The following reply was made to PR usb/156596; it has been noted by GNATS.

From: Colin Percival <>
To:, Hans Petter Selasky <>
Subject: Re: usb/156596: [ehci] Extremely high interrupt rate on ehci/uhci
 IRQ16 80% cpu utilization on CPU0
Date: Fri, 08 Jul 2011 17:26:37 -0700

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 Hi again,
 The attached patch seems to fix the problem while not breaking anything on my
 laptop.  It makes handles the case of EHCI_REMOVE_QH(sqh, last) with sqh == 
 and sqh->prev == NULL by setting last = NULL -- the idea being that if sqh is
 the only entry in the queue, we still ought to delete it even though it doesn't
 have a predecessor.
 I have no idea if this is correct, since I don't know the rest of the code in
 this file.  Maybe we should also delete the first entry in a queue even when it
 isn't also the last entry; maybe the problem lies somewhere else entirely.  I'm
 hoping that someone who knows this code will be able to figure out the right
 fix easily now that I have an apparently-working hack.
 Colin Percival
 Security Officer, FreeBSD | | The power to serve
 Founder / author, Tarsnap | | Online backups for the truly paranoid
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 --- sys/dev/usb/controller/ehci.c.orig 2011-07-08 16:48:10.000000000 +0000
 +++ sys/dev/usb/controller/ehci.c      2011-07-08 16:49:18.000000000 +0000
 @@ -1177,7 +1177,9 @@
                sqh->prev = 0;
 -      }
 +      } else if (last == sqh)
 +              last = NULL;
        return (last);
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