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Date:	Wed, 24 Oct 2012 23:19:07 +0200
From:	Bjørn Mork <>
To:	"Shawn J. Goff" <>,
	netdev <>
Subject: Re: qmi-wwan bug

"Shawn J. Goff" <> wrote:

>I've backported qmi-wwan to 2.6.39 (it's here: 
>, and it mostly 
>works, but I've come across a problem. The modem will sometimes stop 
>responding to any qmi data (but the AT commands on the TTY ports keep 
>working). This only happens when there is significant traffic flowing 
>through the device (downloading a large file) while at the same time,
>commands are sent to one of the TTY ports (I first noticed with my own 
>modem query program, but I can reproduce it using microcom to send 
>"ATI\r" in a loop). 

Using the tty ports should be completely independent of any qmi activity from the host perspective. I am tempted to claim this indicates a firmware bug.

> I see this problem with different devices from 
>different manufacturers. 

Which still may use pretty much the same firmware, although a little less likely.

> I've only made it happen on my kernel - I
>on 3.6.2, but it seems to not happen there.

Good. But I have a feeling that you switched more than just the kernel. Do you see the issue if you run your backport on the same hardware you tested 3.6.2 on?

 > I've also tried using a 
>similar modem that uses a different driver (sierra-net) and that
>have the same problem.

Well, that is an entirely different firmware application and driver, even if the hardware is similar or even identical.

>When it is in failure, if I try to ping an address, the system sends
>several an ARP requests but gets no response. To get the device to 
>respond again, I have to administratively set the wwan interface down, 
>then up, use libqmi to get the connection going again, then dhcp to get
>an address.

Which sounds like the connection died. Does QMI work at this point, or is that dead too?

>I also have some USB traces of the failure and recovery process. I'm
>familiar with CDC or QMI, so it's not yet clear to me exactly what's 
>happening, but it looks like the modem just stops sending anything on 
>its QMI endpoint for no reason.
>How might I dive further into the issue? So far, my next step is to
>into CDC and QMI and try to decypher the USB traces. If anyone is 
>interested, I can share a tcpdump or a USB trace.

I think a test of the backport on the same host hardware you run 3.6.2 on would be the best place to start.

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