I have resumed trying to figure out why the newly written OpenChrome DRM code to support VIA Technologies VT1632(A) DVI transmitter is not working properly. In particular, why I get this weird color display issue.

For this, I used Wyse X90L mobile thin client and C00X thin client. Unfortunately, I got identical results. Out of desperation, I tried MSI Fuzzy CN700 mainboard that came inside MSI Axis 700. This one has CN700 chipset, which is basically P4M800 Pro chipset for VIA x86 processor. I bought this mainboard on eBay from a seller in Europe in Year 2016. I only bought the mainboard only with the fitting back I/O plate. While the color being displayed is still somewhat weird, I was able to finally get proper color display by manipulating CR9B[2:0]. These register bits apparently manipulates the clock output timing of DVP1 (Digital Video Port 1). I do not recall the existing OpenChrome DDX needing this hack for the DVI to display properly for this particular mainboard.

I am starting to suspect that I need to look into how the PLLs are being set up. I have observed that when I do register dump between OpenChrome DDX vs. DRM, I do notice the PLL values to be somewhat different. If I understand it correctly (and I can be wrong), OpenChrome DDX and DRM have different PLL calculation code. Maybe that’s why they behave differently when this DVI transmitter is in use.