libusb usb_interrupt_read hangs under FreeBSD

Hans Petter Selasky hselasky at
Tue Jul 17 05:56:10 UTC 2007

On Monday 16 July 2007 17:44, Xiaofan Chen wrote:
> On 7/15/07, Hans Petter Selasky <hselasky at> wrote:
> > > 2) For the host, how does it know that the buffer data is still correct
> > > if the buffer is not cleared?
> >
> > Clear stall should only clear the data toggle!
> >
> > You need a second control command to reset the buffers and/or the
> > protocol!
> >
> > > 2) What cause the stall to happen in the first place?
> >
> > It is either a wrong data-toggle bit or a protocol error. The device can
> > send stall at any time!
> Thanks a lot for the detailed explanation.
> If it is a protocol error for the control endpoint 0 (EP0), the host will
> not need to send a clear stall feature request to EP0. 

Yes, that is correct, because the data toggle is reset to 0 starting at the 
SETUP packet which uses a separate PID.

> Even if it is sent 
> (shall we consider it a bug of the USB stack if that is the case?), the
> current PICkit 2 firmware will filter out it and ignore it.

No, that is not a bug.

> So I think we can narrow it down to the wrong data-toggle bit. I will
> dig further. I'd like to convince the PICKit 2 firmware developer
> that something is wrong even though it is now working under
> FreeBSD. Could we see the reason for the stall from the following
> USB log?

Imagine you abort your program and start it again. Then outstanding data in 
the device FIFOs should be removed first! Then you clear the stall. Clearing 
of stall, should not clear the data in the buffers! These two events are 

> I am using the alternative stack from Hans and 6.2 Stable version. So
> maybe there is a difference here.

With some modifications.


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