linux-kernel - Re: Allocating buffers for USB transfers (again)

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Date:	Wed, 10 Aug 2011 16:15:52 -0700
From:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
To:	Daniel Mack <zonque@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Florian Mickler <florian@...kler.org>,
	Oliver Neukum <oliver@...kum.org>, linux-usb@...r.kernel.org,
	alsa-devel@...a-project.org, Takashi Iwai <tiwai@...e.de>,
	Clemens Ladisch <clemens@...isch.de>, pedrib@...il.com,
	William Light <wrl@...est.net>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org, Robert Hancock <hancockrwd@...il.com>
Subject: Re: Allocating buffers for USB transfers (again)

On Wed, Aug 10, 2011 at 05:33:02PM +0200, Daniel Mack wrote:
> On 08/10/2011 04:32 PM, Alan Stern wrote:
> >Looking at the driver's current code, it appears that your patch
> >does not fix the bug properly.  Using discontiguous regions in the
> >transfer buffer is perfectly okay.  The real problem is later on,
> >where you do:
> >
> >if (send_it) { out->number_of_packets = FRAMES_PER_URB;
> >
> >This should be
> >
> >out->number_of_packets = outframe;
> >
> >The way it is now, the USB stack will try to use data from all the
> >frame descriptors, and the last few will be stale because the loop
> >doesn't set them.
> 
> That's actually true, even though it doesn't seem to cause any trouble.
> I tested everything here of course, and the output URBs return back from
> the USB stack with their length fields zeroed out, which then
> causes the stack to send packets with zero-length fields at the end.

Actually, it causes system hangs when the driver is loaded on a device
attached to a USB 3.0 port, as Alan Stern pointed out:

https://bugzilla.kernel.org/show_bug.cgi?id=40702

Please don't submit zero-length transfers.  The xHCI driver just isn't
able to handle it.  Arguably, it probably should have just rejected your
URB when it found a zero length buffer, so I'll probably be submitting a
patch to fix that.

Sarah Sharp
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