To take the model I understand: SAS; the links are managed at the phy
level, so the power policy should be set there and thus should probably
be a property of the phy object, which doesn't even exist in the SCSI
model, it only exists in the transport class. It strikes me that even
for ATA, the same thing is probably true.
Now, I can see that the power management models of all the transports
might share some similarities (particularly at this three stage granular
level); if so, it might make sense to export helpers from the mid-layer
for the transport classes to use for this.