Sierra EM7455 broadband modem does not receive data

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Sierra EM7455 built-in usb modem on Lenovo ThinkPad T470 does not work.
Modem is recognized by the system and it can also connect to the network, but there is no data exchange.
RHEL 7.3 operating system is updated via yum.
Modem works fine with Fedora 26 on the same machine.

I am attaching the output of ModemManager --debug.

Thanks for any suggestion.

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Looks like ModemManager is trying to run the modem using both /dev/ttyUSB2 and /dev/cdc-wdm0 device interfaces at the same time. I used to have the same problem on my Debian system.

The problem is, Sierra modems can have multiple control interfaces using different protocols, and ModemManager cannot always tell if you have one modem with /dev/ttyUSB interface and another with /dev/cdc-wdm interface, or just one modem that offers both interfaces.

The /dev/ttyUSB interface for Sierra modems is provided by the qcserial kernel module, while the /dev/cdc-wdm interface is provided by the qmi_wwan module. A third module, cdc_mbim, could also be used with some Sierra modem versions/configurations.

I suggest you choose one module and blacklist the others. For example, if you choose to use the qcserial module, you could create /etc/modprobe.d/sierra.conf file with the following contents:

blacklist qmi_wwan
blacklist cdc_mbim

If the modem now works better, you've found the solution. If it doesn't work, try blacklisting the qcserial module instead and enabling one of the other two. For me, the qcserial interface seemed to be the most stable one, but your mileage may vary.

If that does not help, google for "sierra device composition". For example, this link might be helpful: http://www.0xf8.org/2015/07/dell-wireless-5809e-support-in-linux-a-followup/

Hello Matti,

thanks for the hint. In fact, blacklisting the modules under /etc/modprobe.d/ had no effect. What did work out was blacklisting cdc_mbim module on bootloader's kernel line (add "modprobe.blacklist=cdc_mbim" to the line).

Thanks again!

OK, then your system is apparently loading the module(s) for the Sierra modem while in initramfs, which is earlier than I expected. In that case, the modprobe.d blacklisting would have worked if you had just updated your initramfs after adding the blacklisting to /etc/modprobe.d/. But the boot option achieves exactly the same thing, just in a different way. Good to hear you got it working!

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