USB: serial: ch341: add hardware flow control RTS/CTS

This adds support for enabling and disabling RTS/CTS hardware flow
control.

Tested using CH341A and CH340E.

Fixes part of the following bug report:
Link: https://bugzilla.kernel.org/show_bug.cgi?id=197109

Signed-off-by: Lode Willems <[e-mail]>
[ johan: prepare index argument once, drop casts ]
Signed-off-by: Johan Hovold <[e-mail]>

drivers/usb/serial/ch341.c

···
 #define CH341_REG_DIVISOR      0x13
 #define CH341_REG_LCR          0x18
 #define CH341_REG_LCR2         0x25
+#define CH341_REG_FLOW_CTL     0x27
 
 #define CH341_NBREAK_BITS      0x01
 
···
 #define CH341_LCR_CS7          0x02
 #define CH341_LCR_CS6          0x01
 #define CH341_LCR_CS5          0x00
+
+#define CH341_FLOW_CTL_NONE    0x00
+#define CH341_FLOW_CTL_RTSCTS  0x01
 
 #define CH341_QUIRK_LIMITED_PRESCALER	BIT(0)
 #define CH341_QUIRK_SIMULATE_BREAK	BIT(1)
···
 	return r;
 }
 
+static void ch341_set_flow_control(struct tty_struct *tty,
+				   struct usb_serial_port *port,
+				   const struct ktermios *old_termios)
+{
+	u16 flow_ctl;
+	int r;
+
+	if (C_CRTSCTS(tty))
+		flow_ctl = CH341_FLOW_CTL_RTSCTS;
+	else
+		flow_ctl = CH341_FLOW_CTL_NONE;
+
+	r = ch341_control_out(port->serial->dev,
+			      CH341_REQ_WRITE_REG,
+			      (CH341_REG_FLOW_CTL << 8) | CH341_REG_FLOW_CTL,
+			      (flow_ctl << 8) | flow_ctl);
+	if (r < 0 && old_termios) {
+		tty->termios.c_cflag &= ~CRTSCTS;
+		tty->termios.c_cflag |= (old_termios->c_cflag & CRTSCTS);
+	}
+}
+
 /* Old_termios contains the original termios settings and
  * tty->termios contains the new setting to be used.
  */
···
 	spin_unlock_irqrestore(&priv->lock, flags);
 
 	ch341_set_handshake(port->serial->dev, priv->mcr);
+
+	ch341_set_flow_control(tty, port, old_termios);
 }
 
 /*