Preferring and providing recipes

The dependency relation between recipes is essential to BitBake and Poky. It is defined inside each recipe file, with a variable that describes on what a recipe depends (DEPENDS) and what a recipe provides to the system (PROVIDES). These two variables together build the dependency graph used by BitBake during dependency resolution.

So, if a recipe called foo_1.0.bb depends on bar, BitBake lists all recipes providing bar. The bar dependency can be satisfied by the following:

• A recipe with the bar_<version>.bb format, because every recipe provides itself
• A recipe where the PROVIDES variable includes bar

A dependency can be satisfied by several recipes (for example, if two or more recipes have PROVIDES += "bar"). In this case, we must inform BitBake which specific provider to use.

The virtual/kernel provider is a clear example where this mechanism is used. The virtual/ namespace is the convention adopted when we have a set of commonly overridden providers.

All...