Domain Validation is a fairly essential element to the SCSI Parallel
Interface (although if you look very few drivers actually do it).  The
premise is that the Parallel Bus, being a transmission line, might not
be correctly tuned to the transfers you want do perform.  DV probes the
parameters of the transport until it finds a setting that works (for the
interested, see http://www.t10.org/ftp/t10/drafts/sdv/sdv-r08b.pdf)

The current code employs rather simplistic DV heuristics, although those
can be improved over time.  The change in scsi_scan.c is so that DV may
be done easily from the slave_configure routine, which is the most
natural place to begin.