This represents the kind of device driver that uses SPI messages to interact with the hardware at the other end of a SPI link. It's called a "protocol" driver because it works through messages rather than talking directly to SPI hardware (which is what the underlying SPI controller driver does to pass those messages). These protocols are defined in the specification for the device(s) supported by the driver.
As a rule, those device protocols represent the lowest level interface supported by a driver, and it will support upper level interfaces too. Examples of such upper levels include frameworks like MTD, networking, MMC, RTC, filesystem character device nodes, and hardware monitoring.
|int(*||probe )(struct spi_device *spi)|
|int(*||remove )(struct spi_device *spi)|
|int(*||resume )(struct spi_device *spi)|
|void(*||shutdown )(struct spi_device *spi)|
|int(*||suspend )(struct spi_device *spi, pm_message_t mesg)|