mbim, signal quality atds_signal_query vs. signal_state_query

[Dan Williams]

On Mon, 2023-01-02 at 13:40 +0100, Aleksander Morgado wrote:
> Hey,
> 
> > quite a while ago, I reported inconsistencies with Telit LE910Cx
> > signal quality query when using mbim (see
> > https://www.mail-archive.com/modemmanager-devel@lists.freedesktop.org/msg06990.html
> > ). Now I had to get back to this issue, since our customers heavily
> > complain about the bad signal quality shown (although the
> > connection is good and fast).
> > 
> > While trying to find a solution I stumbled across the
> > atds_signal_query message and experimentally used it instead the
> > signal_state_query mbim message. The result is, that the signal
> > quality is now much higher and matches what we see when not using
> > mbim (and matches what we see on the same location using other LTE
> > modems).
> > 
> > So my questions are:
> > 
> > Is there any documentation about the atds_signal_query command? I
> > did not find any.
> 
> ATDS is an AT&T specific extension that most manufacturers implement.
> I truly don't recall where I found it, but it is likely somewhere out
> there.

I originally got it from the Wireshark MBIM dissector:

https://github.com/freedesktop/libmbim/commit/1f86135579311bb22d391bde39024f8fb1d014b8

which was based on "AT&T Windows 8 Extended API Requirements" which is
no longer easily available (that I can find).

https://github.com/wireshark/wireshark/commit/6c0972bcd6e76bb4e22f729463ccfa5dad84eaff

I don't think ATDS really advanced much so standard MBIM is probably a
better bet these days.

Dan

> 
> > Do I have to expect problems when I prefer using atds_signal_query
> > over signal_state_query
> 
> If you're doing a query, operation-wise it should be equivalent.
> 
> > How to submit that change to modemmanager so that it can be used by
> > others?
> 
> Please note that you can also enable the "extended signal quality
> support" in the Signal interface. If you do that on a MBIM capable
> modem, it will default to use either Microsoft extensions service or
> otherwise the ATDS service. So there is already some support there if
> you want to use it.
> 
> If you would like to change the default signal quality loading logic
> in MBIM so that it uses ATDS instead of the original MBIM service,
> this one is trickier, because, which one would you use? What if some
> other modem has the original MBIM command working better than the
> ATDS
> one? The ATDS one also lacks support for threshold configuration so
> no
> indication support, it is exclusively a polling-based operation which
> isn't ideal. I'd suggest you try with the Signal interface first to
> see if that fits your needs.
> 
> Also, why would those values be so different? This is something Telit
> should probably investigate.
> 
> > Should the command be used for this particular modem only, for all
> > telit modems or even for all mbim modems? (In the patch I currently
> > use, I use that command on all mbim modems that have the command
> > available)
> > 
> 
> Is the patch completely removing the original method? Or do you apply
> the ATDS method instead of the original one for your device?
> Something
> like this could be gated via a new udev tag that modifies the default
> behavior (e.g. see ID_MM_QMI_CONNECTION_STATUS_POLLING_ENABLE for a
> similar thing with a different default behavior change in QMI
> modems).
>