Sierra dual QMI/MBIM

[Bjørn Mork]

Markus Gothe <nietzsche at lysator.liu.se> writes:

> Maybe Björn already mentioned this.
>
> Have USBCOMP=14 on a 340U, MBIM takes precedance when choosing default
> config.

Yes, that is due to Linux preferring class functions over vendor
specific.

> I switch it to QMI mode, I can connect etc. However no data flows on
> the interface.
>
> Seems like the first driver to initialize it also sets the mode
> temporary in fw (until reboot).

I haven't seen that exact behaviour, but I guess it's likely that some
firmware versions fail to initialize QMI mode after having seen the
MBIM/NCM commands sent by the driver during probing and setup.

My experience is that you can switch to QMI mode as long as you haven't
sent any MBIM commands.  But I have only verified that on one firmware
revision on the MC7710.  Other combinations could very well show
different symptoms.

In any case: The firmware is incapable of switching modes, despite the
multi-config appearing to allow this.  The firmware should be switched
to the desired mode on discovery.  Changing modes requires a firmware
reset and a new discovery phase.

I'd call this a firmware bug, but I'm sure the vendor would say
"functions as designed" :-)

> My current workaround is to assign it driver info to NULL in
> cdc_mbim.c... Not very convenient.
>
> Anybody knows a better workaround?

Using an udev rule to select the mode on device discovery should prevent
the "wrong" driver from binding to it.


Bjørn