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Subject[PATCH v4 4/9] tty: n_gsm: fix missing timer to handle stalled links
Date
From: Daniel Starke <daniel.starke@siemens.com>

The current implementation does not handle the situation that no data is in
the internal queue and needs to be sent out while the user tty fifo is
full.
Add a timer that moves more data from user tty down to the internal queue
which is then serialized on the ldisc. This timer is triggered if no data
was moved from a user tty to the internal queue within 10 * T1.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
drivers/tty/n_gsm.c | 43 +++++++++++++++++++++++++++++++++++--------
1 file changed, 35 insertions(+), 8 deletions(-)

Stable backport remark has been removed compared to v3. No other changes applied.

Link: https://lore.kernel.org/all/20220530144512.2731-4-daniel.starke@siemens.com/

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index b51e2023d88d..58bf4b4aea78 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -244,6 +244,7 @@ struct gsm_mux {
struct list_head tx_list; /* Pending data packets */

/* Control messages */
+ struct timer_list kick_timer; /* Kick TX queuing on timeout */
struct timer_list t2_timer; /* Retransmit timer for commands */
int cretries; /* Command retry counter */
struct gsm_control *pending_cmd;/* Our current pending command */
@@ -833,6 +834,7 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
list_add_tail(&msg->list, &gsm->tx_list);
gsm->tx_bytes += msg->len;
gsm_data_kick(gsm, dlci);
+ mod_timer(&gsm->kick_timer, jiffies + 10 * gsm->t1 * HZ / 100);
}

/**
@@ -885,9 +887,6 @@ static int gsm_dlci_data_output(struct gsm_mux *gsm, struct gsm_dlci *dlci)
size = len + h;

msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
- /* FIXME: need a timer or something to kick this so it can't
- * get stuck with no work outstanding and no buffer free
- */
if (!msg)
return -ENOMEM;
dp = msg->data;
@@ -964,9 +963,6 @@ static int gsm_dlci_data_output_framed(struct gsm_mux *gsm,

size = len + overhead;
msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
-
- /* FIXME: need a timer or something to kick this so it can't
- get stuck with no work outstanding and no buffer free */
if (msg == NULL) {
skb_queue_tail(&dlci->skb_list, dlci->skb);
dlci->skb = NULL;
@@ -1062,9 +1058,9 @@ static int gsm_dlci_modem_output(struct gsm_mux *gsm, struct gsm_dlci *dlci,
* renegotiate DLCI priorities with optional stuff. Needs optimising.
*/

-static void gsm_dlci_data_sweep(struct gsm_mux *gsm)
+static int gsm_dlci_data_sweep(struct gsm_mux *gsm)
{
- int len;
+ int len, ret = 0;
/* Priority ordering: We should do priority with RR of the groups */
int i = 1;

@@ -1087,7 +1083,11 @@ static void gsm_dlci_data_sweep(struct gsm_mux *gsm)
/* DLCI empty - try the next */
if (len == 0)
i++;
+ else
+ ret++;
}
+
+ return ret;
}

/**
@@ -1806,6 +1806,30 @@ static void gsm_dlci_command(struct gsm_dlci *dlci, const u8 *data, int len)
}
}

+/**
+ * gsm_kick_timer - transmit if possible
+ * @t: timer contained in our gsm object
+ *
+ * Transmit data from DLCIs if the queue is empty. We can't rely on
+ * a tty wakeup except when we filled the pipe so we need to fire off
+ * new data ourselves in other cases.
+ */
+static void gsm_kick_timer(struct timer_list *t)
+{
+ struct gsm_mux *gsm = from_timer(gsm, t, kick_timer);
+ unsigned long flags;
+ int sent = 0;
+
+ spin_lock_irqsave(&gsm->tx_lock, flags);
+ /* If we have nothing running then we need to fire up */
+ if (gsm->tx_bytes < TX_THRESH_LO)
+ sent = gsm_dlci_data_sweep(gsm);
+ spin_unlock_irqrestore(&gsm->tx_lock, flags);
+
+ if (sent && debug & 4)
+ pr_info("%s TX queue stalled\n", __func__);
+}
+
/*
* Allocate/Free DLCI channels
*/
@@ -2261,6 +2285,7 @@ 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);

/* Free up any link layer users and finally the control channel */
@@ -2293,6 +2318,7 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
struct gsm_dlci *dlci;
int ret;

+ timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
init_waitqueue_head(&gsm->event);
spin_lock_init(&gsm->control_lock);
@@ -2699,6 +2725,7 @@ static int gsmld_open(struct tty_struct *tty)

gsmld_attach_gsm(tty, gsm);

+ timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);

return 0;
--
2.34.1
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