I was working with the SieraWireless LTE device drivers recently. I encountered a strange issue with one of the drivers called GobiSerial. I have compiled this driver code in the same way as described here. Still, when I try to insert this into the Pi kernel, an error occurs. It results in the ensuing error:

pi@raspberrypi:~ $ sudo insmod GobiSerial.ko
 insmod: ERROR: could not insert module GobiSerial.ko: Unknown symbol in module
 pi@raspberrypi:~ $ dmesg
 [   65.759262] GobiSerial: Unknown symbol usb_serial_generic_open (err -2)
 [   65.759290] GobiSerial: Unknown symbol usb_serial_suspend (err -2)
 [   65.759328] GobiSerial: Unknown symbol usb_serial_generic_resume (err -2)
 [   65.759348] GobiSerial: Unknown symbol usb_serial_deregister_drivers (err -2)
 [   65.759407] GobiSerial: Unknown symbol usb_serial_generic_write (err -2)
 [   65.759427] GobiSerial: Unknown symbol usb_serial_register_drivers (err -2)

To debug this issue, I first want to know if the symbols like usb_serial_generic_open exist in the Pi kernel. Are they there or not? Apparently not, but just to verify that I would use the /proc/kallsyms tool provided by Linux Kernel. This utility prints all the symbols (exported) available in the kernel space. I would use grep to locate the missing symbols from the available symbols in the kernel. Here is the process:

pi@raspberrypi:~ $ cat /proc/kallsyms | grep -i "usb_serial_generic_open"
pi@raspberrypi:~ $

The command does not return anything. This indicates that my hunch was correct that the symbols are not there (exported/available) in the Pi Kernel.
Now, by the look of it the symbols should be coming from a Kernel module/driver usb_serial. This is my hunch by the look of the symbols which are missing. My next hunch is some Kernel module are not inserted unless there is a reason for it. Like if you haven’t inserted anything on the USB serial the usb_serial driver won’t be inserted into the kernel. Let’s try this. I will attach the SieraWireless LTE device into the USB port of the Pi. Before proceeding, I usually clean my Kernel messages buffer as shown below. sudo dmesg -c I do this so that I can focus solely on the kernel messages for the inserted device. Now insert the device, wait for a second and then try the dmesg again, and you should be able to see something like this on the console:

 [  252.661876] usb 1-1.3: reset low-speed USB device number 4 using dwc_otg
 [  253.511884] usb 1-1.5: reset low-speed USB device number 5 using dwc_otg
 [  261.551829] usb 1-1.2: new high-speed USB device number 8 using dwc_otg
 [  261.682434] usb 1-1.2: config 1 has an invalid interface number: 8 but max is 3
 [  261.682451] usb 1-1.2: config 1 has no interface number 1
 [  261.683111] usb 1-1.2: New USB device found, idVendor=1199, idProduct=9091, bcdDevice= 0.06
 [  261.683123] usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
 [  261.683133] usb 1-1.2: Product: Sierra Wireless EM7565 Qualcomm® Snapdragon™ X16 LTE-A
 [  261.683143] usb 1-1.2: Manufacturer: Sierra Wireless, Incorporated
 [  261.683153] usb 1-1.2: SerialNumber: UF92377624041506
 [  261.686019] QMAP Disabled
 [  261.688132] GobiNet 1-1.2:1.8 eth1: register 'GobiNet' at usb-3f980000.usb-1.2, GobiNet Ethernet Device, d2:bd:e1:49:f1:08
 [  261.689996] USB Speed : USB 2.0
 [  261.805791] usbcore: registered new interface driver usbserial_generic
 [  261.805832] usbserial: USB Serial support registered for generic
 [  261.806590] usbcore: registered new interface driver cdc_wdm
 [  261.814689] usbcore: registered new interface driver qcserial
 [  261.814756] usbserial: USB Serial support registered for Qualcomm USB modem
 [  261.815707] qcserial 1-1.2:1.0: Qualcomm USB modem converter detected
 [  261.816223] usb 1-1.2: Qualcomm USB modem converter now attached to ttyUSB0
 [  261.819226] qcserial 1-1.2:1.2: Qualcomm USB modem converter detected
 [  261.819635] usb 1-1.2: Qualcomm USB modem converter now attached to ttyUSB1
 [  261.819970] qcserial 1-1.2:1.3: Qualcomm USB modem converter detected
 [  261.820236] usb 1-1.2: Qualcomm USB modem converter now attached to ttyUSB2
 [  261.820411] usbcore: registered new interface driver qmi_wwan
 [  261.920268] IPv6: ADDRCONF(NETDEV_UP): eth1: link is not ready

