Introduction to SCSI drivers

host adapter

card that converts computer-independent SCSI communications protocol to computer-specific information that your computer can process.

SCSI bus

cable that connects a host adapter to a series of SCSI controllers and their associated devices.

SCSI device class

peripheral driver type, such as disk drives or tape drives. SCSI provides a computer with device independence within a class of devices. The scsi(HW) manual page lists the device classes that can be added to a host computer without modifying generic system hardware or software.

SCSI target

refers to the embedded controller for the SCSI peripheral device being addressed. The target ID is the address of the embedded controller.

logical unit

the specific entity that is addressed for an I/O operation. The Logical Unit Number (LUN) is encoded in its minor number and is usually 0 except for a bridge controller, which is an embedded controller that controls peripheral devices with their own embedded controllers.

SCSI initiators

device that can initiate an I/O operation. The SCSI implementation defines the host adapter to be the initiator for all I/O operations. The SCSI protocol provides for the connection of multiple initiators; that is, devices that can initiate an operation, and multiple targets. The supported arrangement is single-initiator, multiple-target.

A SCSI periheral driver communicates with the SCSI host adapter server instead of directly with the hardware. The SCSI peripheral driver contains entry-point routines to pass user-level requests to open and close the device or to read or write data. The SCSI peripheral driver populates a request structure with this information, then passes that information to the SCSI host adapter driver. The host adapter driver handles the direct communication with the hardware.

Communication on the SCSI bus is only allowed between two devices at a time: the initiator (defined to be the SCSI host adapter) and the target (defined to be the embedded controller on the SCSI peripheral device).

The following diagram illustrates the SCSI interface:

SCSI interface

Up to seven SCSI-1 or fifteen SCSI-2 embedded SCSI controllers can be on a bus. Each controller can address up to eight physical devices, for a possible total of up to 120 devices. Multiple devices contending for the bus are handled by an arbitration process in which the priority is assigned based on the device's SCSI ID.

Each SCSI host adapter and peripheral driver has a unique major device number. SCSI host adapters and SCSI peripheral drivers are installed into the link kit the same as other drivers. In addition, the peripheral device driver information is put into the mscsi(F) file. When a new kernel is built, the build software uses the mscsi information about SCSI peripheral devices and the information in the sdevice(F) file about SCSI host adapter devices to create configuration tables for each. This is discussed further in ``SCSI configuration tables''.