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From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Subject: Re: uvcvideo failure under xHCI
Date: Thu, 16 Jun 2011 10:07:49 +0200
Cc: linux-media@vger.kernel.org, linux-usb@vger.kernel.org,
	Andiry Xu <andiry.xu@amd.com>,
	Alan Stern <stern@rowland.harvard.edu>
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Hi Sarah,

On Thursday 16 June 2011 04:59:57 Sarah Sharp wrote:
> On Wed, Jun 15, 2011 at 06:39:57PM -0700, Sarah Sharp wrote:
> > 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:
> ...
> 
> > 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.
> 
> ...
> 
> > 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?
> 
> Ok, looking at both EHCI and UHCI, they seem to set the urb->status to
> zero, regardless of what they set the frame descriptor field to.
> 
> Alan, does that seem correct?

According to Documentation/usb/error-codes.txt, host controller drivers should 
set the status to -EXDEV. However, no device drivers seem to handle that, 
probably because the EHCI/UHCI drivers don't use that error code.

Drivers are clearly out of sync with the documentation, so we should fix one 
of them.

> I've created a patch to do the same thing in the xHCI driver, and I seem
> to be getting consistent video with xHCI debugging turned on, despite
> lots of Missed Service Interval events.

-- 
Regards,

Laurent Pinchart