So I looked closer: uart_get_baud_rate() in turn calls out to
tty_termios_baud_rate() which does some fuzzy matching to the
fix baudrate table and simply "snaps" the baudrate to the closest
match if it is "close enough".

The fuzzing is getting done when re-encoding and because the code
encodes/re-decodes stuff.

I wonder how to fix this ... I'm planning to do what the comment
in tty_ioctl says:

If you support the ancient alt speed hacks then its not quite so neat.

Alan