quoted

That allows using standard properties in DT instead of vendor
specific "rx_delay"/"tx_delay". This results in a much better board
description and shows how far boards derive from the the standard
2ns (which can use rgmii-id without any extra delay specification).

Oh, your approach was also my initial thought.

I first asked whether it would be more appropriate to implement this
in the PHY or in the GMAC, and Andrew told me it should preferably be 
done in the PHY. But I'm not sure whether all PHYs support this
operation.

Andrew, would it be possible to implement this in the GMAC for cases
where the PHY does not support it? Thanks.

As i commented to one of the patches, the PHY will be adding 2ns by
default when 'rgmii-id' is used. If you read the RGMII standard, you
also see there is quite a wide tolerance for these delays. So unless
there is a badly designed PCB which needs something well away from
2ns, i don't think it is necessary.

     Andrew