hal_pdir_t(3) Manual Page

An input pin. The owning component can only read the pin. Multiple input pins can be connected to the same signal.

An output pin. The owning component can read and write the pin. Only one (1) output pin can be connected to a signal.

A bidirectional pin. The owning component can read and write the pin. Multiple bidirectional pins can be connected to the same signal. It must be ensured that at least one component will be writing data onto the signal.

A bidirectional parameter. User-space programs can read and write the parameter. The owning component can read and write the parameter. Parameters cannot be connected to signals.

An output parameter. User-space programs can only read the parameter. The component can read and write the parameter. Parameters cannot be connected to signals.