From owner-freebsd-hackers  Sun Nov 16 00:42:53 1997
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To: Mark Mayo <mark@vmunix.com>
cc: hackers@FreeBSD.ORG
Subject: Re: de underflow errors. huh? 
In-reply-to: Your message of "Sun, 16 Nov 1997 00:17:10 EST."
             <19971116001710.02627@vmunix.com> 
From: David Greenman <dg@root.com>
Reply-To: dg@root.com
Date: Sun, 16 Nov 1997 00:45:44 -0800
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>I posted this questions 2 weeks ago on -questions, and 1 week ago
>on -stable (since it's happening on a -STABLE machine..) but I
>haven't got a response yet. So here it is for the -hackers crowd:
>
>I've got a firewall machine with 2 ethernet cards. One Digital 21040
>based card on de0, and one Intel Pro/100B on fxp0. Everything seems
>to be working fine, with the exception of numerous errors on the
>console:
>
>de0: abnormal interrupt: transmit underflow
>
>I have no clue what this means. Packets seem to be flowing through
>the interface nicely, and there is no noticeable packet loss.
>If anyone has any ideas what could be causing this, or if I should
>give a hoot, please let me know.

   It's indicating that the PCI bus was sufficiently busy enough to cause
the transmit DMA to be stalled too long. The proper response to this condition
in the device driver it to dynamically increase the transmit threshold (the
number of bytes that are DMA'ed onto the card before the transmission is
actually started on the wire). The fxp driver does this, the de driver
doesn't. I think Matt Thomas might have fixed this in a later rev., but I'm
not sure about that.
   Anyway, other than the annoying console messages and perhaps a packet
drop when it happens, the problem can be ignored.

-DG

David Greenman
Core-team/Principal Architect, The FreeBSD Project