At the end of each data phase, the residual is always
   correct, so you update the residual as the number of sg segments left and the
   number of bytes left to transfer in the current segment at that time.  [...]

What you describe sounds more or less equivalent to deferring the implied
RESTORE POINTERS until a new data phase is started.  While this may work,
it's not clear that it's any more compliant with the SCSI standard.

It seems like the only `correct' solution is to rely on having correct
residue counts returned from the device.  This only happens for a limited
subset of device types, but if that's what the standard requires...