[PATCH v2] USB: serial: CH341: add hardware flow control RTS/CTS

[Lode Willems <me@lodewillems.com>]

This adds support for enabling and disabling
RTS/CTS hardware flow control.
Tested using CH341A and CH340E.

Fixes part of the following bug report:
Link: https://bugzilla.kernel.org/show_bug.cgi?id=197109

Signed-off-by: Lode Willems <me@lodewillems.com>
---

Changes in v2:
 - Change the CH341_FLOW_CTL_* values to be 8 bits wide
 - Set flow control setting unconditionally
 - Prepare parameters and do one control request
 - Drop the error message

 drivers/usb/serial/ch341.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index d10e4c4848a0..c255a8437e4a 100644
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -63,6 +63,7 @@
 #define CH341_REG_DIVISOR      0x13
 #define CH341_REG_LCR          0x18
 #define CH341_REG_LCR2         0x25
+#define CH341_REG_FLOW_CTL     0x27
 
 #define CH341_NBREAK_BITS      0x01
 
@@ -77,6 +78,9 @@
 #define CH341_LCR_CS6          0x01
 #define CH341_LCR_CS5          0x00
 
+#define CH341_FLOW_CTL_NONE    0x00
+#define CH341_FLOW_CTL_RTSCTS  0x01
+
 #define CH341_QUIRK_LIMITED_PRESCALER	BIT(0)
 #define CH341_QUIRK_SIMULATE_BREAK	BIT(1)
 
@@ -478,6 +482,28 @@ static int ch341_open(struct tty_struct *tty, struct usb_serial_port *port)
 	return r;
 }
 
+static void ch341_set_flow_control(struct tty_struct *tty,
+				   struct usb_serial_port *port,
+				   const struct ktermios *old_termios)
+{
+	int r;
+	u16 flow_ctl;
+
+	if (C_CRTSCTS(tty))
+		flow_ctl = ((u16)CH341_FLOW_CTL_RTSCTS << 8) | CH341_FLOW_CTL_RTSCTS;
+	else
+		flow_ctl = ((u16)CH341_FLOW_CTL_NONE << 8) | CH341_FLOW_CTL_NONE;
+
+	r = ch341_control_out(port->serial->dev,
+			      CH341_REQ_WRITE_REG,
+			      ((u16)CH341_REG_FLOW_CTL << 8) | CH341_REG_FLOW_CTL,
+			      flow_ctl);
+	if (r < 0 && old_termios) {
+		tty->termios.c_cflag &= ~CRTSCTS;
+		tty->termios.c_cflag |= (old_termios->c_cflag & CRTSCTS);
+	}
+}
+
 /* Old_termios contains the original termios settings and
  * tty->termios contains the new setting to be used.
  */
@@ -546,6 +572,8 @@ static void ch341_set_termios(struct tty_struct *tty,
 	spin_unlock_irqrestore(&priv->lock, flags);
 
 	ch341_set_handshake(port->serial->dev, priv->mcr);
+
+	ch341_set_flow_control(tty, port, old_termios);
 }
 
 /*
-- 
2.47.1