Some questions about qmi/wwan

[Robert Jones]

Hello,

Resurrecting an old thread as I'd like to expand on a previous
question. For reference I'm using a Sierra Wireless MC7354 and qmicli
v1.18.0.

> 2014-07-27 14:06 GMT+02:00 Aleksander Morgado <aleksander at aleksander.es>:
> > On Fri, Jul 25, 2014 at 9:00 PM, Dan Williams <dcbw at redhat.com> wrote:
> > - Once the kernel module is loaded, I get two /dev/cdc-wdm devices (0
> and
> >> 1). What are those for? The one that seems to be working with qmicli is
> >> /dev/cdc-wdm1. I also get two network interfaces, wwp0s20u6 and
> >> wwp0s20u6i3, the second one is the one that I can use dhcp client with
> and
> >> get an IP. Which is the purpose of the other one?
> >
> > I'll leave that to Bjorn and Aleksander, but on some devices there is a
> > second non-operative QMI configuration.  Could be a firmware quirk.
>
> Newer Sierra modems like the MC73XX series will expose two pairs of
> /dev/cdc-wdm (QMI) and wwan ports (usb interfaces 8 and 10). If you
> try to play with both you'll likely see that both reply to QMI but
> only one of the pairs gets correctly connected...
>
> The thing is that one pair comes by default with 802.3 ethernet frames
> configuration in the wwan interface, which is the one that the
> qmi_wwan kernel driver expects. The other pair comes with raw-ip
> packet configuration in the wwan interface, so you won't be able to
> use that pair directly with the qmi_wwan driver, *unless* you request
> to change that via e.g. qmicli
> --device-open-net="net-802-3|net-no-qos-header" or qmicli
> --wda-set-data-format="802-3". ModemManager does the device-open-net
> setup itself, so it can use both pairs directly.

I've been asked what the difference is between these two /dev/cdc-wdm*
devices and even after reading this thread I'm still a bit confused.
The impression I get is that there is in fact no practical difference
between these two exposed channels/devices other than their default
(raw-ip vs 802-3) configuration.

If there is no difference besides that, what is the purpose of these
two separate channels as far as a developer/end user is concerned?

Thanks,
Robert