Some devices take undesired actions when RTS and DTR are

asserted. Some development boards for example will reset on DTR,
and some radio interfaces will transmit on RTS.

This patch allows "stty -f /dev/ttyu9.init -rtsdtr" to prevent
RTS and DTR from being asserted on open(), allowing these devices
to be used without problems.

Reviewed by:    imp
Differential Revision:  https://reviews.freebsd.org/D20031

bin/stty/modes.c

@@ -91,6 +91,8 @@ static const struct modes cmodes[] = {
 	{ "-rtsflow",	0, CRTS_IFLOW },
 	{ "mdmbuf",	MDMBUF, 0 },
 	{ "-mdmbuf",	0, MDMBUF },
+	{ "rtsdtr",	0, CNO_RTSDTR },
+	{ "-rtsdtr",	CNO_RTSDTR, 0 },
 	{ NULL,		0, 0 },
 };
 

bin/stty/print.c

@@ -184,6 +184,12 @@ print(struct termios *tp, struct winsize *wp, int ldisc, enum FMT fmt)
 	put("-dsrflow", CDSR_OFLOW, 0);
 	put("-dtrflow", CDTR_IFLOW, 0);
 	put("-mdmbuf", MDMBUF, 0);	/* XXX mdmbuf ==  dtrflow */
+	if (on(CNO_RTSDTR))
+		bput("-rtsdtr");
+	else {
+		if (fmt >= BSD)
+			bput("rtsdtr");
+	}
 
 	/* special control characters */
 	cc = tp->c_cc;

bin/stty/stty.1

@@ -145,6 +145,8 @@ Assume a line without (with) modem
 control.
 .It Cm crtscts Pq Fl crtscts
 Enable (disable) RTS/CTS flow control.
+.It Cm rtsdtr Pq Fl -rtsdtr
+Enable (disable) asserting RTS/DTR on open.
 .El
 .Ss Input Modes:
 This corresponds to the c_iflag in the termios structure.

share/man/man4/termios.4

@@ -1185,6 +1185,8 @@ flow control of output */
 /* RTS flow control of input */
 .It Dv MDMBUF
 /* flow control output via Carrier */
+.It Dv CNO_RTSDTR
+/* Do not assert RTS or DTR automatically */
 .El
 .Pp
 The
@@ -1267,6 +1269,12 @@ If
 is set then output flow control is controlled by the state
 of Carrier Detect.
 .Pp
+If
+.Dv CNO_RTSDTR
+is set then the RTS and DTR lines will not be asserted when the device
+is opened.
+As a result, this flag is only useful on initial-state devices.
+.Pp
 If the object for which the control modes are set is not an asynchronous
 serial connection, some of the modes may be ignored; for example, if an
 attempt is made to set the baud rate on a network connection to a

sys/dev/uart/uart_tty.c

@@ -285,13 +285,16 @@ uart_tty_param(struct tty *tp, struct termios *t)
 		parity = UART_PARITY_NONE;
 	if (UART_PARAM(sc, t->c_ospeed, databits, stopbits, parity) != 0)
 		return (EINVAL);
-	UART_SETSIG(sc, SER_DDTR | SER_DTR);
+	if ((t->c_cflag & CNO_RTSDTR) == 0)
+		UART_SETSIG(sc, SER_DDTR | SER_DTR);
 	/* Set input flow control state. */
 	if (!sc->sc_hwiflow) {
 		if ((t->c_cflag & CRTS_IFLOW) && sc->sc_isquelch)
 			UART_SETSIG(sc, SER_DRTS);
-		else
+		else {
-			UART_SETSIG(sc, SER_DRTS | SER_RTS);
+			if ((t->c_cflag & CNO_RTSDTR) == 0)
+				UART_SETSIG(sc, SER_DRTS | SER_RTS);
+		}
 	} else
 		UART_IOCTL(sc, UART_IOCTL_IFLOW, (t->c_cflag & CRTS_IFLOW));
 	/* Set output flow control state. */

sys/dev/usb/serial/umcs.c

@@ -499,7 +499,9 @@ umcs7840_cfg_open(struct ucom_softc *ucom)
 	 * Enable DTR/RTS on modem control, enable modem interrupts --
 	 * documented
 	 */
-	sc->sc_ports[pn].sc_mcr = MCS7840_UART_MCR_DTR | MCS7840_UART_MCR_RTS | MCS7840_UART_MCR_IE;
+	sc->sc_ports[pn].sc_mcr = MCS7840_UART_MCR_IE;
+	if (ucom->sc_tty == NULL || (ucom->sc_tty->t_termios.c_cflag & CNO_RTSDTR) == 0)
+		sc->sc_ports[pn].sc_mcr |= MCS7840_UART_MCR_DTR | MCS7840_UART_MCR_RTS;
 	if (umcs7840_set_UART_reg_sync(sc, pn, MCS7840_UART_REG_MCR, sc->sc_ports[pn].sc_mcr))
 		return;
 

sys/dev/usb/serial/usb_serial.c

@@ -796,7 +796,8 @@ ucom_open(struct tty *tp)
 	    &sc->sc_start_task[0].hdr, 
 	    &sc->sc_start_task[1].hdr);
 
-	ucom_modem(tp, SER_DTR | SER_RTS, 0);
+	if (sc->sc_tty == NULL || (sc->sc_tty->t_termios.c_cflag & CNO_RTSDTR) == 0)
+		ucom_modem(tp, SER_DTR | SER_RTS, 0);
 
 	ucom_ring(sc, 0);
 

sys/kern/tty.c

@@ -93,7 +93,7 @@ static const char	*dev_console_filename;
 			FLUSHO|NOKERNINFO|NOFLSH)
 #define TTYSUP_CFLAG	(CIGNORE|CSIZE|CSTOPB|CREAD|PARENB|PARODD|\
 			HUPCL|CLOCAL|CCTS_OFLOW|CRTS_IFLOW|CDTR_IFLOW|\
-			CDSR_OFLOW|CCAR_OFLOW)
+			CDSR_OFLOW|CCAR_OFLOW|CNO_RTSDTR)
 
 #define	TTY_CALLOUT(tp,d) (dev2unit(d) & TTYUNIT_CALLOUT)
 
@@ -332,7 +332,8 @@ ttydev_open(struct cdev *dev, int oflags, int devtype __unused,
 		if (TTY_CALLOUT(tp, dev) || dev == dev_console)
 			tp->t_termios.c_cflag |= CLOCAL;
 
-		ttydevsw_modem(tp, SER_DTR|SER_RTS, 0);
+		if ((tp->t_termios.c_cflag & CNO_RTSDTR) == 0)
+			ttydevsw_modem(tp, SER_DTR|SER_RTS, 0);
 
 		error = ttydevsw_open(tp);
 		if (error != 0)

sys/sys/_termios.h

@@ -143,6 +143,7 @@
 #define	CDTR_IFLOW	0x00040000	/* DTR flow control of input */
 #define	CDSR_OFLOW	0x00080000	/* DSR flow control of output */
 #define	CCAR_OFLOW	0x00100000	/* DCD flow control of output */
+#define	CNO_RTSDTR	0x00200000	/* Do not assert RTS or DTR automatically */
 #endif