copper NICs, a link change event is posted whenever MII autopolling is
toggled off and on, which happens whenever someone calls
bge_miibus_readreg() or bge_miibus_writereg() to access the PHY
registers. This means anytime someone called the SIOCGIFMEDIA ioctl
on a bge interface, the link would reset. Even a simple "ifconfig bge0"
would do it, though other apps like dhclient or the PPPoE daemon could
trigger it as well. An obvious symptom of this problem is lots of
"bgeX: gigabit link up" messages appearing on the console for no
apparent reason.
Through experimentation, I determined that when a real link change
event occurs, the BGE_MIMODE_AUTOPOLL in the BGE_MI_MODE register
is always set, so now if we have a copper NIC and an link change
event occurs and the BGE_MIMODE_AUTOPOLL bit is clear, we ignore
the event.
Note that this does not apply to the original BCM5700 chip since we
use a different method for sensing link changes with that chip (the
status block method was broken), nor to fiber optic NICs since they
don't use the GMII PHY access registers.
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