NOS and FIFO UARTs

["Mike Bilow, mikebw@idsvax.ids.com" <mikebw@IDS.JVNC.NET>]

I have identifed a problem with NOS support of FIFO UARTs similar to
the National Semiconductor 16550AFN.  While this chip works, some of
the second source chips do not.

When National released the 16550AN (no "F"), they found that the FIFOs
were broken, so they released a "fixed" chip and marked it 16550AFN.
They added a test bit so that software could tell if it found the old
chip or the new chip and act accordingly.  NOS correctly implements
this and will not enable the FIFO unless the test bit indicating the
new chip is present.

Naturally, this test bit is not implemented by other manufacturers,
since they never made the bad chips!  My system uses a UMC82C550,
which is a CMOS clone of the NS16550AFN -- except for the "fixed"
bit.  I can enable the FIFO manually if I want, and most other
software does not follow the NAN-293 procedure as rigidly as NOS.

Solution: I think it is desirable to continue to refuse to automatically
enable the FIFO without the "fixed" bit, but it is a good idea to allow
an optional parameter on the attach line ("f") that forces the FIFO on
regardless of the "fixed" bit.  This would allow those of us with
second source chips to have a manual override.

-- Mike