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Re: kern/46596: ehci interrupt storm
The following reply was made to PR kern/46596; it has been noted by GNATS.
From: Andreas Gustafsson <gson%gson.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
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
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