Index: com.c
===================================================================
RCS file: /cvs/src/sys/dev/ic/com.c,v
retrieving revision 1.169
diff -u -p -r1.169 com.c
--- com.c	14 May 2018 19:25:54 -0000	1.169
+++ com.c	24 Jun 2019 01:08:29 -0000
@@ -925,11 +925,16 @@ comstart(struct tty *tp)
 		n = q_to_b(&tp->t_outq, buffer,
 		    min(sc->sc_fifolen, sizeof buffer));
 		for (i = 0; i < n; i++) {
+			while (!ISSET(com_read_reg(sc, com_lsr), LSR_TXRDY))
+				;
 			com_write_reg(sc, com_data, buffer[i]);
 		}
 		bzero(buffer, n);
-	} else if (tp->t_outq.c_cc != 0)
+	} else if (tp->t_outq.c_cc != 0) {
+		while (!ISSET(com_read_reg(sc, com_lsr), LSR_TXRDY))
+			;
 		com_write_reg(sc, com_data, getc(&tp->t_outq));
+	}
 out:
 	splx(s);
 	return;