[PATCH v4 11/13] USB: ch341: improve B0 handling

[Grigori Goronzy]

Check for B0 in a more idiomatic way and make sure to not enable
RTS/CTS hardware flow control in B0 as it may override the control
lines.  Also make sure to only enable RTS/DTR if there's a transition
from B0.

v2: use c_cflag macros.
v3: rebase.

Signed-off-by: Grigori Goronzy <greg@xxxxxxxxxxxx>
---
 drivers/usb/serial/ch341.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index 22cfd88..3ce2041 100644
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -387,10 +387,12 @@ static void ch341_set_termios(struct tty_struct *tty,
 	if (C_CSTOPB(tty))
 		ctrl |= CH341_LCR_STOP_BITS_2;
 
-	if (priv->baud_rate) {
-		spin_lock_irqsave(&priv->lock, flags);
-		priv->line_control |= (CH341_BIT_DTR | CH341_BIT_RTS);
-		spin_unlock_irqrestore(&priv->lock, flags);
+	if (C_BAUD(tty) != B0) {
+		if (old_termios && (old_termios->c_cflag & CBAUD) == B0) {
+			spin_lock_irqsave(&priv->lock, flags);
+			priv->line_control |= (CH341_BIT_DTR | CH341_BIT_RTS);
+			spin_unlock_irqrestore(&priv->lock, flags);
+		}
 		r = ch341_init_set_baudrate(port->serial->dev, priv, ctrl);
 		if (r < 0 && old_termios) {
 			priv->baud_rate = tty_termios_baud_rate(old_termios);
@@ -404,7 +406,7 @@ static void ch341_set_termios(struct tty_struct *tty,
 
 	ch341_set_handshake(port->serial->dev, priv->line_control);
 
-	if (C_CRTSCTS(tty)) {
+	if (C_CRTSCTS(tty) && C_BAUD(tty) != B0) {
 		r = ch341_control_out(port->serial->dev, CH341_REQ_WRITE_REG,
 				CH341_REG_RTSCTS | ((uint16_t)CH341_REG_RTSCTS << 8),
 				0x0101);
-- 
1.9.1