patch "tty: n_gsm: Fix long delays with control frame timeouts in ADM mode" added to tty-linus

This is a note to let you know that I've just added the patch titled

tty: n_gsm: Fix long delays with control frame timeouts in ADM mode

to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From: Tony Lindgren <[email protected]>
Date: Sat, 7 Apr 2018 10:19:50 -0700
Subject: tty: n_gsm: Fix long delays with control frame timeouts in ADM mode

Commit ea3d8465ab9b ("tty: n_gsm: Allow ADM response in addition to UA for control dlci") added support for DLCI to stay in Asynchronous Disconnected Mode (ADM). But we still get long delays waiting for commands to other DLCI to complete:

--> 5) C: SABM(P)

Q>  0) C: UIH(F)
Q>  0) C: UIH(F)
Q>  0) C: UIH(F)

...

This happens because gsm_control_send() sets cretries timer to T2 that is by default set to 34. This will cause resend for T2 times for the control frame. In ADM mode, we will never get a response so the control frame, so retries are just delaying all the commands.

Let's fix the issue by setting DLCI_MODE_ADM flag after detecting the ADM mode for the control DLCI. Then we can use that in gsm_control_send() to set retries to 1. This means the control frame will be sent once allowing the other end at an opportunity to switch from ADM to ABM mode.

Note that retries will be decremented in gsm_control_retransmit() so we don't want to set it to 0 here.

---
 drivers/tty/n_gsm.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

index 3b3e1f6632d7..7b1f8636f8e9 100644
@@ -121,6 +121,9 @@ struct gsm_dlci {
 	struct mutex mutex;
 
 	/* Link layer */
+	int mode;
+#define DLCI_MODE_ABM		0	/* Normal Asynchronous Balanced Mode */
+#define DLCI_MODE_ADM		1	/* Asynchronous Disconnected Mode */
 	spinlock_t lock;	/* Protects the internal state */
 	struct timer_list t1;	/* Retransmit timer for SABM and UA */
 	int retries;

@@ -1364,7 +1367,13 @@ static struct gsm_control *gsm_control_send(struct gsm_mux *gsm,
 	ctrl->data = data;
 	ctrl->len = clen;
 	gsm->pending_cmd = ctrl;
-	gsm->cretries = gsm->n2;
+
+	/* If DLCI0 is in ADM mode skip retries, it won't respond */
+	if (gsm->dlci[0]->mode == DLCI_MODE_ADM)
+		gsm->cretries = 1;
+	else
+		gsm->cretries = gsm->n2;
+
 	mod_timer(&gsm->t2_timer, jiffies + gsm->t2 * HZ / 100);
 	gsm_control_transmit(gsm, ctrl);
 	spin_unlock_irqrestore(&gsm->control_lock, flags);

@@ -1472,6 +1481,7 @@ static void gsm_dlci_t1(struct timer_list *t)
 			if (debug & 8)
 				pr_info("DLCI %d opening in ADM mode.\n",
 					dlci->addr);
+			dlci->mode = DLCI_MODE_ADM;
 			gsm_dlci_open(dlci);
 		} else {
 			gsm_dlci_close(dlci);

2.17.0