On Wed, Nov 13, 2024 at 07:08:27PM +0100, Lode Willems wrote: > This adds support for enabling and disabling > RTS/CTS hardware flow control. > Tested using a CH340E in combination with a CP2102. > > Fixes part of the following bug report: > Link: https://bugzilla.kernel.org/show_bug.cgi?id=197109 > > Signed-off-by: Lode Willems <me@xxxxxxxxxxxxxxx> Thanks for the patch and sorry about the late feedback on this one. I wanted to give it a spin with the devices I have here (ch340g and ch341a). Appears to the modem control lines are not wired up on the ch341a evaluation board I have, but the device accepts the request and stops transmitting with hardware flow control enabled. With ch340g in loopback, I also see it refuse to transmit unless cts is asserted. I was not able to get the device to deassert rts when attempting to fill its receive buffers, however. Perhaps the hardware does not support this, but is this something you tried? > --- > drivers/usb/serial/ch341.c | 41 ++++++++++++++++++++++++++++++++++++++ > 1 file changed, 41 insertions(+) > > diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c > index d10e4c4848a0..62237f657320 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 0x000 > +#define CH341_FLOW_CTL_RTSCTS 0x101 The registers are eight bits wide, but the driver writes two at a time. So this should just be 0x00 and 0x01. > + > #define CH341_QUIRK_LIMITED_PRESCALER BIT(0) > #define CH341_QUIRK_SIMULATE_BREAK BIT(1) > > @@ -478,6 +482,41 @@ 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; > + > + if (old_termios && > + C_CRTSCTS(tty) == (old_termios->c_cflag & CRTSCTS)) > + return; Just drop this and set the requested setting unconditionally. > + > + if (C_CRTSCTS(tty)) { > + r = ch341_control_out(port->serial->dev, CH341_REQ_WRITE_REG, > + CH341_REG_FLOW_CTL | > + ((u16)CH341_REG_FLOW_CTL << 8), > + CH341_FLOW_CTL_RTSCTS); > + if (r < 0) { > + dev_err(&port->dev, > + "%s - failed to enable flow control: %d\n", > + __func__, r); > + tty->termios.c_cflag &= ~CRTSCTS; > + } > + } else { > + r = ch341_control_out(port->serial->dev, CH341_REQ_WRITE_REG, > + CH341_REG_FLOW_CTL | > + ((u16)CH341_REG_FLOW_CTL << 8), > + CH341_FLOW_CTL_NONE); > + if (r < 0) { > + dev_err(&port->dev, > + "%s - failed to disable flow control: %d\n", > + __func__, r); > + tty->termios.c_cflag |= CRTSCTS; > + } > + } Please rewrite this so that you prepare the value and index parameters based on the termios settings and then do one control request. If it fails you can restore the old setting (if old_termios is non-NULL). And please drop the redundant __func__ from the error message (even if the driver still uses it in some other functions still). > +} > + Johan
