u-blox plugin, questions while implementing support for TOBY-L210

[Ulrich Mohr]

Hey,

I am using ModemManager with u-blox TOBY-L210 for a while now with older 
versions of ModemManager (1.7.x). I did some patches and home-brew code 
to make that combination work.
I now want to switch to newer versions of MM (1.18), benefit from the 
good work on MM, and do it "the right way" this time, so I can 
contribute to MM :-)

I tested the new version without modifications and found two "problems":

  * The default bearer interface is not implemented for ublox modems
  * The device does only sometimes get a LTE connection, sometimes it
    doesn't.

While the reason for the first problem is quite obvious ;-), I did some 
analysis on the second one and found the reason why it does not work:
Whenever there is a PDP context matching the APN, besides the context 
with CID 4 -- which is used by the modem to automatically create a 
connected bearer when it can connect using the default EPS bearer -- MM 
selects that PDP context and tries to establish a connection using it. 
But since there is already a connected context with CID 4, connecting a 
secondary context to the same APN may fail (depending on the SIM card, 
due to MNO restrictions I guess, see 
https://www.u-blox.com/sites/default/files/LTE-initial-default-bearer_AppNote_%28UBX-20015573%29.pdf). 

Now the question is why there is a second PDP context when trying to 
connect. The answer is that this context has been created by MM when it 
tries to connect while the default bearer is not connected: CID 4 is 
unused when the initial bearer is not connected, so MM creates a second 
one that gets in the way when we later on try to connect a connection 
and the initial bearer is there again

Therefore I did some modification in MM to make the Toby work:

  * I implemented the the initial EPS bearer setting interface
    (functions  set_initial_eps_bearer_settings*,
    load_initial_eps_bearer_settings*) similar to the icera plugin. I
    used the at+ucgdflt command for reading and writing the initial EPS
    bearer setting (this command is available on Toby-L2 and MPCI-L2 only).
  * I implemented the profile list interface (functions list_profiles*)
    to prefer CID 4 when MM tries to reuse a PDP context for connecting

I got it up and running. But I want to push that changes into MM, so 
there are some open questions regarding the implementation I want to 
discuss.

  * The Toby L210 does not allow to read back the username to use for
    the initial bearer. Therefore, comparing the username after writing
    the initial bearer fails. I omitted comparing the username for now
    when comparing the previous EPS bearer settings with the new ones.
    This change is not in the ublox plugin, so I doubt this is a good
    idea. How to handle this properly?
  * The ucgdflt command is not available for other ublox modems, so the
    implementation I did only applies to Toby-L2 and MPCI-L2. This
    information must be gathered somehow. I see two possibilities: using
    the at+ucfgdlt=? to test the availability of the command (when would
    I do that?), or introduce a flag similar to the ubandsel flag inside
    the band configuration struct. What would be the method to prefer?
  * Is implementing the profile list interface the right approach? The
    fact that MM selects an already activated PDP context (cid 4), tries
    to deactivate it (which fails, but is ignored), tries to activate it
    again (which fails, but is ignored as well) and then uses it
    "smells" and indicates to me that this might not be the right
    solution. A better approach might be that MM accepts using an
    already activated PDP right away without trying to disconnect and
    connect again, but this would mean that there are massive changes
    required in MM to support that. What is your opinion on that?

I found other small issues during work (build warnings, const issues 
etc), but I guess this email is long enough already, so I postpone that 
.... ;-)

I really look forward for any answers and ideas.

I hope I managed to describe the problem and my solution so that one can 
understand it, otherwise just ask. I can also provide the code with 
changes I did....)

-- 
Best regards / Mit freundlichen Grüßen / Salutations distinguées

Ulrich Mohr

SEMEX-EngCon GmbH
Carl-Merz-Strass 26
76275 Ettlingen
email:  u.mohr at semex-engcon.com
___________________________________________
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Commercial register: Mannheim, HRB 718881
Company domicile: Ettlingen

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