[p750mmx]

I see the 3D remark only for the 1080p/720p resolutions.

Kodi says "Hardware based" when playing a MVC video? When starting a video and when going to 3D info during playing.

I also got the CSL adapter that works for you, but it gives me only black picture (does not work). Likewise this second PS8121E adapter.

So, my theory is that the CSL adapter is actually passive and therefore only working with a DP++ (dual-mode) port, in which case that port effectively is HDMI and the adaptre just maps the voltage levels. And on my PC the port does not support DP++ hence the black picture.  Your PC manual shows DP++ port symbols on the rear panel picture, so i guess it does support DP++.

It would be interesting to know if your active adapter also works for 3D. It could either not work as in my case, or it could work, but effectively the adapter also switches to passive, or it could work and actually do actively convert DP to HDMI with 3D, but then i am at a loss what else could be different on my PC why its not working.

Did you try the active adapter and if so to which result ? Your mail wasn't fully conclusive on that.

Oh well, i'll have to try on different PCs to see what difference it could make. This PC itself was bought for the HDMI 1.4a 3D TV, so i don't worry too much about it, but i wouldn't want to buy another one without figuring this out first. Next kodi box should really do 3D and 4k/HDR from the hardware - ideally via a single HDMI port. Maybe i should try one of them ugly Intel NUC with fixed up LSPCON firmware... *sigh*. 

Excuse for the delay

The DP = type++, as I wrote in my reaction regarding the HP TFF PC. I used "the other one" adapter, the one that's not clear on being active or not. But as I wrote before, that doesn't matter if the connection is to an HDMI 1.4 port, it only matters when connected to an HDMI2.x port and one wants to get 2160p/60Hz resolution as an output.

To test if the DP++ port does anything on the Hardware Based 3D option in Kodi, I connected another Kaby Lake system (with Pentium G4560), to my 3D test monitor that has a "normal" DP1.2 port. I used the same doubtful active adapter with this and Hardware Based is still an option and MVC media is still playing fine (and shows Hardware Based under info when playing).
So no difference between DP1.2 and DP1.2++ when connected to a HDMI 1.4 port.