipe except for GPLv2 / v3 versions
Anything not in OE-core can easily be provided by another layer; other layers can also be used to support older versions that have been removed from OE-core.
For items shared amongst multiple layers that cannot be included (or immediately included) in OE-core, there is the meta-openembedded layer. This exists in a repository, also called meta-openembedded, which contains a number of other more focused layers (meta-efl, meta-gnome, etc.). This layer will need to be managed carefully over time to avoid it turning into just a newer version of OE-classic.
As OE-core can be used to build working images entirely on its own, you can get started with it immediately. See this page for instructions.
OpenEmbedded maintains an list of layers that can be used with OE-core - see LayerIndex.
The classes and BitBake configuration used in OE-core requires a few changes for recipes brought over from OE-classic (and for new recipes written by developers used to working with OE-classic):
- LIC_FILES_CHKSUM is mandatory, and will be checked at the end of do_configure. It is used in order to specify one or more files or sections of files from the source that can be used to detect if the license of the software has been changed in a newer version.
- Legacy staging (do_stage) is no longer supported and BitBake will error out on parse of recipes that use it. Items previously in do_stage should be moved to do_install as appropriate.
- The LICENSE field, whilst usually included in most recipes in the past anyway, is now mandatory. It should be as specific and as correct as possible (e.g. "GPLv2", not just "GPL") and try to be consistent with other recipes.
- Any calls to oenote, oewarn, oefatal etc. need to be replaced with the "bb" equivalent, i.e. bbnote, bbwarn, bbfatal etc.
- Recent versions of BitBake no longer allow comments in multi-line strings, so the tradition of being able to comment out lines within a multi-line SRC_URI or when setting configure flags etc. can no longer continue. Remove the commented out lines or move them to beyond where the string terminates.
To aid maintainability, the following recommendations are made for creating new layers:
- Avoid "overlaying" entire recipes (i.e. copying the entire recipe into your layer and modifying it). Use .bbappend files to override the parts of the recipe you need to modify.
- Avoid duplicating include files - use .bbappends for each recipe that uses the inc file, or if you're introducing a new recipe that requires it, use the full path to refer to the original (e.g. require recipes-core/somepackage/somefile.inc instead of require somefile.inc). If you're finding you have to overlay the inc file it may indicate a deficiency in the inc file in the layer it originally belongs to, which should be addressed instead. An example here would be the way Qt 4 database support plugins are configured - OE-core doesn't have MySQL or PostgreSQL, however meta-oe does, so meta-oe uses .bbappends to modify a variable QT_SQL_DRIVER_FLAGS to enable the appropriate plugins. This variable was added to qt4.inc in OE-core specifically to allow meta-oe to be able to control which plugins are built.