| Sometimes we may need to force a device to operate in a specific mode. For example, a fast SCSI may be set to work as a "normal" SCSIalthough we need to remember that the device must support the capability that we are trying to enable or disable. For example, we cannot enable fast SCSI on a "normal" SCSI device. A number of different SCSI options are available and are shown in Table 17.7. These can be set to affect the device functions: Table 17.7. SCSI OptionsType | Meaning |
|---|
0x1 | Debug (target driver) | 0x2 | Debug (library) | 0x4 | Debug (host adaptor) | 0x8 | Disconnect/reconnect | 0x10 | Linked commands | 0x20 | Synchronous transfer capability | 0x40 | Parity support | 0x80 | Tagged command support | 0x100 | Fast SCSI support | 0x200 | Wide SCSI support |
For example, to support fast SCSI, tagged commands, global parity, synchronous transfer, linked commands, and global disconnect/reconnect (i.e., everything except wide SCSI and the debug options), we would apply the following values: scsi_options=0x1f8 If we wanted this to affect every device on the machine, we would add the entry to the /etc/system file as follows: helium# grep scsi_options /etc/system set scsi_options=0x1f8 helium# If, on the other hand, we only had a single type of bus that this should affect, we would add the value to the card's configuration file. For example, if the SCSI card used the FAS driver named /kernel/drv/fas, then we would add the following entry to fas.conf: helium# grep scsi_options /kernel/drv/fas.conf scsi_options=0x1f8; helium# We could also isolate a particular device on that bus by applying further constraints if we wish. Creating the correct constraints can become quite complicated, but the various ways of applying them can be found in the manual pages for the relevant driver. | 
