[01/20] tty: n_gsm: fix missing mux reset on config change at responder

From: Daniel Starke <daniel.starke@siemens.com>

Currently, only the initiator resets the mux protocol if the user requests
new parameters that are incompatible to those of the current connection.
The responder also needs to reset the multiplexer if the new parameter set
requires this. Otherwise, we end up with an inconsistent parameter set
between initiator and responder.
Revert the old behavior to inform the peer upon an incompatible parameter
set change from the user on the responder side by re-establishing the mux
protocol in such case.

Fixes: 509067bbd264 ("tty: n_gsm: Delete gsm_disconnect when config requester")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Thu, Apr 14, 2022 at 02:42:20AM -0700, D. Starke wrote:
> From: Daniel Starke <daniel.starke@siemens.com>
> 
> Currently the peer is not informed about the initial state of the modem
> control lines after a new DLCI has been opened.
> Fix this by sending the initial modem control line states after DLCI open.
> 
> Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
> Cc: stable@vger.kernel.org
> Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
> ---
>  drivers/tty/n_gsm.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index f3fb66be8513..e9a7d9483c1f 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -369,7 +369,11 @@ static const u8 gsm_fcs8[256] = {
>  #define INIT_FCS	0xFF
>  #define GOOD_FCS	0xCF
>  
> +/*
> + * Prototypes
> + */

We know they are prototypes, no need to say it :)

>  static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len);
> +static int gsmtty_modem_update(struct gsm_dlci *dlci, u8 brk);
>  
>  /**
>   *	gsm_fcs_add	-	update FCS
> @@ -1479,6 +1483,8 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
>  		pr_debug("DLCI %d goes open.\n", dlci->addr);
>  	/* Register gsmtty driver,report gsmtty dev add uevent for user */
>  	tty_register_device(gsm_tty_driver, dlci->addr, NULL);
> +	if (dlci->addr) /* Send current modem state */
> +		gsmtty_modem_update(dlci, 0);

Please do not put comments at the end of a line like this.

thanks,

greg k-h

On Thu, Apr 14, 2022 at 02:42:24AM -0700, D. Starke wrote:
> From: Daniel Starke <daniel.starke@siemens.com>
> 
> gsm_carrier_raised() returns always 1 if the kernel module parameter
> 'debug' has its second least significant bit set. This changes the behavior
> of the module instead of just adding some debug output.
> Remove this 'debug' gated early out to avoid debug settings from changing
> the program flow.
> 
> Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
> Cc: stable@vger.kernel.org

Why is this relevant for stable backporting?  It's a debugging feature
only, and you are changing how it previously worked :(