[PATCH 4.4 18/44] tty: n_gsm: Fix DLCI handling for ADM mode if debug & 2 is not set

From: Greg Kroah-Hartman
Date: Mon Apr 30 2018 - 16:33:10 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Tony Lindgren <tony@xxxxxxxxxxx>

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: linux-serial@xxxxxxxxxxxxxxx
Cc: Alan Cox <alan@xxxxxxxxxxxxxxxx>
Cc: Dan Williams <dcbw@xxxxxxxxxx>
Cc: Jiri Prchal <jiri.prchal@xxxxxxxxxxx>
Cc: Jiri Slaby <jslaby@xxxxxxx>
Cc: Marcel Partap <mpartap@xxxxxxx>
Cc: Merlijn Wajer <merlijn@xxxxxxxxxx>
Cc: Michael Nazzareno Trimarchi <michael@xxxxxxxxxxxxxxxxxxxx>
Cc: Michael Scott <michael.scott@xxxxxxxxxx>
Cc: Pavel Machek <pavel@xxxxxx>
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Cc: Russ Gorby <russ.gorby@xxxxxxxxx>
Cc: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
Cc: Sebastian Reichel <sre@xxxxxxxxxx>
Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
Cc: stable <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

drivers/tty/n_gsm.c | 11 +++++++++++
1 file changed, 11 insertions(+)

--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2891,11 +2891,22 @@ static int gsmtty_modem_update(struct gs
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;