Hi Laurent,
I think this issue has been happening for a while now, but my recent
patches to remove most of the xHCI debugging have finally allowed me to
use a webcam under xHCI with debugging on. Unfortunately, it doesn't
work very well.
When I plug in a webcam under an xHCI host controller in 3.0-rc3+
(basically top of Greg's usb-linus branch) with xHCI debugging turned
on, the host controller occasionally cannot keep up with the isochronous
transfers, and it tells the xHCI driver that it had to "skip" several
microframes of transfers. These "Missed Service Intervals" aren't
supposed to be fatal errors, just an indication that something was
hogging the PCI memory bandwidth.
The xHCI driver then sets the URB's status to -EXDEV, to indicate that
some of the iso_frame_desc transferred, and sets at least one frame's
status to -EXDEV:
static int skip_isoc_td(struct xhci_hcd *xhci, struct xhci_td *td,
struct xhci_transfer_event *event,
struct xhci_virt_ep *ep, int *status)
{
struct xhci_ring *ep_ring;
struct urb_priv *urb_priv;
struct usb_iso_packet_descriptor *frame;
int idx;
ep_ring = xhci_dma_to_transfer_ring(ep, le64_to_cpu(event->buffer));
urb_priv = td->urb->hcpriv;
idx = urb_priv->td_cnt;
frame = &td->urb->iso_frame_desc[idx];
/* The transfer is partly done */
*status = -EXDEV;
frame->status = -EXDEV;
/* calc actual length */
frame->actual_length = 0;
/* Update ring dequeue pointer */
while (ep_ring->dequeue != td->last_trb)
inc_deq(xhci, ep_ring, false);
inc_deq(xhci, ep_ring, false);
return finish_td(xhci, td, NULL, event, ep, status, true);
}
The urb->status causes uvcvideo code in uvc_status.c:uvc_status_complete() to
fail with the message:
Jun 15 17:37:11 talon kernel: [ 117.987769] uvcvideo: Non-zero status (-18) in video completion handler.
It doesn't resubmit the isochronous URB in that case, and the userspace
video freezes. If I restart the application, the video comes back until
the next Missed Service Interval event from the xHCI driver. Ideally,
the video driver would just resubmit the URB, and the xHCI host
controller would complete transfers as best it can. I think the frames
with -EXDEV status should be treated like short transfers.
I've grepped through drivers/media/video, and it seems like none of the
drivers handle the -EXDEV status. What should the xHCI driver be
setting the URB's status and frame status to when the xHCI host
controller skips over transfers? -EREMOTEIO?
Or does it need to set the URB's status to zero, but only set the
individual frame status to -EXDEV?
Sarah Sharp
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