Looks like the default Linux kernel serial drivers been just inserted. Curious huh? Check the Kernel to see if our missing symbol is back. Execute this step:

 
 pi@raspberrypi:~ $ cat /proc/kallsyms | grep -i "usb_serial_generic_open"
 00000000 r __ksymtab_usb_serial_generic_open    [usbserial]
 00000000 r __kstrtab_usb_serial_generic_open    [usbserial]
 00000000 t usb_serial_generic_open    [usbserial]

Hurrah!! It is back. Now try inserting the GobiSerial driver module again:

pi@raspberrypi:~ $ sudo insmod GobiSerial.ko
pi@raspberrypi:~ $

Seems like the prior error didn’t. Lets check the dmesg again, and here we go:

[  326.647915] usbserial: USB Serial support registered for GobiSerial
[  326.647943] GobiSerial: 2019-11-22/SWI_2.39:GobiSerial

Issue resolved.
I initially thought it was straightforward. But later I realised that attaching the device to the RPI HW requires insmod-ing the qcserial kernel module. It also involves insmod-ing the qmi_wwan kernel module. These are Qualcomm drivers. Still, with the device EM7565, the release notes prohibit having qcserial and qmi_wwan drivers in the kernel. They prescribe blacklisting these in the next file:
/etc/modprobe.d/blacklist-modem.conf.

$ sudo vim /etc/modprobe.d/blacklist-modem.conf

# Uncomment these entries in order to blacklist unwanted modem drivers

# blacklist snd-atiixp-modem

# blacklist snd-intel8x0m

# blacklist snd-via82xx-modem

blacklist qcserial

blacklist qmi_wwan

If you do that, the qcserial and qmi_wwan drivers won’t be loaded. As a result, the drivers will not start. This happens when you attach your EM7565 device to the USB port. So, once I do that and restart the computer my above resolution to the problem stops working again :(.

Actually, the qcserial driver loads the usbserial. If you remove the qcserial from the equation, it all stops working again.

So finally what I did was to:

sudo insmod /lib/modules/4.19.75-v7+/kernel/drivers/usb/serial/usbserial.ko

sudo insmod GobiSerial.ko

and it works fine.

This works but not yet the best way to sort this problem. To solve this problem cleanly, first install the GobiNet and GobiSerial. Then execute depmod to resolve the dependencies. Follow the below steps for this:

$ export OUTPUTDIR=/lib/modules/`uname -r`/kernel/drivers/net/usb/

$ mkdir -p $OUTPUTDIR

$ cp -f GobiNet.ko $(OUTPUTDIR)

$ cp -f GobiSerial.ko $(OUTPUTDIR)

$ depmod

$ modprobe GobiNet

$ modprobe GobiSerial

Once you have completed these steps, restart the device. Everything should be in place now.
Do these actions to confirm:

pi@raspberrypi:~ $ lsmod | grep -i "Gobi*"

GobiSerial             20480  1

usbserial              40960  3 GobiSerial

GobiNet               438272  0

When we do depmod, it resolves the dependencies of the various modules in the Kernel /lib/modules/ folder. Then at the time of modprobe the dependency drivers/modules are loaded first and then the actual module of interest.

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