[Bug 265714] igc(4) drops link under high traffic

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=265714

            Bug ID: 265714
           Summary: igc(4) drops link under high traffic
           Product: Base System
           Version: 13.1-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: mike@sentex.net

Created attachment 235773
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=235773&action=edit
diff of output of sysctl dev.igc before and after link drop

Using a combo of netperf3 on 2 back to back RELENG_13 machines, the link will
drop both at 1G and 2.5G link speeds.  Trying without FC does seem to reduce
the instances of the drop, but it still happens.
Running a simple shell script to do

TARGET=10.1.255.168
I=/usr/local/bin/iperf3
REPDIR=/var/tmp/reports

while true 
do
d=`date "+%s"`
$I -t 60 -c $TARGET
sleep 2
/sbin/sysctl -a dev.igc.1 > $REPDIR/${d}a
$I -P4 -t 60 -c $TARGET
/sbin/sysctl -a dev.igc.1 > $REPDIR/${d}b
sleep 2
$I -R -t 60 -c $TARGET
/sbin/sysctl -a dev.igc.1 > $REPDIR/${d}c
sleep 2
$I  -R -P4 -t 60 -c $TARGET
/sbin/sysctl -a dev.igc.1 > $REPDIR/${d}d
done

will result in random drops of the link

igc1: link state changed to DOWN
igc1: link state changed to UP
igc1: link state changed to DOWN
igc1: link state changed to UP
igc1: link state changed to DOWN
igc1: link state changed to UP


its not clear if its the receiver or sender thats dropping it as the 2 machines
are on xover cable. Discussion at 
https://lists.freebsd.org/archives/freebsd-stable/2022-August/000835.html

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