have been in the code all along, but were masked by having a fifo depth of
one byte at the hardware level, so everything kinda worked by accident.
The hardware interrupts when the TX fifo is half empty, so set
sc->sc_txfifosz to 8 bytes (half the hardware fifo size) to match. This
eliminates dropped characters on output.
Restructure the read loop to consume all the bytes in the fifo by using
the "rx fifo empty" bit of the flags register rather than the "rx ready"
bit of the interrupt status register. The rx-ready interrupt is cleared
when the number of bytes in the fifo fall below the interrupt trigger
level, leaving the fifo half full every time receive routine was called.
Now it loops until the fifo is completely empty every time (including
when the function is called due to a receive timeout as well as for
fifo-full).
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