Um, before you folks go off and implement something weird, let me
introduce some reality here:

1) ATA-4 IS NOT A STANDARD.  There is a DRAFT (available from the
   usual location on fission.dt.wdc.com in PDF format) but it is NOT a
   standard.

2) There are no hard disks which do not support CHS translation.  So
   for the moment, ignore anything related to such disks.

3) Some of the features you mention (e.g. all of wdp_cur*) do not
   exist before ATA-2 or ATA-3, and therefore cannot just be used on
   all devices.

4) There is no standard -- or even particularly reliable -- way to
   detect what ATA revision a device supports.  And please don't start
   throwing suggestions at me; I asked on the ATA mailing list and
   that was the answer I got.  (I did get a suggestion for a klugy
   heuristic, though, which I can probably dig up.)

5) Regarding the code I `turned off': it caused a divide-by-0 fault
   when accessing a *SCSI* disk with no MBR (which I believe I
   documented).

The reason the wd driver always uses LBA mode when available is
precisely to avoid the strange CHS semantics with large disks.