tty: n_gsm: Allow ADM response in addition to UA for control dlci
[ Upstream commit ea3d8465 ] 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>
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