tty: n_gsm: Don't block input queue by waiting MSC

[ Upstream commit 3cf0b3c ]

Currently gsm_queue() processes incoming frames and when opening
a DLC channel it calls gsm_dlci_open() which calls gsm_modem_update().
If basic mode is used it calls gsm_modem_upd_via_msc() and it
cannot block the input queue by waiting the response to come
into the same input queue.

Instead allow sending Modem Status Command without waiting for remote
end to respond. Define a new function gsm_modem_send_initial_msc()
for this purpose. As MSC is only valid for basic encoding, it does
not do anything for advanced or when convergence layer type 2 is used.

Fixes: 4847380 ("tty: n_gsm: fix missing update of modem controls after DLCI open")
Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>
Link: https://lore.kernel.org/r/20250827123221.1148666-1-seppo.takalo@nordicsemi.no
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: SeppoTakalo

drivers/tty/n_gsm.c

@@ -461,6 +461,7 @@ static int gsm_send_packet(struct gsm_mux *gsm, struct gsm_msg *msg);
 static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr);
 static void gsmld_write_trigger(struct gsm_mux *gsm);
 static void gsmld_write_task(struct work_struct *work);
+static int gsm_modem_send_initial_msc(struct gsm_dlci *dlci);
 
 /**
  *	gsm_fcs_add	-	update FCS
@@ -2174,7 +2175,7 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
 		pr_debug("DLCI %d goes open.\n", dlci->addr);
 	/* Send current modem state */
 	if (dlci->addr) {
-		gsm_modem_update(dlci, 0);
+		gsm_modem_send_initial_msc(dlci);
 	} else {
 		/* Start keep-alive control */
 		gsm->ka_num = 0;
@@ -4161,6 +4162,28 @@ static int gsm_modem_upd_via_msc(struct gsm_dlci *dlci, u8 brk)
 	return gsm_control_wait(dlci->gsm, ctrl);
 }
 
+/**
+ * gsm_modem_send_initial_msc - Send initial modem status message
+ *
+ * @dlci channel
+ *
+ * Send an initial MSC message after DLCI open to set the initial
+ * modem status lines. This is only done for basic mode.
+ * Does not wait for a response as we cannot block the input queue
+ * processing.
+ */
+static int gsm_modem_send_initial_msc(struct gsm_dlci *dlci)
+{
+	u8 modembits[2];
+
+	if (dlci->adaption != 1 || dlci->gsm->encoding != GSM_BASIC_OPT)
+		return 0;
+
+	modembits[0] = (dlci->addr << 2) | 2 | EA; /* DLCI, Valid, EA */
+	modembits[1] = (gsm_encode_modem(dlci) << 1) | EA;
+	return gsm_control_command(dlci->gsm, CMD_MSC, (const u8 *)&modembits, 2);
+}
+
 /**
  *	gsm_modem_update	-	send modem status line state
  *	@dlci: channel