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

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: linux-serial@vger.kernel.org
Cc: Alan Cox <alan@llwyncelyn.cymru>
Cc: Dan Williams <dcbw@redhat.com>
Cc: Jiri Prchal <jiri.prchal@aksignal.cz>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Michael Nazzareno Trimarchi <michael@amarulasolutions.com>
Cc: Michael Scott <michael.scott@linaro.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Russ Gorby <russ.gorby@intel.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 7b1f8636f8e9b44690f68b4f03cf0c722ea30d41..1dbe27c9946c16578a6ad0f82313c4adf797aef0 100644 (file)
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2871,11 +2871,22 @@ 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;
 }