5 Yocto Project Compatible

5.1 Introduction

After the introduction of layers to OpenEmbedded, it quickly became clear that while some layers were popular and worked well, others developed a reputation for being “problematic”. Those were layers which didn’t interoperate well with others and tended to assume they controlled all the aspects of the final output. This usually isn’t intentional but happens because such layers are often created by developers with a particular focus (e.g. a company’s BSP) whilst the end users have a different one (e.g. integrating that BSP into a product).

As a result of noticing such patterns and friction between layers, the project developed the “Yocto Project Compatible” badge program, allowing layers following the best known practises to be marked as being widely compatible with other ones. This takes the form of a set of “yes/no” binary answer questions where layers can declare if they meet the appropriate criteria. In the second version of the program, a script was added to make validation easier and clearer, the script is called yocto-check-layer and is available in OpenEmbedded-Core (OE-Core).

See Making Sure Your Layer is Compatible With Yocto Project for details.

5.2 Benefits

The Yocto Project Layer Model is powerful and flexible: it gives users the ultimate power to change pretty much any aspect of the system but as with most things, power comes with responsibility. The Yocto Project would like to see people able to mix and match BSPs with distro configs or software stacks and be able to merge succesfully. Over time, the project identified characteristics in layers that allow them to operate well together. “anti-patterns” were also found, preventing layers from working well together.

The intent of the compatibility program is simple: if the layer passes the compatibility tests, it is considered “well behaved” and should operate and cooperate well with other compatible layers.

The benefits of compatibility can be seen from multiple different user and member perspectives. From a hardware perspective (a BSP Layer), compatibility means the hardware can be used in many different products and use cases without impacting the software stacks being run with it. For a company developing a product, compatibility gives you a specification / standard you can require in a contract and then know it will have certain desired characteristics for interoperability. It also puts constraints on how invasive the code bases are into the rest of the system, meaning that multiple different separate hardware support layers can coexist (e.g. for multiple product lines from different hardware manufacturers). This can also make it easier for one or more parties to upgrade those system components for security purposes during the lifecycle of a product.

5.3 Validating a layer

The badges are available to members of the Yocto Project (as member benefit) and to open source projects run on a non-commercial basis. However, anyone can answer the questions and run the script.

The project encourages all layer maintainers to review the questions and the output from the script against their layer, as the way some layers are constructed often has unintended consequences. The questions and the script are designed to highlight known issues which are often easy to solve. This makes layers easier to use and therefore more popular.

It is intended that over time, the tests will evolve as new best known practices are identified, and as new interoperability issues are found, unnecessarily restricting layer interoperability. If anyone becomes aware of either type, please let the project know through the technical calls, the mailing lists or through the Technical Steering Committee (TSC). The TSC is responsible for the technical criteria used by the program.

Layers are divided into three types:

  • “BSP” or “hardware support” layers contain support for particular pieces of hardware. This includes kernel and boot loader configuration, and any recipes for firmware or kernel modules needed for the hardware. Such layers usually correspond to a MACHINE setting.

  • “distro” layers defined as layers providing configuration options and settings such as the choice of init system, compiler and optimisation options, and configuration and choices of software components. This would usually correspond to a DISTRO setting.

  • “software” layers are usually recipes. A layer might target a particular graphical UI or software stack component.

Here are key best practices the program tries to encourage:

  • A layer should clearly show who maintains it, and who change submissions and bug reports should be sent to.

  • Where multiple types of functionality are present, the layer should be internally divided into sublayers to separate these components. That’s because some users may only need one of them and separability is a key best practice.

  • Adding a layer to a build should not modify that build, unless the user changes a configuration setting to activate the layer, by selecting a MACHINE, a DISTRO or a DISTRO_FEATURES setting.

The project does test the compatibility status of the core project layers on its Autobuilder.

The official form to submit compatibility requests with is at https://www.yoctoproject.org/ecosystem/branding/compatible-registration/. Applicants can display the badge they get when their application is successful.