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

[Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>]

On Mon, 3 Dec 2007, Andrew Bird wrote:

> OK, I have the terminology wrong here. But what I meant to say is that if I 
> reduce the size of the URBs I submit as a test (noting your comment below), 
> the USB packet flow from the device stops at some point. If I reassemble 
> those USB packets together, I can see that they do not form a complete IP 
> packet, it is truncated. Hence my supposition that the USB packet flow 
> stopped from the device, not the IP packet flow into the device.

Yes, it does sound likely.

> > 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.
> Checking out the device its Bulk in and out endpoints are advertised as 64 
> bytes. Originally the URBs were set large enough to contain a full Ethernet 
> packet at over 1500 bytes. Presumably the USB core was coalesing those USB 
> packets either to fill my URB, or until an empty USB packet was sent 
> indicating end of data. In my test I reduced the size down to 64 bytes, so is 
> that OK?

It is.  Anything smaller than 64 bytes would be bad.

> > > 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.
> I was hoping there might be a generic USB level ping or get status command I 
> could send to its bulk-out endpoint, just to tickle the other end.

There are no generic Bulk-out commands in USB.  There are some generic 
Control requests (like Get-Device-Descriptor); they may or may not 
accomplish what you want.

Alan Stern


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