serial: zs: Fix a transmit lockup in console output

Transmit interrupts are disabled and the transmit buffer drained in the
course of console output so that polled transmission is possible.  That
however causes a lost transmit interrupt as the TxIP bit in RR3 is only
set on a transmit buffer full-to-empty transition and then iff transmit
interrupts are enabled at the same time.  Consequently if console output
disturbs a regular transmission in progress, the TxIP bit is never set
again and the transmission locks up waiting for a transmit interrupt.

Fix the problem by restarting transmission manually rather than waiting
for a transmit interrupt that will never happen.
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/zs.c

@@ -1181,6 +1181,10 @@ static void zs_console_write(struct console *co, const char *s,
 	if (txint & TxINT_ENAB) {
 		zport->regs[1] |= TxINT_ENAB;
 		write_zsreg(zport, R1, zport->regs[1]);
+
+		/* Resume any transmission as the TxIP bit won't be set.  */
+		if (!zport->tx_stopped)
+			zs_raw_transmit_chars(zport);
 	}
 	spin_unlock_irqrestore(&scc->zlock, flags);
 }