Re: [PATCH v2 4/7] USB: serial: ftdi_sio: Do not reset baud rate to 9600 Baud on error

["Pali Rohár" <pali@kernel.org>]

On Sunday 24 July 2022 14:20:58 Johan Hovold wrote:
> On Tue, Jul 12, 2022 at 01:53:03PM +0200, Marek Behún wrote:
> > From: Pali Rohár <pali@kernel.org>
> > 
> > If setting new baud rate fails, all other drivers leave the device at
> > previous baud rate, only ftdi_sio resets to 9600 Baud.
> 
> > Change this behavior to that of other drivers so that /dev/ttyUSB*
> > devices behave in the same way.
> 
> These statements are not true. Several USB serial driver set 9600 baud
> on errors for historical reasons. Yet others clamp. It's inconsistent at
> best.
> 
> > Signed-off-by: Pali Rohár <pali@kernel.org>
> > Tested-by: Marek Behún <kabel@kernel.org>
> > Signed-off-by: Marek Behún <kabel@kernel.org>
> > ---
> > Greg pointed out that this make break some people's workflow, that they
> > may depend on this behavior.
> > 
> > This is of course possible, although IMO improbable: it would be weird
> > to depend on the fall back to 9600 Baud on error, instead of expecting
> > that the baud rate didn't change at all (like in other /dev/ttyUSB*
> > drivers).
> > 
> > Nonetheless if someone complains that they workflow got broken, we will
> > need to revert this.
> 
> >  static u32 get_ftdi_divisor(struct tty_struct *tty,
> > -						struct usb_serial_port *port)
> > +			    struct usb_serial_port *port, speed_t old_baud)
> >  {
> >  	struct ftdi_private *priv = usb_get_serial_port_data(port);
> >  	struct device *dev = &port->dev;
> > @@ -1322,6 +1322,8 @@ static u32 get_ftdi_divisor(struct tty_struct *tty,
> >  			__func__, priv->custom_divisor, baud);
> >  	}
> >  
> > +	if (!baud)
> > +		baud = old_baud;
> >  	if (!baud)
> >  		baud = 9600;
> 
> This looks like it breaks B0 handling. Either way, an unrelated change.

Unreleased to this change.

> >  	switch (priv->chip_type) {
> > @@ -1330,8 +1332,12 @@ static u32 get_ftdi_divisor(struct tty_struct *tty,
> >  		if (div_value == -1) {
> >  			dev_dbg(dev, "%s - Baudrate (%d) requested is not supported\n",
> >  				__func__,  baud);
> > -			baud = 9600;
> > +			baud = old_baud ? old_baud : 9600;
> >  			div_value = ftdi_sio_baud_to_divisor(baud);
> > +			if (div_value == -1) {
> > +				baud = 9600;
> > +				div_value = ftdi_sio_baud_to_divisor(baud);
> > +			}
> >  			div_okay = 0;
> >  		}
> >  		break;
> > @@ -1340,8 +1346,8 @@ static u32 get_ftdi_divisor(struct tty_struct *tty,
> >  			div_value = ftdi_232am_baud_to_divisor(baud);
> >  		} else {
> >  			dev_dbg(dev, "%s - Baud rate too high!\n", __func__);
> > -			baud = 9600;
> > -			div_value = ftdi_232am_baud_to_divisor(9600);
> > +			baud = (old_baud >= 183 && old_baud <= 3000000) ? old_baud : 9600;
> 
> And please avoid using the ternary operator which tend to just hurt
> readability.
> 
> Looks like this needs to be refactored in some way.
> 
> > @@ -1525,7 +1536,7 @@ static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss)
> >  		if (priv->flags & ASYNC_SPD_MASK)
> >  			dev_warn_ratelimited(&port->dev, "use of SPD flags is deprecated\n");
> >  
> > -		change_speed(tty, port);
> > +		change_speed(tty, port, 0);
> 
> Zero has a special meaning (B0).
> 
> >  	}
> >  	mutex_unlock(&priv->cfg_lock);
> >  	return 0;
> > @@ -2774,9 +2785,12 @@ static void ftdi_set_termios(struct tty_struct *tty,
> >  		/* Drop RTS and DTR */
> >  		clear_mctrl(port, TIOCM_DTR | TIOCM_RTS);
> >  	} else {
> > +		speed_t old_baud =
> > +			old_termios ? tty_termios_baud_rate(old_termios) : 0;
> 
> Just use 9600 if you don't have an old termios. The termios rate should
> be valid in that case.

Ok