USB: serial: cp210x: add support for software flow control

When data is transmitted between two serial ports, the phenomenon of
data loss often occurs. The two kinds of flow control commonly used in
serial communication are hardware flow control and software flow
control.

In serial communication, If you only use RX/TX/GND Pins, you can't do
hardware flow. So we often used software flow control and prevent data
loss. The user sets the software flow control through the application
program, and the application program sets the software flow control mode
for the serial port chip through the driver.

For the cp210 serial port chip, its driver lacks the software flow
control setting code, so the user cannot set the software flow control
function through the application program. This adds the missing software
flow control.
Signed-off-by: Wang Sheng Long <shenglong.wang.ext@siemens.com>
Link: https://lore.kernel.org/r/20210104094502.3942-1-china_shenglong@163.com
[ johan: rework properly on top of recent termios changes ]
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -377,6 +377,16 @@ static struct usb_serial_driver * const serial_drivers[] = {
 #define CONTROL_WRITE_DTR	0x0100
 #define CONTROL_WRITE_RTS	0x0200
 
+/* CP210X_(GET|SET)_CHARS */
+struct cp210x_special_chars {
+	u8	bEofChar;
+	u8	bErrorChar;
+	u8	bBreakChar;
+	u8	bEventChar;
+	u8	bXonChar;
+	u8	bXoffChar;
+};
+
 /* CP210X_VENDOR_SPECIFIC values */
 #define CP210X_READ_2NCONFIG	0x000E
 #define CP210X_READ_LATCH	0x00C2
@@ -1074,11 +1084,38 @@ static void cp210x_disable_event_mode(struct usb_serial_port *port)
 	port_priv->event_mode = false;
 }
 
+static int cp210x_set_chars(struct usb_serial_port *port,
+		struct cp210x_special_chars *chars)
+{
+	struct cp210x_port_private *port_priv = usb_get_serial_port_data(port);
+	struct usb_serial *serial = port->serial;
+	void *dmabuf;
+	int result;
+
+	dmabuf = kmemdup(chars, sizeof(*chars), GFP_KERNEL);
+	if (!dmabuf)
+		return -ENOMEM;
+
+	result = usb_control_msg(serial->dev, usb_sndctrlpipe(serial->dev, 0),
+				CP210X_SET_CHARS, REQTYPE_HOST_TO_INTERFACE, 0,
+				port_priv->bInterfaceNumber,
+				dmabuf, sizeof(*chars), USB_CTRL_SET_TIMEOUT);
+
+	kfree(dmabuf);
+
+	if (result < 0) {
+		dev_err(&port->dev, "failed to set special chars: %d\n", result);
+		return result;
+	}
+
+	return 0;
+}
+
 static bool cp210x_termios_change(const struct ktermios *a, const struct ktermios *b)
 {
 	bool iflag_change;
 
-	iflag_change = ((a->c_iflag ^ b->c_iflag) & INPCK);
+	iflag_change = ((a->c_iflag ^ b->c_iflag) & (INPCK | IXON | IXOFF));
 
 	return tty_termios_hw_change(a, b) || iflag_change;
 }
@@ -1086,13 +1123,29 @@ static bool cp210x_termios_change(const struct ktermios *a, const struct ktermio
 static void cp210x_set_flow_control(struct tty_struct *tty,
 		struct usb_serial_port *port, struct ktermios *old_termios)
 {
+	struct cp210x_special_chars chars;
 	struct cp210x_flow_ctl flow_ctl;
 	u32 flow_repl;
 	u32 ctl_hs;
 	int ret;
 
-	if (old_termios && C_CRTSCTS(tty) == (old_termios->c_cflag & CRTSCTS))
+	if (old_termios &&
+			C_CRTSCTS(tty) == (old_termios->c_cflag & CRTSCTS) &&
+			I_IXON(tty) == (old_termios->c_iflag & IXON) &&
+			I_IXOFF(tty) == (old_termios->c_iflag & IXOFF)) {
+		return;
+	}
+
+	if (I_IXON(tty) || I_IXOFF(tty)) {
+		memset(&chars, 0, sizeof(chars));
+
+		chars.bXonChar = START_CHAR(tty);
+		chars.bXoffChar = STOP_CHAR(tty);
+
+		ret = cp210x_set_chars(port, &chars);
+		if (ret)
 			return;
+	}
 
 	ret = cp210x_read_reg_block(port, CP210X_GET_FLOW, &flow_ctl,
 			sizeof(flow_ctl));
@@ -1118,6 +1171,16 @@ static void cp210x_set_flow_control(struct tty_struct *tty,
 		flow_repl |= CP210X_SERIAL_RTS_SHIFT(CP210X_SERIAL_RTS_ACTIVE);
 	}
 
+	if (I_IXOFF(tty))
+		flow_repl |= CP210X_SERIAL_AUTO_RECEIVE;
+	else
+		flow_repl &= ~CP210X_SERIAL_AUTO_RECEIVE;
+
+	if (I_IXON(tty))
+		flow_repl |= CP210X_SERIAL_AUTO_TRANSMIT;
+	else
+		flow_repl &= ~CP210X_SERIAL_AUTO_TRANSMIT;
+
 	dev_dbg(&port->dev, "%s - ulControlHandshake=0x%08x, ulFlowReplace=0x%08x\n",
 			__func__, ctl_hs, flow_repl);