Trying to make sense of builtin modem's interfaces

[szatox]

Long story short, I want to make voice calls with a builtin modem, and the conventional wisdom of "use ttyUSB0 to initiate call, and play sound from data incomming on ttyUSB1" doesn't work; most likely because it's an mbim modem and not an emulated analogue.
So, I'm currently trying to understand what am I even working with. THIS seems to be relevant, but doesn't really tells me what to do next.

I can make and answer voice calls using modemmanager or gnome-calls.
I wasn't able to connect audio streams to mic/speakers. I currently don't know why: I'm on pipewire, gnome-calls attempts to use pulseaudio interface I've never tested before, and callaudiod is doing some stupid things, so it might very well be the culprit, but I'll be happy to use an shell script with ffmpeg instead, so didn't debug this part very hard for now.

This Fibocom has 4 interfaces (details will come later)
1.0 using driver cdc-mbim, connected to "usbmisc" /dev/cdc-wdm0. wwan0 uses this interface (fuser reports mbim_proxy talking to this device)
1.1 using driver cdc_mbim
1.2 using driver cdc_acm, connected to terminal /dev/ttyACM0 (fuser reports ModemManager)
1.3 using driver cdc_acm

I couldn't figure out what uses the odd interfaces.
Funny thing, I tried opening both terminals with screen; ttyACM0 prints some probably stats every second, and cdc-wdm0 instantly closes screen.
I think I should be able to get modemmanager to hand me audio streams in some way, but I don't really know what to look for.

At this stage I can't really make a coherent question, so just give me your thoughts. Anything you think might help.

[gentoo_ram]

The cellular modules I've seen that support voice calls also enumerate USB-Audio devices. One for Headphone and one for Mic. That's what I've seen from modules based on Qualcomm chipsets. Maybe this one only really supports data calls.

[szatox]

I know data-only modems exist, but if that was the case, shouldn't it be unavailable for voice calls on PSTN side?
Well, it is worth verifying, I'll look into that. I don't recall my Huawei modems showing up this way though, and I know they were voice-enabled. They even came with a dialler (for windows, obviously) on a builtin virtual cdrom.
Those were emulated analogue modems though, so showed up with 3 tty interfaces, which brings us back the conventional wisdom I mentioned at the very beginning, making it out of scope here. I'll consider it a plan C
Gotta compare it with a pinephone modem too. Definitely a voice-enabled one.


Edit:
/usr/lib/udev/rules.d/77-mm-fibocom-port-types.rules contains udev rules for a bunch of fibocom modems, but none of those seem to match my exact device. This might be another missing piece, or might just mean this modem doesn't need additional setup.
Any idea what do those rules actually do?

[szatox]

I did some digging on the internet, and it actually is damn hard to find exact information on that fibocom inside my thinkpad.
Bing claims it is voice-enabled, but I couldn't find it on the website linked as the source; the datasheet for L830-EB I found only listed L830-EB-02 as applicable device and says it offers digital audio in PCM and I2S modes.

For reference, pinephone is equipped with a qmi modem, which presents itself to the system as /dev/cdc-wdm0 via module qmi_wwan+ /dev/ttyUSB[0-3] via module usbserial

[szatox]

Did more digging; apparently modem is currently running in AT mode and MBIM interface is disabled. I guess that's why I couldn't open that cdc terminal with screen.
hw-probe reports the modem only as "detected" instead of "working", which is funny, since I use basically all the time. I wonder how these other systems make it, didn't have much luck with liveCDs though https://linux-hardware.org/?id=usb:2cb7-0210

Stumbled over https://www.kernel.org/doc/html/latest/networking/cdc_mbim.html
In particular, section on DSS looks interesting, though it does not provide nearly enough information to be immediately usable, but if it's possible to get GPS information from wwan0 interface, it should also be possible to make phone call this way. Might be worth exploring.
_________________
Make Computing Fun Again

[szatox]

I dug up a spare parts table somewhere (?) at Lenovo website.
This particular modem is listed as "ThinkPad Fibocom Intel XMM7262 L830-EB CAT6 WWAN Module", which - according to Intel's scrap of a spec - features VoLTE. Which makes sense, considering I can both, make and answer calls with it, but it is explicitly listed this time.

In the meantime I played with drivers a bit:
Disabling mbim driver makes wwan0 go away and I lose connection to the internet. ModemManager stops recognizing the device as a modem.
Disabling acm driver on the other hand makes the modem lose voice capabilities.

I saw a datasheet with a hardware diagram for a similar modem offered in the same line of laptops, also for M.2 socket.
Among other things, it has separate lanes for legacy USB, USB3, I2C (for hardware codecs) and I2S/PCM (alternate function). Too bad, I currently don't have any information where those audio pins are connected on the other end.
Well, at least I got to the point where I know what I'm looking for.
_________________
Make Computing Fun Again

[szatox]

So, I actually did something sane for a change
Looked up m.2 socket pin map and compared it to the pinout in the sister modem spec i found. They go in the same slot so I figured it should be good enough.
M.2 is made in 3 variants with different keys. M.2 with key B matches the modem spec.

[sanitariu]

I am playing with my fibocom modem too.
You can use your serial port to switch between ecm/mbim modes so you can use ppp/wwan0/usb0 interfaces for internet.
You can switch with commands using minicom like:

AT+GTUSBMODE=?
+GTUSBMODE: (17-27,29,66)

AT+GTUSBMODE=18
AT+GTUSBMODE=17

Mine is default 17.
I did an error switching to mode 23 and now i am missing serial access so i can not switch back to mode 17 (mbim) or 18 (ecm).
Do you know a way to make it back to 17 ?

[szatox]

What kind of modem is it? Something similar to mine? Did you brick it to the point even cutting of power completely doesn't help?
I can't remove battery from my laptop, but its BIOS offers an option to put it into a service mode, which should be just as good for this purpose.

I don't know AT commands for this device. It didn't seem like they would be very helpful without the hardware part anyway, so I've been looking for this information first.
Anyway, might as well summarize the latest bits of information I found:
Those 4 pins I mentioned earlier are designated audio lines in PCIe M.2 socket specification. The protocol is not pre-determined, and has been left up to the vendor to decide.
Kernel reports 4 I2S pins, though I don't really understand what those kernel interface files actually mean nor how to use those pins.
Not even how to confirm they are the pins I'm looking for (e.g. do they belong to this particular socket).

[sanitariu]

Modem is Fibocom

[sanitariu]

I think i found and solved the problem.
For some reason after switching mode AT+GTUSBMODE=23 my device is recognised as

[logrusx]

It might not be a bug but just another very similar device being supported instead of your own. Maybe it's worth filing a kernel bug just so somebody helps you figure it out and with a bit of luck contribute to a new device being supported. Or fixing a bug if that is what it is after all.

Best Regards,
Georgi

[sanitariu]

No, the device is the same it just changes somehow the mode in which is detected maybe. Original is

Bus 001 Device 003: ID 1508:1000 Fibocom Fibocom Modem

After wrong mode become

Bus 001 Device 003: ID 1508:1000 Fibocom Fibocom Modem

1000 is missing in kernel option driver. So you are out of your modems and not AT
commands to return it back to mode 17/18.