Here's POWER's training code for DDR4:

https://git.raptorcs.com/git/talos-hostboot/tree/src/import/...

pgeorgi has a valid point in that if you go for the cheapest off the shelf building block type DDR4 solution for your silicon design (won't name names here, but it's a widely known vendor in the silicon block space), those controllers come with mandated binary-only firmware. IBM (and apparently Marvell?) both didn't use that cheap off the shelf solution and also decided to release their training code. Kudos to both companies for bucking the trend here!