[PATCH 3.16 204/370] USB: serial: ch341: fix baud rate and line-control handling

Ben Hutchings
2017-03-10T13:59:08Z

From: Johan Hovold <johan@kernel.org>

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 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/usb/serial/ch341.c | 24 +++++++++++++++++-------
1 file changed, 17 insertions(+), 7 deletions(-)

--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -132,8 +132,8 @@ static int ch341_control_in(struct usb_d
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;
@@ -156,9 +156,19 @@ static int ch341_init_set_baudrate(struc
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;
}
@@ -231,7 +241,7 @@ static int ch341_configure(struct usb_de
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;

@@ -366,7 +376,7 @@ static void ch341_set_termios(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);