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: 4e46c410e050 ("USB: serial: ch341: reinitialize chip on
reconfiguration")
Signed-off-by: Johan Hovold <johan@kernel.org>

1 files changed, 17 insertions, 7 deletions

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index eabdd05a2147..8d7b0847109b 100644
--- 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;
@@ -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);