tty: n_gsm: Fix DLCI handling for ADM mode if debug & 2 is not set

commit b2d89ad9c9682e795ed6eeb9ed455789ad6cedf1 upstream.

At least on droid 4 with control channel in ADM mode, there is no response
to Modem Status Command (MSC). Currently gsmtty_modem_update() expects to
have data in dlci->modem_rx unless debug & 2 is set. This means that on
droid 4, things only work if debug & 2 is set.

Let's fix the issue by ignoring empty dlci->modem_rx for ADM mode. In
the AMD mode, CMD_MSC will never respond and gsm_process_modem() won't
get called to set dlci->modem_rx.

And according to ts_127010v140000p.pdf, MSC is only relevant if basic
option is chosen, so let's test for that too.

Fixes: ea3d8465ab9b ("tty: n_gsm: Allow ADM response in addition to UA for control dlci")
Cc: [email protected]
Cc: Alan Cox <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Jiri Prchal <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Marcel Partap <[email protected]>
Cc: Merlijn Wajer <[email protected]>
Cc: Michael Nazzareno Trimarchi <[email protected]>
Cc: Michael Scott <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Peter Hurley <[email protected]>
Cc: Russ Gorby <[email protected]>
Cc: Sascha Hauer <[email protected]>
Cc: Sebastian Reichel <[email protected]>
Signed-off-by: Tony Lindgren <[email protected]>
Cc: stable <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/tty/n_gsm.c

@@ -2875,11 +2875,22 @@ static int gsmtty_modem_update(struct gsm_dlci *dlci, u8 brk)
 static int gsm_carrier_raised(struct tty_port *port)
 {
 	struct gsm_dlci *dlci = container_of(port, struct gsm_dlci, port);
+	struct gsm_mux *gsm = dlci->gsm;
+
 	/* Not yet open so no carrier info */
 	if (dlci->state != DLCI_OPEN)
 		return 0;
 	if (debug & 2)
 		return 1;
+
+	/*
+	 * Basic mode with control channel in ADM mode may not respond
+	 * to CMD_MSC at all and modem_rx is empty.
+	 */
+	if (gsm->encoding == 0 && gsm->dlci[0]->mode == DLCI_MODE_ADM &&
+	    !dlci->modem_rx)
+		return 1;
+
 	return dlci->modem_rx & TIOCM_CD;
 }