I just picked a Asus MG279Q and set it up last night and below is what I have done so far.
1. Connected the monitor with the supplied miniDP<-->DP cable which I connect the miniDP end to my monitor and the regular DP end to one of my 290X cards.
2. I enable freesync in the OSD of the monitor,
3. Set my screen refresh rate in windows to 90Hz,
4. Enabled freesync in the radeon settings.
I downloaded the Windmill demo just to make sure that freesync was enabled, and to my surprise i doesn't seem to work.
The windmill demo recognizes my monitor to be freesync compatible, but the option to enable freesync in the demo is grayed out. Also if I use the testpattern mode of the demo and turn of VSYNC I can see noticeable tearing.
But if I turn off crossfire in the radeon settings and start up the demo, the freesync on/off option is now available to me and there is no tearing when it is turned on (even if I turn off VSYNC). I can also turn freesync on and off through the radeon settingspanel with the same affect.
This leads me to think that freesync doesn't work with the crimson drivers if crossfire is turned on, atleast not for me. I have tried reinstalling the 15.11.1 drivers by uninstalling them in the normal fashion and then running DDU (Display Driver uninstaller) and reinstallning the drivers to no affect.
Anybody else have this issue? Or have any ideas what I would try? Im going to see if overclocking my 290X's might be causing the issue.
EDIT: I removed my overclocked setting in MSI afterburner and set it so it doesnt run at startup. I also did a third reinstall by uninstalling and after running the "AMD Cleanup Utility" to remove the drivers, and reinstall them. Now freesync and crossfire is working in the demo, however the on/off toggle i stiil greyed out which seems to be a bug/problem with just the demo that other people are experiencing.
MSI X99A SLI KRAIT Edition| Intel i7 5820K @4,5 GHz| Custom Watercooling| Samsung 950 PRO 512GB M.2 NVMe| HyperX Predator DDR4 2666MHz 16GB 14-14-14-36| Sapphire TRI-X R9 290X @ 1150 MHz GPU 1500 Memory| Sapphire TRI-X R9 290X @ 1150 MHz GPU 1500 Memory| ASUS MG279Q 27" Monitor| Creative SoundBlaster Recon3D PCIe| Cooler Master Vanguard 1000W| Fractal Design Define S| Windows 10 Pro 64bit