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    SubjectRe: Allocating buffers for USB transfers (again)
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    On Wed, 2011-08-10 at 16:15 -0700, Sarah Sharp 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
    >
    > 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.
    >

    I think queue a zero-length TRB to xhci host is OK. I've not tested it,
    but the issue here seems is caused by td->last_trb = NULL. Check
    count_isoc_trbs_needed(), num_trbs will be 0 if the packet length is
    zero and (addr & (TRB_MAS_BUFF_SIZE - 1)) is zero. We can not return
    num_trbs as 0 to xhci_queue_isoc_tx(), which caused a td added to ep's
    td list, while it's not actually queued to ep ring and last_trb is not
    set.

    In order to avoid this, we just make sure count_isoc_trbs_needed()
    always return 1 or larger numbers, instead of reject the urb. Is that
    feasible?

    Thanks,
    Andiry




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