USB: serial: cp210x: fix CP2102N-A01 modem control

CP2102N revision A01 (firmware version <= 1.0.4) has a buggy
flow-control implementation that uses the ulXonLimit instead of
ulFlowReplace field of the flow-control settings structure (erratum
CP2102N_E104).

A recent change that set the input software flow-control limits
incidentally broke RTS control for these devices when CRTSCTS is not set
as the new limits would always enable hardware flow control.

Fix this by explicitly disabling flow control for the buggy firmware
versions and only updating the input software flow-control limits when
IXOFF is requested. This makes sure that the terminal settings matches
the default zero ulXonLimit (ulFlowReplace) for these devices.

Link: https://lore.kernel.org/r/[e-mail]
Reported-by: David Frey <[e-mail]>
Reported-by: Alex Villacís Lasso <[e-mail]>
Tested-by: Alex Villacís Lasso <[e-mail]>
Fixes: f61309d9c96a ("USB: serial: cp210x: set IXOFF thresholds")
Cc: [e-mail] # 5.12
Signed-off-by: Johan Hovold <[e-mail]>

drivers/usb/serial/cp210x.c

···
 	u8			gpio_input;
 #endif
 	u8			partnum;
+	u32			fw_version;
 	speed_t			min_speed;
 	speed_t			max_speed;
 	bool			use_actual_rate;
+	bool			no_flow_control;
 };
 
 enum cp210x_event_state {
···
 
 /* CP210X_VENDOR_SPECIFIC values */
 #define CP210X_READ_2NCONFIG	0x000E
+#define CP210X_GET_FW_VER_2N	0x0010
 #define CP210X_READ_LATCH	0x00C2
 #define CP210X_GET_PARTNUM	0x370B
 #define CP210X_GET_PORTCONFIG	0x370C
···
 static void cp210x_set_flow_control(struct tty_struct *tty,
 		struct usb_serial_port *port, struct ktermios *old_termios)
 {
+	struct cp210x_serial_private *priv = usb_get_serial_data(port->serial);
 	struct cp210x_port_private *port_priv = usb_get_serial_port_data(port);
 	struct cp210x_special_chars chars;
 	struct cp210x_flow_ctl flow_ctl;
 	u32 flow_repl;
 	u32 ctl_hs;
 	int ret;
+
+	/*
+	 * Some CP2102N interpret ulXonLimit as ulFlowReplace (erratum
+	 * CP2102N_E104). Report back that flow control is not supported.
+	 */
+	if (priv->no_flow_control) {
+		tty->termios.c_cflag &= ~CRTSCTS;
+		tty->termios.c_iflag &= ~(IXON | IXOFF);
+	}
 
 	if (old_termios &&
 			C_CRTSCTS(tty) == (old_termios->c_cflag & CRTSCTS) &&
···
 		port_priv->crtscts = false;
 	}
 
-	if (I_IXOFF(tty))
+	if (I_IXOFF(tty)) {
 		flow_repl |= CP210X_SERIAL_AUTO_RECEIVE;
-	else
+
+		flow_ctl.ulXonLimit = cpu_to_le32(128);
+		flow_ctl.ulXoffLimit = cpu_to_le32(128);
+	} else {
 		flow_repl &= ~CP210X_SERIAL_AUTO_RECEIVE;
+	}
 
 	if (I_IXON(tty))
 		flow_repl |= CP210X_SERIAL_AUTO_TRANSMIT;
 	else
 		flow_repl &= ~CP210X_SERIAL_AUTO_TRANSMIT;
-
-	flow_ctl.ulXonLimit = cpu_to_le32(128);
-	flow_ctl.ulXoffLimit = cpu_to_le32(128);
 
 	dev_dbg(&port->dev, "%s - ctrl = 0x%02x, flow = 0x%02x\n", __func__,
 			ctl_hs, flow_repl);
···
 	priv->use_actual_rate = use_actual_rate;
 }
 
+static int cp210x_get_fw_version(struct usb_serial *serial, u16 value)
+{
+	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
+	u8 ver[3];
+	int ret;
+
+	ret = cp210x_read_vendor_block(serial, REQTYPE_DEVICE_TO_HOST, value,
+			ver, sizeof(ver));
+	if (ret)
+		return ret;
+
+	dev_dbg(&serial->interface->dev, "%s - %d.%d.%d\n", __func__,
+			ver[0], ver[1], ver[2]);
+
+	priv->fw_version = ver[0] << 16 | ver[1] << 8 | ver[2];
+
+	return 0;
+}
+
+static void cp210x_determine_quirks(struct usb_serial *serial)
+{
+	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
+	int ret;
+
+	switch (priv->partnum) {
+	case CP210X_PARTNUM_CP2102N_QFN28:
+	case CP210X_PARTNUM_CP2102N_QFN24:
+	case CP210X_PARTNUM_CP2102N_QFN20:
+		ret = cp210x_get_fw_version(serial, CP210X_GET_FW_VER_2N);
+		if (ret)
+			break;
+		if (priv->fw_version <= 0x10004)
+			priv->no_flow_control = true;
+		break;
+	default:
+		break;
+	}
+}
+
 static int cp210x_attach(struct usb_serial *serial)
 {
 	int result;
···
 
 	usb_set_serial_data(serial, priv);
 
+	cp210x_determine_quirks(serial);
 	cp210x_init_max_speed(serial);
 
 	result = cp210x_gpio_init(serial);