In article <199803271938.OAA14149@ginger.cmf.nrl.navy.mil> you write:
> AFAIK, the com driver (which I believe is the only supported UART
> that has a hardware FIFO) already does this for lower baud rates;
> anything 1200 baud or lower, it sets the fifo trigger to 1.

The zs chips have a hardware FIFO, but the zs driver (at least - dunno
about other drivers we may have for the zilog chips) always uses a trigger
depth of 1 (which is all that's supported on the 8530).  Some machines
(alpha?) have the '230 which can interrupt at depth 4 (for a larger 8
byte FIFO).  The zs driver didn't use this feature though, last I
looked. 

There may be other chips that do, but com and zs seem to cover the vast
majority of NetBSD machines...