[alsa-devel] Allocating buffers for USB transfers (again)

Daniel Mack zonque at gmail.com
Thu Aug 11 02:57:41 CEST 2011

On Thu, Aug 11, 2011 at 1:15 AM, Sarah Sharp
<sarah.a.sharp at linux.intel.com> wrote:
> 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

Yes, I've noticed this.

> 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.

According to the spec, sending zero-length frames should be fine, no?
Is there any particular reason why XCHI can't handle this while EHCI
can? And does my patch fix the driver for XHCI?


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