Re: [PATCH 1/3] tty: n_gsm: add keep alive support

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Feb 01, 2023 at 09:01:49AM +0100, D. Starke wrote:
> From: Daniel Starke <daniel.starke@siemens.com>
> 
> n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
> See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
> The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
> the newer 27.010 here. Chapters 5.4.6.3.4 and 5.1.8.1.3 describe the test
> command which can be used to test the mux connection between both sides.
> 
> Currently, no algorithm is implemented to make use of this command. This
> requires that each multiplexed upper layer protocol supervises the
> underlying muxer connection to handle possible connection losses.
> 
> Introduce an ioctl parameter and functions to optionally enable keep alive
> handling via the test command as described in chapter 5.4.6.3.4. A single
> incrementing octet is being used to distinguish between multiple retries.
> Retry count and interval are taken from the general parameters N2 and T2.
> 
> Note that support for the test command is mandatory and already present in
> the muxer implementation since the very first version.
> 
> Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
> ---
>  drivers/tty/n_gsm.c         | 89 +++++++++++++++++++++++++++++++++----
>  include/uapi/linux/gsmmux.h |  3 +-
>  2 files changed, 83 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index 5783801d6524..98577b54f1fd 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -318,13 +318,19 @@ struct gsm_mux {
>  	struct gsm_control *pending_cmd;/* Our current pending command */
>  	spinlock_t control_lock;	/* Protects the pending command */
>  
> +	/* Keep-alive */
> +	struct timer_list ka_timer;	/* Keep-alive response timer */
> +	u8 ka_num;			/* Keep-alive match pattern */
> +	int ka_retries;			/* Keep-alive retry counter */
> +
>  	/* Configuration */
> -	int adaption;		/* 1 or 2 supported */
> -	u8 ftype;		/* UI or UIH */
> -	int t1, t2;		/* Timers in 1/100th of a sec */
> -	unsigned int t3;	/* Power wake-up timer in seconds. */
> -	int n2;			/* Retry count */
> -	u8 k;			/* Window size */
> +	int adaption;			/* 1 or 2 supported */
> +	u8 ftype;			/* UI or UIH */
> +	int t1, t2;			/* Timers in 1/100th of a sec */
> +	unsigned int t3;		/* Power wake-up timer in seconds. */
> +	int n2;				/* Retry count */
> +	u8 k;				/* Window size */
> +	unsigned int keep_alive;	/* Control channel keep-alive in ms */
>  
>  	/* Statistics (not currently exposed) */
>  	unsigned long bad_fcs;
> @@ -1897,11 +1903,13 @@ static void gsm_control_response(struct gsm_mux *gsm, unsigned int command,
>  						const u8 *data, int clen)
>  {
>  	struct gsm_control *ctrl;
> +	struct gsm_dlci *dlci;
>  	unsigned long flags;
>  
>  	spin_lock_irqsave(&gsm->control_lock, flags);
>  
>  	ctrl = gsm->pending_cmd;
> +	dlci = gsm->dlci[0];
>  	command |= 1;
>  	/* Does the reply match our command */
>  	if (ctrl != NULL && (command == ctrl->cmd || command == CMD_NSC)) {
> @@ -1916,10 +1924,57 @@ static void gsm_control_response(struct gsm_mux *gsm, unsigned int command,
>  	/* Or did we receive the PN response to our PN command */
>  	} else if (command == CMD_PN) {
>  		gsm_control_negotiation(gsm, 0, data, clen);
> +	/* Or did we receive the TEST response to our TEST command */
> +	} else if (command == CMD_TEST && clen == 1 && *data == gsm->ka_num) {
> +		gsm->ka_retries = -1; /* trigger new keep-alive message */
> +		if (dlci && !dlci->dead)
> +			mod_timer(&gsm->ka_timer,
> +				  jiffies + gsm->keep_alive * HZ / 100);
>  	}
>  	spin_unlock_irqrestore(&gsm->control_lock, flags);
>  }
>  
> +/**
> + * gsm_control_keep_alive	-	check timeout or start keep-alive
> + * @t: timer contained in our gsm object
> + *
> + * Called off the keep-alive timer expiry signaling that our link
> + * partner is not responding anymore. Link will be closed.
> + * This is also called to startup our timer.
> + */
> +
> +static void gsm_control_keep_alive(struct timer_list *t)
> +{
> +	struct gsm_mux *gsm = from_timer(gsm, t, ka_timer);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&gsm->control_lock, flags);
> +	if (gsm->ka_num && gsm->ka_retries == 0) {
> +		/* Keep-alive expired -> close the link */
> +		if (debug & DBG_ERRORS)
> +			pr_info("%s keep-alive timed out\n", __func__);
> +		spin_unlock_irqrestore(&gsm->control_lock, flags);
> +		if (gsm->dlci[0])
> +			gsm_dlci_begin_close(gsm->dlci[0]);
> +	} else if (gsm->keep_alive && gsm->dlci[0] && !gsm->dlci[0]->dead) {
