tty: n_gsm: fix missing update of modem controls after DLCI open

[ Upstream commit 4847380250 ]

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: e1eaea46bb ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220420101346.3315-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/tty/n_gsm.c

@@ -371,6 +371,7 @@ static const u8 gsm_fcs8[256] = {
 #define GOOD_FCS	0xCF
 
 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
@@ -1489,6 +1490,9 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
 	dlci->state = DLCI_OPEN;
 	if (debug & 8)
 		pr_debug("DLCI %d goes open.\n", dlci->addr);
+	/* Send current modem state */
+	if (dlci->addr)
+		gsmtty_modem_update(dlci, 0);
 	wake_up(&dlci->gsm->event);
 }