# ethtool --show-loopback eth0
Loopback endpoints for eth0:
  id  owner  depth  direction          name       enabled
   0  mac       1   host->loop->host   mac        off
   1  phy       2   host->loop->host   phy-pcs    on
   2  module    3   line->loop->line   cmis-far   off

# Enable endpoint 2
ethtool --set-loopback eth0 id 2 on

We have to be careful about what is ABI here. Initially you plan to
implement module, so you will have:

# ethtool --show-loopback eth0
Loopback endpoints for eth0:
  id  owner  depth  direction          name       enabled
   0  module    1   line->loop->line   cmis-far   off

ethtool --set-loopback eth0 id 0 on

And then somebody implements loopback at the mac:

# ethtool --show-loopback eth0
Loopback endpoints for eth0:
  id  owner  depth  direction          name       enabled
   0  mac       1   host->loop->host   mac        off
   1  module    2   line->loop->line   cmis-far   off

You script doing

ethtool --set-loopback eth0 id 0 on

Suddenly does something else.

Is this an ABI break? How do we make this reliable so implementing
more loopbacks at different levels does not change how you use
--set-loopback?

	Andrew