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

From: Lode Willems
Date: Wed Dec 18 2024 - 12:37:07 EST


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@xxxxxxxxxxxxxxx>
---

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