it empties all of the 256 byte incoming fifo, as it can spend more time
processing one port than intended, especially if data is streaming in
at 115.2K. The port fifo will be emptied and dumped into the tty system
and left until next time. I've been running this for quite some time on
one of my systems here.
Also, if the tty layer is blocked or full it lets the hardware assert the
flow control rather than loosing the data.
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