On Mon, 17 Jul 2006, Aras Vaichas wrote:

> Eventually they ran the test with the laptop disconnected and then 
> reconnected 
> our device to see if it still worked after each type of test. It took a while 
> longer, but we passed.

That's good.

> >>* Is there something in the USB host management code that ignores a USB 
> >>device 
> >>which is acting strangely?
> > 
> > Yes, there is.  If a device fails to enumerate after 4 attempts, the USB 
> > stack will ignore it until a disconnect event occurs.  However, at each 
> > reconnect everything should restart from the beginning.  So when those 
> > electrical pulses stopped, the device should have been enumerated 
> > correctly.
> 
> Is that in the USB specification?

No, it's just the way the Linux USB stack is written.  Other operating 
systems will behave differently.

> > It would be better to tackle the shortcomings of the host by fixing the 
> > host's software.  To start with, you should compile the host's kernel with 
> > CONFIG_USB_DEBUG and then post the host's system log for the test period.
> 
> I'll try and include some form of host logging for the next barrage of tests 
> if 
> I can. I couldn't do anything this time because we couldn't delay the tests 
> and 
> miss our pre-booked (and expensive) testing window.
> 
> I'll try and think of a way to simulate this test in the office if I can. I 
> think the test was a short 1000V pulse, every 300ms on the power line. It 
> must 
> have caused the ground to bounce around relative to the laptop which detected 
> it as a USB disconnect (I guess). I could probably rig up something similar 
> that applies the same principle to the USB cable (not with a 1000V though!).

These rapid repeated disconnects are a kind of stress testing the Linux
USB host code is hardly ever exposed to.  It's not surprising that 
eventually it would fail.

Alan Stern



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