980 Ti / MacPro 3.1 4k image too small. SwitchResX requires SIP disable from CMD R boot... no screen. Reinstall stock card?

[heffalumpy]

Just got a Geforce 980 Ti and 4k tv for my old MacPro 2008. 4k image is too small, and no controls for scaling, so trying to use SwitchResX, but it requires disabling SIP on OSX 10.11.6 (can't update this hardware beyond that) which requires CMD R at boot. Unfortunately, no screen comes up when I do this.

Is this because the 980Ti can't display the recovery mode boot screen? It is "mac flashed", and does display option boot for drive selection. Is something else going on, or am I going to need to reinstall the factory card just to access recovery mode just to disable SIP?

 

[heffalumpy]

Fixed the scaling issue. My new high speed HDMI cable arrived. Swapping that did nothing, but my new active DP to HDMI adapter came too, and when I come out of the DP port on the 980 TI into the adapter, 4k 60 hz shows up as the default option... and it's sized correctly.

It appears that the issue is some sort of limitation on the (supposedly) HDMI 2.0 output of the 980Ti. No matter. It's got 3 DP outputs, and I only actually need one.

Also someone in another forum pointed out a link to a blind fix for the SIP issue which I'll leave for anyone with similar issues who stumbles on this thread:
https://forums.macrumors.com/threads/cmp-getting-dell-5k-working-in-sierra.2026628/#post-25075208

Just need to figure out now how to output rec709 gamut in 8 bit at 4:4:4 as none of the color choices seem to be indicating the 4:4:4 part.