There are two fixes that can be committed separately, one is to set em_mac_min correctly. I've reviewed uses of em_mac_min, especially the boundary conditions, and think this is fully correct but would like another set of eyes on them.
The other is to set the packet buffer correctly for 82547 class chips.
PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236119
Reported by: Jeff Gibbons <jgibbons@protogate.com>