Something like:

  struct {
     enum type;   // MAC, PHY, SFP
     int type_id; // if type=PHY - phy id
     int depth;   // counting from CPU, first loopback opportunity is 1
                  // second is 2, etc.
     bool direction; // towards CPU/host vs towards network
     char name[16];  // "pcs", "far", "near", "post-fec", whatever
  }

Lets see what comes from the drawing board, but i was more thinking
about expanding the bitmap this proposal already has, extending it to
other layers. As use cases are implemented, we define the bits needed
in the map. The ethtool kAPI has the needed infrastructure to map bits
to names, it is used for link modes etc, and that can be used here. So
the ethtool(1) part should be reasonably generic.

    Andrew