Re: [Libusbx-devel] usbdevfs: BULK_CONTINUATION flag does not work with XHCI controller

[Sarah Sharp <sarah.a.sharp@xxxxxxxxxxxxxxx>]

On Fri, May 25, 2012 at 04:54:01PM -0400, Alan Stern wrote:
> On Fri, 25 May 2012, Hans de Goede wrote:
> > Here is what I believe is happening, at 2 libusb splits the
> > bulk in transfer into 2 packets, 1 of 16k and 1 of 4k, setting the
> > USBDEVFS_URB_SHORT_NOT_OK flag on both and the
> > USBDEVFS_URB_BULK_CONTINUATION flag on the second.
> > 
> > With the EHCI controller all then works as intended, the first packet
> > transfers 36 bytes and returns a status of EREMOTEIO, the second
> > packet gets cancelled by drivers/usb/core/devio.c, transferring 0 bytes
> > and returns a status of ECONNRESET and we all live happily ever after :)
> 
> That's the expected result.
> 
> > With the XHCI controller however, the first packet transfers 36 bytes
> > and returns a status of EREMOTEIO, as it should, but the second
> > packet transfers the 13 next bytes and returns a status of ECONNRESET.
> 
> However that is wrong.
> 
> > IOW the second packet of the split bulk transfer reads more data
> > despite the first packet returning an error status and the
> > USBDEVFS_URB_BULK_CONTINUATION flag being set.
> > 
> > Thus the second packets has consumed the data the device had ready
> > which should have been read by the next bulk transfer from the guest
> > pov.
> > 
> > All in all it seems that the cancel of further packets done by
> > drivers/usb/core/devio.c: cancel_bulk_urbs()
> > comes too late when the device is on an XHCI controller, it seems that
> > the controller is already "executing" the next bulk transfer *before*
> > the completion handler of the previous one has completed.
> 
> No, the cancellation doesn't come too late.  Rather, the endpoint queue
> did not get stopped as it should when the EREMOTEIO error occurred.

Why would the xHCI endpoint queue get stopped after an EREMOTEIO error?
The upper layers never clear a halt on a short bulk packet, so the xHCI
driver has to restart the ring itself after a short packet.

Sarah Sharp
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