grab_port fails for AT/PPP port (stuck) in data mode

[Norbert van Bolhuis]

Hi Aleksander,


On 8/31/21 9:58 AM, Aleksander Morgado wrote:
> Hey Norbert,
> 
>> I'm creating a plugin for Sierra Wireless HL78xx LTE-M modem.
>> Actually, it is already partly done.
>>
>> The HL78xx has its own TCP/TLS stack/interface. To use it as
>> "generic linux network interface" though, PPP is used for the AT/PPP capable
>> ttyACM1 port. ttyACM0 is the AT control port.
>>
>> Note: I'm talking about usb (cdc_acm) 'virtual' tty ports. The serial UART
>>         port (UART1) can be used for PPP as well, but that is another use-case.
>>
>> The problem I currently have is that the plugin fails to grab
>> the ttyACM1 port if this port is in data mode. It enters data mode
>> after the ATD "dial-in" command ("ATD*99***<cid>#1").
>>
>> The same problem occurs when the ATD "dial-in" command itself, it times
>> out when the ttyACM1 port is in data mode.
>>
>> Afaict, there are 2 possible solutions.
>> -1- implement some proper switching between data and command mode
>> -2- MM uses ttyACM0 only
>>
>> I wonder whether -1- is a good option since I don't see any code dealing with
>> data<->command mode switching. Do you think this is needed in my case?
>> if yes, How do I issue "+++" to enter AT command mode in (any custom) grab_port?
>>
>> about -2-, the ATD "dial-in" command probably isn't necessary (since MM
>> already register with COPS and create/activates the PDP-context/bearer with
>> CGDCONT/CGACT). Do you think this is a good option?
>> We do use NetworkManager to start pppd with the right port (ttyACM1), will this still work?
>>
> 
> I think the question here would be why ttyACM1 is in data mode. MM
> expects all ports in control mode, and then decides which one to
> switch into data mode with an ATD call. MM does not expect the ports
> to be already in data mode.
> 


I'd expect MM to handle any case.

What if the user disconnects and then reconnects?
Currently this doesn't work because once ttyACM1 is in data mode the
disconnect (CGACT=0/etc... on ttyACM0) doesn't make it switch to control mode
and so the ATD call times out.

So it looks like I need to go for option -2- or am I missing something?

Thanks!

Regards,
Norbert