USB: serial: console: fix reported terminal settings

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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?

thanks,

greg k-h