Just adding to this....

The code we (myself and Rados) are running here includes the latest EC21 patches for the DTR quirk.

It seems to get one command through but flunks on subsequent calls.  Some of the other libqmi commands are failing -get-manufacturer isn't working either.

While we suspect the issue is actually in the Linux kernel driver, We're at a bit of a dead end on this one atm, any tips for where to look?

Steps to replicate on Quectel EC21

After build, have tried:
modprobe -r qmi_wwan
modprobe qmi_wwan
ifconfig wwan0 down
echo Y > /proc/..../ -> enable raw_ip on wwan0
ifconfig wwan0 up

After this the interface looks like this:
wwan0     Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00                           -00
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:17 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:3803 (3.7 KiB)

Prior to the raw_ip mode, the interface looked like this:
wwan0     Link encap:Ethernet  HWaddr 9e:ec:13:2x:xx:xx
          inet6 addr: fe80::30c6:7fc2:c62:a1a2/64 Scope:Link
          UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:24 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:6127 (5.9 KiB)

And libqmi is timing out again.  It pretty much locks up completely, I have to use modprobe -r qmi_wwan to regain control.

(In reply to barfle from comment #1)
> Just adding to this....
> 
> The code we (myself and Rados) are running here includes the latest EC21
> patches for the DTR quirk.

That means an unmodified v4.12-rc1 kernel at the moment. But from the way you word this, I get the impression that you could be running something else.  Please clarify.

There have been some important fixes to the cdc-wdm driver lately, for bugs which could produce symptoms similar to what you describe.

The most recent we have tried so far is:

Linux raspberrypi 4.11.0-v7+ #1 SMP Mon May 15 13:09:57 BST 2017 armv7l GNU/Linux

That's the latest Linux build available for Raspbian (this is a Raspberry Pi Compute Module device on a custom mainboard).

We're going to try either to backport some of the cdc and qmi_wwan patches from 4.12, or to build create our own 4.12 branch of the raspbian build, maybe that helps.

(In reply to Bjørn Mork from comment #4)
> (In reply to barfle from comment #1)
> > Just adding to this....
> > 
> > The code we (myself and Rados) are running here includes the latest EC21
> > patches for the DTR quirk.
> 
> That means an unmodified v4.12-rc1 kernel at the moment. But from the way
> you word this, I get the impression that you could be running something
> else.  Please clarify.
> 
> There have been some important fixes to the cdc-wdm driver lately, for bugs
> which could produce symptoms similar to what you describe.

@Barfle has backported your commit from 4.12 to 4.11: https://github.com/torvalds/linux/commit/19445816996d1a89682c37685fe95959631d9f32 . It solved the issue we've reported. Raspberry Pi kernel developers started catching up to 4.12.y branch yesterday and the above commit is included. The issue is now solved and it can be closed. Thank you for helping out @Bjorn.