Hi Stefan
Setting the skews in DT would indeed be preferable in my opinion.
+1 from me.
Agreed. I'll focus on a devicetree based implementation, and editing phy values during board debug and bringup can be addressed with debug/development notes. I'd be willing to publish these somewhere with some suggestions about the best place for something like this to reside based on our experience.
You can do that with the 'mii' command as well I think, but I might be wrong.
Yes. For testing or board bringup this might really serve. Even though this setting via environment as proposed from Vince is more elegant and less hackish. And easier to adjust/tune for "normal users".
This was the intent of having environment variables - to make trying different skew values easy for our customers. Not all developers/users are Uboot experts and need help from time to time. I think this can also be addressed by writing use case notes on how to edit phy registers using the mii command for debug/bringup purposes.
The default values should come from the DT, once this is all in place. But I think that for initial board bringup / testing such a method, to override those values via environment variables can be quite helpful.
After reading through most of the comments, I agree with Marek it's possible (and probably best) to support overriding phy registers through use of the mii command. Non expert users (such as hardware developers bringing up a new board) could make use of notes to help them know how to try different settings.
All the best!
Vince