The HPS code uses a HAL (Hardware Abstraction Layer) interface to access the HPS hardware device. The dev.h interface defines the HAL API. This HAL library implements the various access methods that are used.
The i2c implementation uses a i2c device to connect to the hardware module.
The ftdi implementation uses the libftdi1 library to remotely interface a minimal I2C connection via FTDI USB interface devices such as FT4232H.
The FakeDev class implements an internal s/w simulator of the HPS hardware for testing and development.
The uart implementation uses a serial protocol to communicate with a remote device.
The retry class is a shim proxy layer that allows calls to be retried upon error.