As described in SCSI Target Drivers, the DDI/DKI divides the software interface to SCSI devices into two major parts:
Target devices and drivers
Host bus adapter devices and drivers
Target device refers to a device on a SCSI bus, such as a disk or a tape drive. Target driver refers to a software component installed as a device driver. Each target device on a SCSI bus is controlled by one instance of the target driver.
Host bus adapter device refers to HBA hardware, such as a PCI SCSI adapter card. Host bus adapter driver refers to a software component that is installed as a device driver. Some examples are the esp driver on a SPARC machine, the ncrs driver on an x86 machine, and the isp driver, which works on both architectures. An instance of the HBA driver controls each of its host bus adapter devices that are configured in the system.
The Sun Common SCSI Architecture (SCSA) defines the interface between the target and HBA components.
The host bus adapter driver is responsible for performing the following tasks:
Managing host bus adapter hardware
Accepting SCSI commands from the SCSI target driver
Transporting the commands to the specified SCSI target device
Performing any data transfers that the command requires
Collecting status
Handling auto-request sense (optional)
Informing the target driver of command completion or failure