USB: serial: console: fix reported terminal settings

[Johan Hovold <johan@kernel.org>]

On Tue, Dec 04, 2018 at 05:15:18PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Dec 04, 2018 at 05:00:36PM +0100, Johan Hovold wrote:
> > The USB-serial console implementation has never reported the actual
> > terminal settings used. Despite storing the corresponding cflags in its
> > struct console, this was never honoured on later tty open() where the
> > tty termios would be left initialised to the driver defaults.
> > 
> > Unlike the serial console implementation, the USB-serial code calls
> > subdriver open() already at console setup. While calling set_termios()
> > before open() looks like it could work for some USB-serial drivers,
> > others definitely do not expect this, so modelling this after serial
> > core is going to be intrusive, if at all possible.
> > 
> > Instead, use a (renamed) tty helper to save the termios data used at
> > console setup, so that the tty termios reflects the actual terminal
> > settings after a subsequent tty open().
> > 
> > Note that the calls to tty_init_termios() (tty_driver_install()) and
> > tty_save_termios() are serialised using the disconnect mutex.
> > 
> > This specifically fixes a regression that was triggered by a recent
> > change adding software flow control to the pl2303 driver: a getty trying
> > to disable flow control while leaving the baud rate unchanged would now
> > also set the baud rate to the driver default (prior to the flow-control
> > change this had been a noop).
> > 
> > Fixes: 7041d9c3f01b ("USB: serial: pl2303: add support for tx xon/xoff flow control")
> > Cc: stable <stable@vger.kernel.org>	# 4.18
> > Reported-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
> > Cc: Florian Zumbiehl <florz@florz.de>
> > Signed-off-by: Johan Hovold <johan@kernel.org>
> > ---
> >  drivers/tty/tty_io.c         | 11 +++++++++--
> >  drivers/usb/serial/console.c |  2 +-
> >  include/linux/tty.h          |  1 +
> >  3 files changed, 11 insertions(+), 3 deletions(-)
> 
> Ah, messy :)
> 
> Want me to take this through my tty tree?

If you prefer. I was planning on including this in a USB-serial pull
request for -rc6 since it fixes a user-reported regression, but perhaps
taking this through your tty-linus branch (which already holds a console
fix) is easier/faster.

We should wait for Jarkko to confirm that this fixes the problem he
reported first, though.

Thanks,
Johan