USB: serial: pl2303: fix tranceiver suspend mode

Add helper function to update register bits instead of overwriting the
entire control register when updating the flow-control settings.

This specifically avoids having the tranceiver suspend mode (bit 0)
depend on the flow control setting.

The tranceiver is currently configured at probe to be disabled during
suspend, but this was overridden when disabling flow control or enabling
xon/xoff.

Fixes: 715f9527 ("USB: flow control fix for pl2303")
Fixes: 7041d9c3 ("USB: serial: pl2303: add support for tx xon/xoff flow control")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/pl2303.c

@@ -145,6 +145,8 @@ MODULE_DEVICE_TABLE(usb, id_table);
 #define UART_OVERRUN_ERROR		0x40
 #define UART_CTS			0x80
 
+#define PL2303_FLOWCTRL_MASK		0xf0
+
 static void pl2303_set_break(struct usb_serial_port *port, bool enable);
 
 enum pl2303_type {
@@ -225,6 +227,29 @@ static int pl2303_vendor_write(struct usb_serial *serial, u16 value, u16 index)
 	return 0;
 }
 
+static int pl2303_update_reg(struct usb_serial *serial, u8 reg, u8 mask, u8 val)
+{
+	int ret = 0;
+	u8 *buf;
+
+	buf = kmalloc(1, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	ret = pl2303_vendor_read(serial, reg | 0x80, buf);
+	if (ret)
+		goto out_free;
+
+	*buf &= ~mask;
+	*buf |= val & mask;
+
+	ret = pl2303_vendor_write(serial, reg, *buf);
+out_free:
+	kfree(buf);
+
+	return ret;
+}
+
 static int pl2303_probe(struct usb_serial *serial,
 					const struct usb_device_id *id)
 {
@@ -694,13 +719,13 @@ static void pl2303_set_termios(struct tty_struct *tty,
 
 	if (C_CRTSCTS(tty)) {
 		if (spriv->quirks & PL2303_QUIRK_LEGACY)
-			pl2303_vendor_write(serial, 0x0, 0x41);
+			pl2303_update_reg(serial, 0, PL2303_FLOWCTRL_MASK, 0x40);
 		else
-			pl2303_vendor_write(serial, 0x0, 0x61);
+			pl2303_update_reg(serial, 0, PL2303_FLOWCTRL_MASK, 0x60);
 	} else if (pl2303_enable_xonxoff(tty, spriv->type)) {
-		pl2303_vendor_write(serial, 0x0, 0xc0);
+		pl2303_update_reg(serial, 0, PL2303_FLOWCTRL_MASK, 0xc0);
 	} else {
-		pl2303_vendor_write(serial, 0x0, 0x0);
+		pl2303_update_reg(serial, 0, PL2303_FLOWCTRL_MASK, 0);
 	}
 
 	kfree(buf);