USB: serial: xr: add TIOCGRS485 and TIOCSRS485 ioctls

Exar devices like XR21B1411 can control an RS485 transceiver by
automatically asserting the RTS#/RS485 pin before sending data
and deasserting it when the last stop bit has been transmitted.
The polarity of the RST#/RS485 signal is configurable and the
hardware also supports half-duplex turn-around delay and
address matching mode.

Add support for enabling and disabling RS-485 mode and
configuring the RST#/RS485 signal polarity using the TIOCGRS485
and TIOCSRS485 ioctls. Support for half-duplex turn-around delay
and address matching mode are left unimplemented for now.

User enables RS-485 mode by setting SER_RS485_ENABLED flag in
struct serial_rs485 flags. User should also set either
SER_RS485_RTS_ON_SEND or SER_RS485_RTS_AFTER_SEND to select the
behaviour of the RTS#/RS485 pin. Setting SER_RS485_RTS_ON_SEND
will drive RTS#/RS485 low during transmission.

Signed-off-by: Jarkko Sonninen <kasper@iki.fi>
[ johan: let SER_RS485_RTS_ON_SEND determine SER_RS485_RTS_AFTER_SEND ]
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/xr_serial.c

@@ -93,6 +93,7 @@ struct xr_txrx_clk_mask {
 #define XR_GPIO_MODE_SEL_DTR_DSR	0x2
 #define XR_GPIO_MODE_SEL_RS485		0x3
 #define XR_GPIO_MODE_SEL_RS485_ADDR	0x4
+#define XR_GPIO_MODE_RS485_TX_H		0x8
 #define XR_GPIO_MODE_TX_TOGGLE		0x100
 #define XR_GPIO_MODE_RX_TOGGLE		0x200
 
@@ -237,6 +238,7 @@ static const struct xr_type xr_types[] = {
 struct xr_data {
 	const struct xr_type *type;
 	u8 channel;			/* zero-based index or interface number */
+	struct serial_rs485 rs485;
 };
 
 static int xr_set_reg(struct usb_serial_port *port, u8 channel, u16 reg, u16 val)
@@ -629,6 +631,7 @@ static void xr_set_flow_mode(struct tty_struct *tty,
 	struct xr_data *data = usb_get_serial_port_data(port);
 	const struct xr_type *type = data->type;
 	u16 flow, gpio_mode;
+	bool rs485_enabled;
 	int ret;
 
 	ret = xr_get_reg_uart(port, type->gpio_mode, &gpio_mode);
@@ -645,7 +648,17 @@ static void xr_set_flow_mode(struct tty_struct *tty,
 	/* Set GPIO mode for controlling the pins manually by default. */
 	gpio_mode &= ~XR_GPIO_MODE_SEL_MASK;
 
-	if (C_CRTSCTS(tty) && C_BAUD(tty) != B0) {
+	rs485_enabled = !!(data->rs485.flags & SER_RS485_ENABLED);
+	if (rs485_enabled) {
+		dev_dbg(&port->dev, "Enabling RS-485\n");
+		gpio_mode |= XR_GPIO_MODE_SEL_RS485;
+		if (data->rs485.flags & SER_RS485_RTS_ON_SEND)
+			gpio_mode &= ~XR_GPIO_MODE_RS485_TX_H;
+		else
+			gpio_mode |= XR_GPIO_MODE_RS485_TX_H;
+	}
+
+	if (C_CRTSCTS(tty) && C_BAUD(tty) != B0 && !rs485_enabled) {
 		dev_dbg(&port->dev, "Enabling hardware flow ctrl\n");
 		gpio_mode |= XR_GPIO_MODE_SEL_RTS_CTS;
 		flow = XR_UART_FLOW_MODE_HW;
@@ -809,6 +822,79 @@ static void xr_cdc_set_line_coding(struct tty_struct *tty,
 	kfree(lc);
 }
 
+static void xr_sanitize_serial_rs485(struct serial_rs485 *rs485)
+{
+	if (!(rs485->flags & SER_RS485_ENABLED)) {
+		memset(rs485, 0, sizeof(*rs485));
+		return;
+	}
+
+	/* RTS always toggles after TX */
+	if (rs485->flags & SER_RS485_RTS_ON_SEND)
+		rs485->flags &= ~SER_RS485_RTS_AFTER_SEND;
+	else
+		rs485->flags |= SER_RS485_RTS_AFTER_SEND;
+
+	/* Only the flags are implemented at the moment */
+	rs485->flags &= SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND |
+			SER_RS485_RTS_AFTER_SEND;
+	rs485->delay_rts_before_send = 0;
+	rs485->delay_rts_after_send = 0;
+	memset(rs485->padding, 0, sizeof(rs485->padding));
+}
+
+static int xr_get_rs485_config(struct tty_struct *tty,
+			       struct serial_rs485 __user *argp)
+{
+	struct usb_serial_port *port = tty->driver_data;
+	struct xr_data *data = usb_get_serial_port_data(port);
+
+	down_read(&tty->termios_rwsem);
+	if (copy_to_user(argp, &data->rs485, sizeof(data->rs485))) {
+		up_read(&tty->termios_rwsem);
+		return -EFAULT;
+	}
+	up_read(&tty->termios_rwsem);
+
+	return 0;
+}
+
+static int xr_set_rs485_config(struct tty_struct *tty,
+			       struct serial_rs485 __user *argp)
+{
+	struct usb_serial_port *port = tty->driver_data;
+	struct xr_data *data = usb_get_serial_port_data(port);
+	struct serial_rs485 rs485;
+
+	if (copy_from_user(&rs485, argp, sizeof(rs485)))
+		return -EFAULT;
+	xr_sanitize_serial_rs485(&rs485);
+
+	down_write(&tty->termios_rwsem);
+	data->rs485 = rs485;
+	xr_set_flow_mode(tty, port, NULL);
+	up_write(&tty->termios_rwsem);
+
+	if (copy_to_user(argp, &rs485, sizeof(rs485)))
+		return -EFAULT;
+
+	return 0;
+}
+
+static int xr_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
+{
+	void __user *argp = (void __user *)arg;
+
+	switch (cmd) {
+	case TIOCGRS485:
+		return xr_get_rs485_config(tty, argp);
+	case TIOCSRS485:
+		return xr_set_rs485_config(tty, argp);
+	}
+
+	return -ENOIOCTLCMD;
+}
+
 static void xr_set_termios(struct tty_struct *tty,
 			   struct usb_serial_port *port,
 			   const struct ktermios *old_termios)
@@ -1010,6 +1096,7 @@ static struct usb_serial_driver xr_device = {
 	.set_termios		= xr_set_termios,
 	.tiocmget		= xr_tiocmget,
 	.tiocmset		= xr_tiocmset,
+	.ioctl			= xr_ioctl,
 	.dtr_rts		= xr_dtr_rts
 };