Re: [PATCH v2 6/7] USB: serial: ftdi_sio: Fix custom_divisor and c_*speed for ASYNC_SPD_CUST

[Johan Hovold <johan@kernel.org>]

On Sun, Jul 24, 2022 at 02:33:51PM +0200, Pali Rohár wrote:
> On Sunday 24 July 2022 14:28:43 Johan Hovold wrote:
> > On Tue, Jul 12, 2022 at 01:53:05PM +0200, Marek Behún wrote:
> > > From: Pali Rohár <pali@kernel.org>
> > > 
> > > When ASYNC_SPD_CUST is used, update custom_divisor and c_*speed fields
> > > so that they correspond to the newly set baud rate value, so that
> > > userspace GET ioctls will see the true baud rate that is being used.
> > 
> > No, this is wrong.
> > 
> > In fact, there's a long-standing bug in this driver which started
> > reporting back the actual baud rate when using SPD_CUST.
> 
> Hello! And this commit is fixing also this bug as a side change.

Ah, indeed it is, or at least to some extent.

> > The rate should be left unchanged at 38400 in that case.
> 
> With this change, rate in c_cflag is unchanged and stays at B38400.

Right.

> What is updated is the real baudrate in c_ispeed and c_ospeed
> extensions.
> 
> It is really wrong? I thought that c_cflag should stay unchanged at
> B38400 when ASYNC_SPD_CUST is used.

Yeah, cflags stay unchanged, but you shouldn't touch those fields when
using the deprecated ASYNC_SPD_CUST hack.

Note that this currently only works because the ftdi driver uses
tty_get_baud_rate() instead of c_ospeed directly which is the
recommended (new) way.
 
> > > +	/* Fix deprecated async-compatible custom_divisor hack and update tty baud rate */
> > > +	if (fix_custom_divisor) {
> > > +		priv->custom_divisor = DIV_ROUND_CLOSEST(priv->baud_base, baud);
> > > +		old_baud = baud;
> > > +		baud = 38400;
> > > +	}
> > > +
> > >  	tty_encode_baud_rate(tty, baud, baud);
> > > +
> > > +	/* For async-compatible custom_divisor store into TCGETS2 c_*speed fields real baud rate */
> > > +	if (fix_custom_divisor)
> > > +		tty->termios.c_ispeed = tty->termios.c_ospeed = old_baud;
> > > +
> > >  	return div_value;
> > >  }
> > >  
> > > @@ -2674,6 +2688,8 @@ static void ftdi_set_termios(struct tty_struct *tty,
> > >  		dev_dbg(ddev, "%s: forcing baud rate for this device\n", __func__);
> > >  		tty_encode_baud_rate(tty, priv->force_baud,
> > >  					priv->force_baud);
> > > +		termios->c_ispeed = termios->c_ospeed =
> > > +			DIV_ROUND_CLOSEST(priv->baud_base, priv->custom_divisor);
> > >  	}
> > >  
> > >  	/* Force RTS-CTS if this device requires it. */

Johan