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

commit 55fa15b5987db22b4f35d3f0798928c126be5f1c upstream.

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: 4e46c410e050 ("USB: serial: ch341: reinitialize chip on
reconfiguration")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index 3e498f4..22e748a 100644 (file)
--- a/drivers/usb/serial/ch341.c
+++ b/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;
@@ -159,9 +159,19 @@ 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;
 }
@@ -240,7 +250,7 @@ static int ch341_configure(struct usb_device *dev, struct ch341_private *priv)
        if (r < 0)
                goto out;
 
-       r = ch341_init_set_baudrate(dev, priv, 0);
+       r = ch341_set_baudrate_lcr(dev, priv, 0);
        if (r < 0)
                goto out;
 
@@ -378,7 +388,7 @@ 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);