Cleanroom reverse engineering for the purpose of publishing new driver code, to avoid legal/IP minefields, is super-expensive. It should be a much narrower scope to determine whether a binary blob's actions are limited to memory training, since there is no requirement to publish reusable source code.

Looks like Collabora is already monitoring the blob, so it's not entirely a mystery:

  At the moment of writing this article, we have identified a few differences from the binary blob previously used, which we can highlight as following:

  Binary BL31 contains some unknown code to get HDMI-RX PHY access working.
  The cpufreq support in binary BL31 is different from TF-A. 

  There could be more issues that are unknown at the moment and users should be aware of it.