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    Subject[PATCH 4.9 263/310] tty: n_gsm: Allow ADM response in addition to UA for control dlci
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Tony Lindgren <tony@atomide.com>


    [ Upstream commit ea3d8465ab9b3e01be329ac5195970a84bef76c5 ]

    Some devices have the control dlci stay in ADM mode instead of the UA
    mode. This can seen at least on droid 4 when trying to open the ts
    27.010 mux port. Enabling n_gsm debug mode shows the control dlci
    always respond with DM to SABM instead of UA:

    # modprobe n_gsm debug=0xff
    # ldattach -d GSM0710 /dev/ttyS0 &
    gsmld_output: 00000000: f9 03 3f 01 1c f9
    --> 0) C: SABM(P)
    gsmld_receive: 00000000: f9 03 1f 01 36 f9
    <-- 0) C: DM(P)
    ...
    $ minicom -D /dev/gsmtty1
    minicom: cannot open /dev/gsmtty1: No error information
    $ strace minicom -D /dev/gsmtty1
    ...
    open("/dev/gsmtty1", O_RDWR|O_NOCTTY|O_NONBLOCK|O_LARGEFILE) = -1 EL2HLT

    Note that this is different issue from other n_gsm -EL2HLT issues such
    as timeouts when the control dlci does not respond at all.

    The ADM mode seems to be a quite common according to "RF Wireless World"
    article "GSM Issue-UE sends SABM and gets a DM response instead of
    UA response":

    This issue is most commonly observed in GSM networks where in UE sends
    SABM and expects network to send UA response but it ends up receiving
    DM response from the network. SABM stands for Set asynchronous balanced
    mode, UA stands for Unnumbered Acknowledge and DA stands for
    Disconnected Mode.

    An RLP entity can be in one of two modes:
    - Asynchronous Balanced Mode (ABM)
    - Asynchronous Disconnected Mode (ADM)

    Currently Linux kernel closes the control dlci after several retries
    in gsm_dlci_t1() on DM. This causes n_gsm /dev/gsmtty ports to produce
    error code -EL2HLT when trying to open them as the closing of control
    dlci has already set gsm->dead.

    Let's fix the issue by allowing control dlci stay in ADM mode after the
    retries so the /dev/gsmtty ports can be opened and used. It seems that
    it might take several attempts to get any response from the control
    dlci, so it's best to allow ADM mode only after the SABM retries are
    done.

    Note that for droid 4 additional patches are needed to mux the ttyS0
    pins and to toggle RTS gpio_149 to wake up the mdm6600 modem are also
    needed to use n_gsm. And the mdm6600 modem needs to be powered on.

    Cc: linux-serial@vger.kernel.org
    Cc: Alan Cox <alan@llwyncelyn.cymru>
    Cc: Jiri Prchal <jiri.prchal@aksignal.cz>
    Cc: Jiri Slaby <jslaby@suse.cz>
    Cc: Marcel Partap <mpartap@gmx.net>
    Cc: Michael Scott <michael.scott@linaro.org>
    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/tty/n_gsm.c | 17 ++++++++++++++---
    1 file changed, 14 insertions(+), 3 deletions(-)

    --- a/drivers/tty/n_gsm.c
    +++ b/drivers/tty/n_gsm.c
    @@ -1467,6 +1467,10 @@ static void gsm_dlci_open(struct gsm_dlc
    * in which case an opening port goes back to closed and a closing port
    * is simply put into closed state (any further frames from the other
    * end will get a DM response)
    + *
    + * Some control dlci can stay in ADM mode with other dlci working just
    + * fine. In that case we can just keep the control dlci open after the
    + * DLCI_OPENING retries time out.
    */

    static void gsm_dlci_t1(unsigned long data)
    @@ -1480,8 +1484,15 @@ static void gsm_dlci_t1(unsigned long da
    if (dlci->retries) {
    gsm_command(dlci->gsm, dlci->addr, SABM|PF);
    mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
    - } else
    + } else if (!dlci->addr && gsm->control == (DM | PF)) {
    + if (debug & 8)
    + pr_info("DLCI %d opening in ADM mode.\n",
    + dlci->addr);
    + gsm_dlci_open(dlci);
    + } else {
    gsm_dlci_close(dlci);
    + }
    +
    break;
    case DLCI_CLOSING:
    dlci->retries--;
    @@ -1499,8 +1510,8 @@ static void gsm_dlci_t1(unsigned long da
    * @dlci: DLCI to open
    *
    * Commence opening a DLCI from the Linux side. We issue SABM messages
    - * to the modem which should then reply with a UA, at which point we
    - * will move into open state. Opening is done asynchronously with retry
    + * to the modem which should then reply with a UA or ADM, at which point
    + * we will move into open state. Opening is done asynchronously with retry
    * running off timers and the responses.
    */


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