> +		if (gsm->ka_retries > 0) {
> +			/* T2 expired for keep-alive -> resend */
> +			gsm->ka_retries--;
> +		} else {
> +			/* Start keep-alive timer */
> +			gsm->ka_num++;
> +			if (!gsm->ka_num)
> +				gsm->ka_num++;
> +			gsm->ka_retries = gsm->n2;
> +		}
> +		gsm_control_command(gsm, CMD_TEST, &gsm->ka_num,
> +				    sizeof(gsm->ka_num));
> +		mod_timer(&gsm->ka_timer,
> +			  jiffies + gsm->t2 * HZ / 100);
> +		spin_unlock_irqrestore(&gsm->control_lock, flags);
> +	}
> +}
> +
>  /**
>   *	gsm_control_transmit	-	send control packet
>   *	@gsm: gsm mux
> @@ -2061,8 +2116,10 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
>  		/* Ensure that gsmtty_open() can return. */
>  		tty_port_set_initialized(&dlci->port, false);
>  		wake_up_interruptible(&dlci->port.open_wait);
> -	} else
> +	} else {
> +		del_timer(&dlci->gsm->ka_timer);
>  		dlci->gsm->dead = true;
> +	}
>  	/* A DLCI 0 close is a MUX termination so we need to kick that
>  	   back to userspace somehow */
>  	gsm_dlci_data_kick(dlci);
> @@ -2078,6 +2135,8 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
>  
>  static void gsm_dlci_open(struct gsm_dlci *dlci)
>  {
> +	struct gsm_mux *gsm = dlci->gsm;
> +
>  	/* Note that SABM UA .. SABM UA first UA lost can mean that we go
>  	   open -> open */
>  	del_timer(&dlci->t1);
> @@ -2087,8 +2146,15 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
>  	if (debug & DBG_ERRORS)
>  		pr_debug("DLCI %d goes open.\n", dlci->addr);
>  	/* Send current modem state */
> -	if (dlci->addr)
> +	if (dlci->addr) {
>  		gsm_modem_update(dlci, 0);
> +	} else {
> +		/* Start keep-alive control */
> +		gsm->ka_num = 0;
> +		gsm->ka_retries = -1;
> +		mod_timer(&gsm->ka_timer,
> +			  jiffies + gsm->keep_alive * HZ / 100);
> +	}
>  	gsm_dlci_data_kick(dlci);
>  	wake_up(&dlci->gsm->event);
>  }
> @@ -2840,6 +2906,7 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
>  	/* Finish outstanding timers, making sure they are done */
>  	del_timer_sync(&gsm->kick_timer);
>  	del_timer_sync(&gsm->t2_timer);
> +	del_timer_sync(&gsm->ka_timer);
>  
>  	/* Finish writing to ldisc */
>  	flush_work(&gsm->tx_work);
> @@ -2987,6 +3054,7 @@ static struct gsm_mux *gsm_alloc_mux(void)
>  	INIT_LIST_HEAD(&gsm->tx_data_list);
>  	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
>  	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
> +	timer_setup(&gsm->ka_timer, gsm_control_keep_alive, 0);
>  	INIT_WORK(&gsm->tx_work, gsmld_write_task);
>  	init_waitqueue_head(&gsm->event);
>  	spin_lock_init(&gsm->control_lock);
> @@ -3003,6 +3071,7 @@ static struct gsm_mux *gsm_alloc_mux(void)
>  	gsm->mru = 64;	/* Default to encoding 1 so these should be 64 */
>  	gsm->mtu = 64;
>  	gsm->dead = true;	/* Avoid early tty opens */
> +	gsm->keep_alive = 0;	/* Disabled */
>  
>  	/* Store the instance to the mux array or abort if no space is
>  	 * available.
> @@ -3046,6 +3115,7 @@ static void gsm_copy_config_values(struct gsm_mux *gsm,
>  	c->mru = gsm->mru;
>  	c->mtu = gsm->mtu;
>  	c->k = gsm->k;
> +	c->keep_alive = gsm->keep_alive;
>  }
>  
>  static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
> @@ -3094,6 +3164,8 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
>  		need_restart = 1;
>  	if (c->mtu != gsm->mtu)
>  		need_restart = 1;
> +	if (c->keep_alive != gsm->keep_alive)
> +		need_restart = true;
>  
>  	/*
>  	 * Close down what is needed, restart and initiate the new
> @@ -3109,6 +3181,7 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
>  	gsm->encoding = c->encapsulation ? GSM_ADV_OPT : GSM_BASIC_OPT;
>  	gsm->adaption = c->adaption;
>  	gsm->n2 = c->n2;
> +	gsm->keep_alive = c->keep_alive;
>  
>  	if (c->i == 1)
>  		gsm->ftype = UIH;
> diff --git a/include/uapi/linux/gsmmux.h b/include/uapi/linux/gsmmux.h
> index cb8693b39cb7..b64360aca1f9 100644
> --- a/include/uapi/linux/gsmmux.h
> +++ b/include/uapi/linux/gsmmux.h
> @@ -19,7 +19,8 @@ struct gsm_config
>  	unsigned int mtu;
>  	unsigned int k;
>  	unsigned int i;
> -	unsigned int unused[8];		/* Padding for expansion without
> +	unsigned int keep_alive;
> +	unsigned int unused[7];		/* Padding for expansion without

"unsigned int" is not really a valid uapi variable type.

Shouldn't this be __u32 instead?

Should you document this field as to what the value is and the units as
you are creating a new user/kernel api here.

And finally, "unused" here is being properly checked to be all 0, right?
If not, then this change can't happen for obvious reasons :(

thanks,

greg k-h