UDA50/RA81 problems....

[Dave Martindale]

Alex White, who worked on the UDA50 driver here, theorized that the
"Get Unit Status" botch was written into the driver at a time
when the UDA50 could queue only 15 outstanding requests, which happens
to be the same as the number of BDP's available on the 780 UBA.
If all of the BDP's are in use, and if you don't use them for anything
other than the UDA50, then the UDA can't handle another request anyway
right now, and sending out the Get Unit Status really just provides you
with a way to get an interrupt at the point that the UDA50 finishes
one of the transfers, coincidentally freeing up a BDP.

This strategy doesn't work if the UDA50 can handle more than 15 requests
(new UDA50's do 22) or if you have some other device using one of the
BDP's.  In this case, you get constant interrupts.