Sierra Wireless EM7455

[Bjørn Mork]

Ralph Plawetzki <ralph at purejava.org> writes:

> Just a short notice that I could not find a way to flip the device to
> QMI mode. I tried the udev rules here [1] but that did not work.
>
> Giving up now.
>
> Kind regards,
> Ralph
>
> [1]
> http://askubuntu.com/questions/747959/sierra-e7455-mobile-broadband-modem

Yes, that's a load of nonsense.  As you found out. That method will
achieve nothing on any MC/EM74xx series modem. It's a copy of an answer
related to an older modem in a special dual configuration mode.  It's
completely pointless when taken out of that context.

I'm afraid the quality of the askubuntu.com site is very low, since it
enocurages answers from this kind of clueless users.  The site gives
credit for volume and don't penalize bogus answers. So you have a number
of users who cut'n'paste whatever google turns up, without understanding
any of it. I actually tried to correct that particular bogus answer
since it seemed like a waste of time for anyone to follow that advice.
But I apparently failed to follow the site policies, so my answer was
deleted.  Correctness is obviously not part of their policy :( My advice
is to use mailing lists instead, or google yourself.  But be aware that
Google hits are often polluted by these "ask" sites..  Annoying things
with no useful content of their own.

Well, back to the real problem and possible soution: You can flip the
modem into QMI mode by using a Sierra vendor specific QMI request, via
the QMI-over-MBIM service.  I don't think this is implemented in any
"proper" tools, but I have a perl script which can do it here:
http://git.mork.no/wwan.git/plain/scripts/swi_setusbcomp.pl

BIG FAT WARNING:  I would not have tried this if I were you.  The script
is never tested on any modem with an OEM vendor PID originally, AFAIK.

This is a Lenovo modem in a Lenovo laptop, right?  If not then I got the
rest wrong.  But in case that is correct:

We know Lenovo has a BIOS whitelist matching this modem.  We do not know
what that whitelisting code is looking for. It used to be simply
VID:PID, but this could have changed.  And if the BIOS doesn't like the
changes, then you will probably not be able to boot the laptop again
without removing the modem physically.  That's a least the way Lenovo
whitelisting used to work.

This means that if the whitelisting code fails to recognize the "new"
modem, then you are unable to go back using that laptop.  You have to
remove the modem and reconfigure it in some other system, which will
need a way to connect an m.2 modem and not have similar whitelisting
functionality.  And since we never tried this, we don't actually know
what it will take to go back either.  It's most likely just reversing
the change, but there could be more to it...

Actually, there is a good chance that the script I link to above doesn't
work for you at all.  I've seen several reports of users trying out
"customized PIDs" and not being able to go back because both the QMI and
AT "flipping" methods stopped working.  Which is probably a feature
since those PIDs are intended to be used in a specific mode - like MBIM
only.  This has been proven possible to work around by flashing a .nvu
file instead, but that's so far from supported land that I'm definitely
not going to recommend it.

I'd stay with the supported MBIM mode if I were you.  You can do most
QMI stuff via the QMI-over-MBIM service.  And networking is supposed to
work.  There isn't any reason to believe that whatever makes it fail in
MBIM mode won't make it fail in QMI mode too.  We need to understand the
problem.

Of course, if you have read this far and still are prepared to test it
out then I am going to do my best to assist on the way, on the hope of
learning something.  But don't say I didn't warn you :)


Bjørn