I've got a 2009 Mac Pro (5.1 flashed) with a Sapphire Radeon Pulse RX 580 8GB GPU that I've acquired to support Mojave. The configuration is fortunately stable since the end of 2018 and has worked as expected with my venerable 30-inch Apple Cinema Display.
However, after a decade of use, the screen was showing signs of pixel fatigue. Today, I have unpacked an LG 34BK95U-W 34 "5K monitor and connected it to the RX 580 via DisplayPort.
- Although the graphics processor and monitor apparently support 5120 x 2160 formats via DisplayPort 1.4, I can not achieve a resolution greater than 3440 x 1440.
- Strangely, I have to set the display on DisplayPort 1.1 to get 3440 x 1440.
- When the monitor is set to DisplayPort 1.4, the best I can do is 2560 x 1080.
Until now, I've unplugged everything, plugged it in and everything reinstalled, reset the SMU and zapped the NVRAM. No joy. Attempts to get support from Radeon and LG have (predictably) failed, and the problem is so specific that I could not find anyone in a similar situation. DisplayPort research, while interesting, has not been enlightening.
I would really like to understand how I was wrong. "Everything looks good on paper." I can pack the product and send it back, but I will still have to deal with the need for a new display and the desire (forgive me) of a big screen.