Re: SARA-R4 unreliable LTE-M connection problem

[Alexander Dahl <ada thorsis com>]

Hello Aleksander, everyone,

Am Montag, 5. Juli 2021, 11:14:38 CEST schrieb Aleksander Morgado:
> > Connection can be established, I see data on the modems RX line, but
> > nothing on the TX line, which means the host sends data to the modem, but
> > gets back nothing.

I tried again. According to scope baudrate on RX of the modem module seems to 
be 115200 in connected state.

> > Can this be a problem with the baudrate? I mean it's a classical serial
> > UART, so I would assume baudrates of host and modem must match. In AT
> > command mode the baudrate is 115200 and ModemManager can successfuly send
> > AT commands and receives reponses. Is it possible NetworkManager and/or
> > pppd change this somehow?
>
> Have you tried to set the ID_MM_TTY_FLOW_CONTROL udev tag in the TTY
> port so that flow control settings are applied by ModemManager before
> the connection is started? E.g. ID_MM_TTY_FLOW_CONTROL set to
> "rts-cts" or to "xon-xoff"
>
> Also, if using 115200, have you set ID_MM_TTY_BAUDRATE to 115200 as
> well as a udev tag in the TTY port? The default in MM is 57600 really.

Yes, all this is set and I compared with the AT command lists for both modems, 
that 115200 with rts-cts are the default settings:

root@unit:~ cat /usr/lib/udev/rules.d/78-mm-whitelist-platform-modem.rules 
ACTION!="add|change|move", GOTO="mm_whitelist_platform_modem_end"
DEVPATH=="/devices/platform/soc/2100000.bus/21f4000.serial/tty/ttymxc4", 
ENV{ID_MM_DEVICE_PROCESS}="1", ENV{ID_MM_TTY_BAUDRATE}="115200", 
ENV{ID_MM_TTY_FLOW_CONTROL}="rts-cts"
LABEL="mm_whitelist_platform_modem_end"

In all ModemManager debug output I did neither see command AT\Q nor AT+IPR, so 
I assume ModemManager does not change this setting?

I'm afraid I did not fully understand yet how baudrates with those LTE modems 
are supposed to work in connected state, when pppd has taken over. 

Can the modem change that baudrate and how would the host follow then? 

Or is it the other way round, can pppd change the baudrate and does the modem 
adapt itself? 

Or does the baudrate remain constant at all time? 

Then if 115200 is the default, that would limit actual throughput, even if the 
modem could send/receive with a higher rate, right?

Greets
Alex