[Bug 1735134] Re: ModemManager uses a wrong plugin for Dell DW5818/5819

[Łukasz Zemczak]

I would say this is much more fitting for an SRU than the big backport.
I would propose trying to release this anyway as the backport can take
longer to get prepared and then reviewed - big upstream releases as SRUs
are never easy to handle.

Looking at the debdiff, it's mostly ok although we would need a DEP-3
header on the patch [1] to give more background on the change that's
being made. Also, the 'Non-maintainer upload' changelog entry is not
required in Ubuntu. Could you fix those and re-attach the patch? Thanks!

[1] http://dep.debian.net/deps/dep3/

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Title:
  ModemManager uses a wrong plugin for Dell DW5818/5819

Status in modemmanager package in Ubuntu:
  New

Bug description:
  [Impact]

  * Dell Wireless DW5818/5819 modems showed an incorrect signal strength
  and were using a ttyUSB* port for data connections instead of the MBIM
  device (which provides better performance).

  Since linux-4.4.0-98, the kernel additionally loads gcserial driver
  for Dell Wireless DW5818/5819. The reason behind it is to support
  firmware switching and upgrading. However, the change makes
  ModemManager use Gobi plugin for this two modules. With Gobi plugin,
  the modules could establish data links, but it failed to retrieve the
  signal state. And it caused the mmcli and nm-applet giving wrong
  signal strength. The modules support the MBIM protocol, so
  ModemManager should use Dell plugin for these two modules.

  I have worked out a patch to forbid these two modules in Gobi plugin,
  and it does work well.

  [Test Case]

  Current MM:
  * Create connection with 
  $ nmcli c add type gsm ifname ttyUSB2 con-name gsmconn apn <operator APN>
  * Without the patched package, mmcli shows, with an active connection (see comment #2):
      primary port: 'ttyUSB2'
      signal quality: '0' (recent)

  Patched MM:
  * Create connection with 
  $ nmcli c add type gsm ifname cdc-wdm0 con-name gsmconn apn <operator APN>
  * With the patched package, mmcli shows, with an active connection (see comment #7):
      primary port: 'cdc-wdm0'
      signal quality: '38' (cached)

  [Regression Potential]

  The patch simply adds the Sierra modems VID/PIDs to the list of
  forbidden ids in the Gobi plugin, so the possibility of a regression
  is very small: only products with said VID/PID will be affected.

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