When the device for which you write the driver takes a seat on a physical bus called the bus controller, it must rely on the driver of that bus called the controller driver, responsible for sharing bus access between devices. The controller driver offers an abstraction layer between your device and the bus. Whenever you perform a transaction (read or write) on an I2C or USB bus, for example, the I2C/USB bus controller transparently takes care of that in the background. Every bus controller driver exports a set of functions to simplify the development of drivers for devices sitting on that bus. This works for every physical bus (I2C, SPI, USB, PCI, SDIO, and so on).

An I2C driver is represented in the kernel as an instance of struct i2c_driver. The I2C client (which represents the device itself) is represented by a struct i2c_client structure.