Re: [PATCH] cdc-acm: acm_init: Set initial BAUD to B0

[Johan Hovold <johan@kernel.org>]

On Fri, Jul 10, 2020 at 11:35:18AM +0200, Joakim Tjernlund wrote:
> BO will disable USB input until the device opens. This will
> avoid garbage chars waiting flood the TTY. This mimics a real UART
> device better.
> For initial termios to reach USB core, USB driver has to be
> registered before TTY driver.
> 
> Signed-off-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>
> Cc: stable@vger.kernel.org
> ---
> 
>  I hope this change makes sense to you, if so I belive
>  ttyUSB could do the same.

No, this doesn't make sense. B0 is used to hang up an already open tty.

Furthermore, this change only affects the initial terminal settings and
won't have any effect the next time you open the same port.

Why not fix your firmware so that it doesn't transmit before DTR is
asserted during open()?

>  drivers/usb/class/cdc-acm.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
> index 751f00285ee6..5680f71200e5 100644
> --- a/drivers/usb/class/cdc-acm.c
> +++ b/drivers/usb/class/cdc-acm.c
> @@ -1999,19 +1999,19 @@ static int __init acm_init(void)
>  	acm_tty_driver->subtype = SERIAL_TYPE_NORMAL,
>  	acm_tty_driver->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;
>  	acm_tty_driver->init_termios = tty_std_termios;
> -	acm_tty_driver->init_termios.c_cflag = B9600 | CS8 | CREAD |
> +	acm_tty_driver->init_termios.c_cflag = B0 | CS8 | CREAD |
>  								HUPCL | CLOCAL;
>  	tty_set_operations(acm_tty_driver, &acm_ops);
>  
> -	retval = tty_register_driver(acm_tty_driver);
> +	retval = usb_register(&acm_driver);
>  	if (retval) {
>  		put_tty_driver(acm_tty_driver);
>  		return retval;
>  	}
>  
> -	retval = usb_register(&acm_driver);
> +	retval = tty_register_driver(acm_tty_driver);
>  	if (retval) {
> -		tty_unregister_driver(acm_tty_driver);
> +		usb_deregister(&acm_driver);
>  		put_tty_driver(acm_tty_driver);
>  		return retval;
>  	}

Johan