Other SCSI Design Considerations

There are some other SCSI design considerations for error recovery that are listed in this section.

Responsibilities of the SCSI Device Driver
In the operating system, it is assumed that other SCSI initiators might be active on the SCSI bus. Usually, the SCSI device driver reserves the SCSI device at open time and releases it at close time (except when told to do otherwise through parameters in the SCSI device driver interface).
SCSI Options to the openx Subroutine
SCSI device drivers in the operating system must support eight defined extended options in their open routine (that is, an openx subroutine). Additional extended options to the open are also allowed, but they must not conflict with predefined open options.
Using the SC_FORCED_OPEN Option
The SC_FORCED_OPEN option causes the SCSI device driver to call the SCSI adapter device driver's Bus Device Reset ioctl (SCIORESET) operation on the first open. This forces the device to release another initiator's reservation.
Using the SC_RETAIN_RESERVATION Option
The SC_RETAIN_RESERVATION option causes the SCSI device driver not to issue the SCSI release command during the close of the device. This guarantees a calling program control of the device (using SCSI reservation) through open and close cycles.
Using the SC_DIAGNOSTIC Option
The SC_DIAGNOSTIC option causes the SCSI device driver to enter Diagnostic mode for the given device. This option directs the SCSI device driver to perform only minimal operations to open a logical path to the device. No SCSI commands should be sent to the device in the open or close routine when the device is in Diagnostic mode. One or more ioctl operations should be provided by the SCSI device driver to allow the caller to issue SCSI commands to the attached device for diagnostic purposes.
Using the SC_NO_RESERVE Option
The SC_NO_RESERVE option causes the SCSI device driver not to issue the SCSI reserve command during the opening of the device and not to issue the SCSI release command during the close of the device. This allows multiple hosts to share the device.
Using the SC_SINGLE Option
The SC_SINGLE option causes the SCSI device driver to issue a normal open, but does not allow another caller to issue another open until the first caller has closed the device. This request gives the caller an exclusive open to the selected device. If this openx is attempted when the device is already open, a return value of -1 is passed, with the errno global variable set to a value of EBUSY.
Closing the SCSI Device
When a SCSI device driver is preparing to close a device through the SCSI adapter device driver, it must ensure that all transactions are complete.
SCSI Error Processing
It is the responsibility of the SCSI device driver to process SCSI check conditions and other returned errors properly.
Device Driver and Adapter Device Driver Interfaces
The SCSI device drivers can have both character (raw) and block special files in the /dev directory.
Performing SCSI Dumps
A SCSI adapter device driver must have a dddump entry point if it is used to access a system dump device. A SCSI device driver must have a dddump entry point if it drives a dump device. Examples of dump devices are disks and tapes.

Parent topic: Small Computer System Interface Subsystem (Parallel SCSI)