But it's called from scsi_add_lun()->scsi_alloc_sdev() before the type
is known.  The type is -1 all the time in scsi_initialise_merge_fn()
and scsi always bounces.

This patch makes it do the right thing - just enable block-highmem for
all scsi devices.

Jens had this to say:

"I guess that block-highmem has been around long enough, that I can
 use the term 'historically' at least in the kernel sense :-)

 This extra check was added for IDE because each device type driver
 (ide-disk, ide-cd, etc) needed to be updated to not assume virtual
 mappings of request data was valid.  I only did that for ide-disk,
 since this is the only one where bounce buffering really hurt
 performance wise.  So while ide-cd and ide-tape etc could have been
 updated, I deemed it uninteresting and not worthwhile.

 Now, this was just carried straight into the scsi counter parts,
 conveniently, because of laziness.  A quick glance at sr shows that it
 too can aviod bouncing easily (no changes needed).  st may need some
 changes, though.  So again, for scsi it was a matter of not impacting
 existing code in 2.4 too much.

 So TYPE_DISK check can be killed in 2.5 if someone does the work of
 checking that it is safe.  I'm not so sure it will make eg your SCSI
 CD-ROM that much faster :-)"