Philippe Gerum authored Mar 26, 2019 and Jan Kiszka committed Apr 01, 2019

We may receive TRDY when the TX-FIFO is not entirely empty, which
breaks the logic in rt_imx_uart_tx_chars(), as we may break early from
the transmit loop when the TXFULL condition is detected, skipping
decrement of ctx->out_npend although a byte was sent.

Because of this, we may end up with ghost bytes in the output buffer
which were actually sent to the UART, but still erroneously counted in
ctx->out_npend. As a result, incorrect output can be observed later on
as bytes from the software FIFO are unduly sent, or such FIFO may
overflow indefinitely due to accumulated errors.

Fix this by testing TXFULL before sending the next byte to the UART.
At this chance, drop the count limit on the FIFO depth: testing TXFULL
is sufficient and robust.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>