it would be nice to reuse the same mapping that is common everywhere. I
suspect the PHY code already has some mechanism to support device
specific since not all PHYs support all link modes either....

phydev->supported is initialised with all the link modes a PHY
supports. The MAC driver when gets a chance to remove any it does not
support,

phylink has similar concepts, the MAC, PHY, and SFP declare what they
each support and then it all gets combined to resolve to a link mode,
to use.

So it should be possible to make this generic with the help of the MAC
driver indicating a mask of what is actually supports.

       Andrew