Re: USB net device unexpectedly stops sending driver bulk in urbs

On Mon, 3 Dec 2007, Andrew Bird wrote:

> > What do you mean by "instantaneously"?  How do you know the data isn't
> > stuck in a network buffer rather than a USB buffer?
> 
> Well I think it is the USB device, because if I lower the size of the receive 
> URB to less than the packet size, the device stops sending URBs mid packet.

This statement is confusing.  For one thing, devices don't send URBs -- 
they send packets.  (Drivers don't send URBs either; they submit them.)  
For another, what does it mean to stop sending packets in mid packet?

Bear in mind that if your driver submits receive URBs with a smaller
transfer length than the device expects, then your driver is ipso facto
at fault.  Any further errors you observe can't be blamed on the
device.

> My thought ATM is that it's the device that is at fault. Assuming it is for 
> the moment, and knowing that sending data along the bulk-out path stimulates 
> the receive path again, what would be a valid thing to send? Ideally I want 
> something that terminates on the USB device, and doesn't get pushed out to 
> the network. 

It's impossible to answer this question without knowing what data the 
device expects to receive on its bulk-out endpoint.

> Well I have tested variants of the driver on two platforms and see the fault 
> on both. Currently I'm trying to make it work on a Broadcom embedded MIPSel 
> device with 2.4.20 kernel. Previously I saw it on x86 with 2.6.22 kernel.
> The device is an Option UMTS card. It is a PCI expressCard that comes with a 
> cardbus caddy containing an ohci usb controller. The device is not one of the 
> usual usbserial type cards, but one that does PPP on the card itself, and 
> ships IP packets out to the host. The 2.6 driver is available from 
> http://www.pharscape.org/component/option,com_forum/Itemid,68/page,viewtopic/t,350/

If you're trying to debug a difficult problem, you should make the rest 
of your environment as trouble-free as possible.  That means using a 
standard host architecture (like x86) with a standard USB host 
controller (not something built into a cardbus caddy), and using the 
most recent and most bug-free kernel you can find (like 2.6.23.x).

Alan Stern


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