USB: serial: ch341: fix baud rate and line-control handling

Revert to using direct register writes to set the divisor and
line-control registers.

A recent change switched to using the init vendor command to update
these registers, something which also enabled support for CH341A
devices. It turns out that simply setting bit 7 in the divisor register
is sufficient to support CH341A and specifically prevent data from being
buffered until a full endpoint-size packet (32 bytes) has been received.

Using the init command also had the side-effect of temporarily
deasserting the DTR/RTS signals on every termios change (including
initialisation on open) something which for example could cause problems
in setups where DTR is used to trigger a reset.

Fixes: 4e46c41 ("USB: serial: ch341: reinitialize chip on
reconfiguration")
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ch341.c

@@ -133,8 +133,8 @@ static int ch341_control_in(struct usb_device *dev,
 	return r;
 }
 
-static int ch341_init_set_baudrate(struct usb_device *dev,
-				   struct ch341_private *priv, unsigned ctrl)
+static int ch341_set_baudrate_lcr(struct usb_device *dev,
+				  struct ch341_private *priv, u8 lcr)
 {
 	short a;
 	int r;
@@ -157,9 +157,19 @@ static int ch341_init_set_baudrate(struct usb_device *dev,
 	factor = 0x10000 - factor;
 	a = (factor & 0xff00) | divisor;
 
-	/* 0x9c is "enable SFR_UART Control register and timer" */
-	r = ch341_control_out(dev, CH341_REQ_SERIAL_INIT,
-			      0x9c | (ctrl << 8), a | 0x80);
+	/*
+	 * CH341A buffers data until a full endpoint-size packet (32 bytes)
+	 * has been received unless bit 7 is set.
+	 */
+	a |= BIT(7);
+
+	r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, a);
+	if (r)
+		return r;
+
+	r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x2518, lcr);
+	if (r)
+		return r;
 
 	return r;
 }
@@ -233,7 +243,7 @@ static int ch341_configure(struct usb_device *dev, struct ch341_private *priv)
 	if (r < 0)
 		goto out;
 
-	r = ch341_init_set_baudrate(dev, priv, priv->lcr);
+	r = ch341_set_baudrate_lcr(dev, priv, priv->lcr);
 	if (r < 0)
 		goto out;
 
@@ -394,7 +404,7 @@ static void ch341_set_termios(struct tty_struct *tty,
 	if (baud_rate) {
 		priv->baud_rate = baud_rate;
 
-		r = ch341_init_set_baudrate(port->serial->dev, priv, ctrl);
+		r = ch341_set_baudrate_lcr(port->serial->dev, priv, ctrl);
 		if (r < 0 && old_termios) {
 			priv->baud_rate = tty_termios_baud_rate(old_termios);
 			tty_termios_copy_hw(&tty->termios, old_termios);