Re: kern/46596: ehci interrupt storm

[Andreas Gustafsson <gson%gson.org@localhost>]

The following reply was made to PR kern/46596; it has been noted by GNATS.

From: Andreas Gustafsson <gson%gson.org@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: Nick Hudson <skrll%netbsd.org@localhost>
Subject: Re: kern/46596: ehci interrupt storm
Date: Tue, 17 Sep 2013 21:19:59 +0300

 A short while ago, I wrote:
 > I have no idea what triggers the transition.
 
 I do now, after finding some discussion about a similar bug in
 FreeBSD:
 
   http://www.freebsd.org/cgi/query-pr.cgi?pr=156596
   http://forums.freebsd.org/showthread.php?t=24952
 
 The crucial hint came from the FreeBSD user "starslab" who noticed
 that the interrupt storm started when pulling the VGA connector from
 the back of the machine.
 
 My machine is connected to a KVM switch, and I have now determined
 that the interrupt storm consistently starts when that machine's VGA
 output is selected for display.  If the machine is already selected at
 boot time, the storm starts at boot, and if not, the storm starts when
 setting the KVM switch to display the machine's VGA output.  This
 happens even if the USB cable used for the K and M in KVM is not
 connected, so it's clearly triggered by some signal transition on the
 VGA port, not the USB port.
 
 I suppose this is kind of a reverse Heisenbug: it only appears when
 the machine's VGA output is observed.
 -- 
 Andreas Gustafsson, gson%gson.org@localhost