linux-kernel - [PATCH 1/2] tty: n_gsm: wait until channel 0 is ready

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Message-Id: <20241228150100.100354-2-ivo.g.dimitrov.75@gmail.com>
Date: Sat, 28 Dec 2024 17:00:59 +0200
From: Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	Tony Lindgren <tony@...mide.com>
Subject: [PATCH 1/2] tty: n_gsm: wait until channel 0 is ready

Currently code does not wait for channel 0 open sequence to complete before
pushing data to the other channels. Also, if userland opens tty, it will
receive EL2NSYNC. Both issues result in hard to predict initialization
sequence and possible userland failures.

Fix that by waiting channel 0 open sequence to complete before attempting
opening of the other channels. Also, if tty open() is attempted while
channel 0 is opening, wait until sequence is complete before returning to
userland.

Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>
Signed-off-by: Tony Lindgren <tony@...mide.com>
---
 drivers/tty/n_gsm.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 252849910588..b92480051e3d 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2244,13 +2244,16 @@ static void gsm_dlci_t1(struct timer_list *t)
 		break;
 	case DLCI_OPENING:
 		if (dlci->retries) {
-			dlci->retries--;
-			gsm_command(dlci->gsm, dlci->addr, SABM|PF);
+			if (!dlci->addr || !gsm->dlci[0] ||
+			    gsm->dlci[0]->state != DLCI_OPENING) {
+				dlci->retries--;
+				gsm_command(dlci->gsm, dlci->addr, SABM|PF);
+			}
+
 			mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
 		} else if (!dlci->addr && gsm->control == (DM | PF)) {
 			if (debug & DBG_ERRORS)
-				pr_info("DLCI %d opening in ADM mode.\n",
-					dlci->addr);
+				pr_info("DLCI 0 opening in ADM mode.\n");
 			dlci->mode = DLCI_MODE_ADM;
 			gsm_dlci_open(dlci);
 		} else {
@@ -2308,7 +2311,9 @@ static void gsm_dlci_begin_open(struct gsm_dlci *dlci)
 		dlci->retries = gsm->n2;
 		if (!need_pn) {
 			dlci->state = DLCI_OPENING;
-			gsm_command(gsm, dlci->addr, SABM|PF);
+			if (!dlci->addr || !gsm->dlci[0] ||
+			    gsm->dlci[0]->state != DLCI_OPENING)
+				gsm_command(gsm, dlci->addr, SABM|PF);
 		} else {
 			/* Configure DLCI before setup */
 			dlci->state = DLCI_CONFIGURE;
@@ -4251,7 +4256,7 @@ static const struct tty_port_operations gsm_port_ops = {
 static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
 {
 	struct gsm_mux *gsm;
-	struct gsm_dlci *dlci;
+	struct gsm_dlci *dlci, *dlci0;
 	unsigned int line = tty->index;
 	unsigned int mux = mux_line_to_num(line);
 	bool alloc = false;
@@ -4274,10 +4279,20 @@ static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
 	perspective as we don't have to worry about this
 	if DLCI0 is lost */
 	mutex_lock(&gsm->mutex);
-	if (gsm->dlci[0] && gsm->dlci[0]->state != DLCI_OPEN) {
+
+	dlci0 = gsm->dlci[0];
+	if (dlci0 && dlci0->state != DLCI_OPEN) {
 		mutex_unlock(&gsm->mutex);
-		return -EL2NSYNC;
+
+		if (dlci0->state == DLCI_OPENING)
+			wait_event(gsm->event, dlci0->state != DLCI_OPENING);
+
+		if (dlci0->state != DLCI_OPEN)
+			return -EL2NSYNC;
+
+		mutex_lock(&gsm->mutex);
 	}
+
 	dlci = gsm->dlci[line];
 	if (dlci == NULL) {
 		alloc = true;
-- 
2.30.2