If IRIX allowed multiple drivers to operate the host adapter, there would be confusion and errors from the conflicting uses. IRIX puts the management of each host adapter under the control of a host adapter driver, whose job is to issue commands on its bus and report the results. The host adapter is tailored to the hardware of the particular host adapter and to the architecture of the host system.
The interface to the host adapter driver is the same no matter what type of hardware the adapter uses. This insulates the individual device drivers from details of the adapter hardware.
The driver for each type of device is responsible for preparing the SCSI command bytes for its device, for passing the command requests to the correct host adapter driver, and for interpreting sense and status data as it comes back.