Devicetree visitors are objects that implement the interfaces in visitor.h. They are responsible to parse the relevant devicetree nodes with respect to a particular devicetree binding and produce either driver metadata and/or driver framework node properties. All |Visitor::Visit| calls are invoked during the devicetree manager |Walk| call.
Visitors for properties/bindings that are mentioned as standard properties in the devicetree specification will be added under the default visitors. Additionally visitors for properties that are generic in Fuchsia platform i.e. properties explicitly mentioned in |fuchsia_hardware_platform_bus::Node| are considered default for Fuchsia and are added to the default visitor set.
Visitors corresponding to driver specific metadata or dependencies are built as shared libraries using the devicetree_visitor GN target. Creating a shared library of the visitor helps to keep the list of visitors dynamic i.e. visitors can be added and removed from the board driver without having to recompile it. This also helps to update and contribute visitors independent of the board driver.
A new visitor will be needed when a new devicetree binding is created. And typically a new devicetree binding is created either because a new metadata was introduced, and/or a composite node needs to be created from the board driver.
fx create devicetree visitor --lang cpp --path <visitor path> can be used as a starting point for writing a new visitor.
All visitors should include a devicetree schema file representing the bindings that it is parsing.
This library provides a constructor that takes in a list of compatible strings and only calls the visitor when the node with the matching compatible string is found.
This library provides a parser object which can be configured to parse all relevant properties for a node. It also reduces some of the complexity involved around parsing phandle references.
Used for combining multiple visitors in to a single object that can be passed to the devicetree manager |Walk| call.
This can be used by the board driver to load shared library visitors packaged with the driver in it's /lib/visitors folder.
A visitor integration test can be created using visitor-test-helper to test the parsing and creation of metadata and bind rules by the visitor. The helper library creates an instance of devicetree manager along with fakes for platform-bus protocols. The DTB containing the test nodes for the visitor is passed during the initialization of the test helper. All of the properties set by the visitor (i.e. metadata and bind rules/properties) are recorded by the test helper and can be used to verify the visitor behavior.