Sierra Wireless EM7455 LTE Card on CentOS7

I had someone approach me trying to get some help. He had a Sierra Wireless LTE card that he wanted to use on CentOS7.  He had Network Manager running, and ModemManager, and he had two kernel modules loaded up called qcserial and qmi_wwan, but ModemManager would not recognize the card. So that's where we start.

I am not a low level expert on drivers these days (I don't do that day in day out), but have had some experience with drivers for wireless devices, such as USB 802.11x sticks. I had a TrendNet one years ago that wouldn't work with Linux until I found some sketch drivers on the web that I compiled and got to work. But, that entailed Network Manager and wpa_supplicant...not ModemManager. This was my first dive into LTE cards and drivers. Furthermore, I did not have the card in my hand, or on my own system.

So, apparently Ubuntu supports these cards natively, but CentOS7 doesn't.

I noticed that CentOS 7 does include a sierra.ko (sierra.ko.xz) module, which I thought should work with a Sierra Wireless EM7455 LTE-A card, which uses a Snapdragon X7 chip. We tested that out, by loading the sierra kernel module manually and starting ModemManager. No luck. Maybe it doesn't work with this EM7455 card? Not sure. I did see some support threads on the sierra.ko kernel module where the module only works for Sierra cards because Sierra does some interesting power management stuff with their driver (they made mention of another option.ko kernel module that should work with most other LTE cards). But this card, the EM7455 is indeed a Sierra LTE card. And the sierra.ko module didn't seem to work.

There are also a couple of other kernel modules that ARE on a CentOS7 box. These are called:

• qcserial
• qmi_wwan

The qcserial module creates a /dev/ttyUSB interface. The qmi_wwan creates a /dev/cdc-wdm interface. My understanding is that the serial interface is a control protocol for commands and statistics while the other is used for data transmission/reception (Tx/Rx). This is all part of a protocol called QMI; a Qualcomm protocol.

If you want to learn more about these protocols, this link below is absolutely fascinating as it discusses distinctions between GSM and CDMA, and the history of CDMA which has ties to Hollywood and a Beautiful Actress. Eventually it gets into QMI.

https://blogs.gnome.org/dcbw/2010/04/15/mobile-broadband-and-qualcomm-proprietary-protocols/

I think what is/was happening, is that when you crank the EM7455 card, these two drivers, qcserial and qmi_wwan are loaded but ModemManager still doesn't recognize the card. Either does NetworkManager.

So - the engineer heard that if he got access to two new drivers, GobiNet and GobiSerial, which are generated from a Sierra Wireless SDK, the card would work. You would need to blacklist the qcserial and qmi_wwan drivers though. The problem: how to get the SDK. I guess there might be some reason why Sierra Wireless doesn't release this SDK, which is probably, maybe, tied to royalties or licensing to Qualcom.

So we eventually obtained the SDK. We compiled it, and it produces, for our x86_64 architecture, two kernel modules:

• GobiNet
• GobiSerial

We (I) created an rpm (separate blog post about rpm package creation) to do all of the voodoo to get these drivers installed, along with the blacklist file, and configure an apn connection to a Verizon LTE access point.

Voila'. The drivers work! I think he said something about it using a ppp interface, though. And we specifically compiled GobiNet to use rawip with a rawip=1 setting on the Makefile.  So we may need to look into that but at least the LTE modem is now working.

By the way. You cannot rely just on Syslog for information about LTE. Because these are kernel drivers, you need to use dmesg to see what these modules are barking out!

So some more testing the engineer will do. But we have something that seems to work. I will wait to hear more feedback.