One monitor works as a primary only (RX 580)

[kikap]

I actually don't even know what to google. I have two monitors, NEC PA272 and Apple LED Display. The NEC is primary (center) and Apple is secondary (on the side). Both plugged into the DP outputs of the XFX RX 580 4GB (reflashed as Sapphire with the same config and timings). Apple uses DP to miniDP-female convertor, which is just a bunch of wires.
If I plug NEC into the topmost DP and Apple into the next (middle) DP Apple doesn't come to life. System doesn't see it, it doesn't even blink, for all practical purposes it doesn't exist. NEC works as a primary.
If I swap them: Apple goes into the first DP and NEC into the second I have a dual monitor configuration as I wanted but the roles are swapped, Apple becomes primary and NEC secondary. I get BIOS on Apple, OpenCore boot menu on Apple, etc. Not that it's a big deal, in MacOS I still have them the way I like it, but the fact that it doesn't work as I intend it to work bothers me. Also, I just can not understand WHY? DP outputs work. Cables work. Monitors work. What doesn't work?!?

P.S. Catalina 10.15.7

Any ideas please.

 

[Edhawk]

Could be a combination of issues:
I have a couple of RX580's and two of the three DP connectors work just fine, on both cards.

The one furthest from the motherboard (top most connector highlighted in the red rectangle in the image below) doesn't work with macOS.



I use the middle two DP connectors to connect my two Dell U2515H displays, running macOS Catalina, Big Sur and Windows 10 on three separate drives. The two displays work fine in all three OS's using these middle DP connections. However, the Primary screen does change between the three OS's, even though the cables/connections are never swapped. In Catalina the Primary screen will be the left-hand display, in Big Sur and Windows it is the Right-hand display.

Your XFX GPU's VBIOS may not initialise/work well with the Apple LED Display set as primary in macOS.

I have assumed you are running Big Sur given your system specs; Apple changed a number of graphics settings, dropping codecs and other elements, which may effect how your Apple LED Display behaves in Big Sur.

There could be a conflict between the topmost DP port and the framebuffer, which the Apple LED can't negotiate as well as the NEC display.

Sorry but I don't have a definitive answer, I doubt anyone here does.

 

[kikap]

Could be a combination of issues:
I have a couple of RX580's and two of the three DP connectors work just fine, on both cards.

The one furthest from the motherboard (top most connector highlighted in the red rectangle in the image below) doesn't work with macOS.

It works for me (with NEC and with Apple). It's the second one (second from the left on your picture) that doesn't want to work with Apple, but works with NEC.

I use the middle two DP connectors to connect my two Dell U2515H displays, running macOS Catalina, Big Sur and Windows 10 on three separate drives. The two displays work fine in all three OS's using these middle DP connections. However, the Primary screen does change between the three OS's, even though the cables/connections are never swapped. In Catalina the Primary screen will be the left-hand display, in Big Sur and Windows it is the Right-hand display.

But before the OS is booted the order is always the same, right? Are both your MacOSes OpenCore? What about OC boot menu? It also appears on different displays for Catalina and BigSur?

Your XFX GPU's VBIOS may not initialise/work well with the Apple LED Display set as primary in macOS.

As secondary. Thanks for the idea. I'm wondering if there's a way to get into the VBIOS and see what does it see connected, before the OS picks up.

It's Sapphire BIOS now. XFX is a non-starter with MacOS, kernel panic early in the boot process (as soon as the driver runs for the first time, boom). I just found a BIOS from Sapphire for the same timings and 4GB, changed the "subvendor ID" (or something) in it and flashed.

I have assumed you are running Big Sur given your system specs; Apple changed a number of graphics settings, dropping codecs and other elements, which may effect how your Apple LED Display behaves in Big Sur.

I'm running Catalina 10.15.7, sorry for the confusion. Updated the post.

There could be a conflict between the topmost DP port and the framebuffer, which the Apple LED can't negotiate as well as the NEC display.

I'm wondering if there's a way to get a debug log out of it, to see if it even tried.

Thanks for responding, there are few pointers to look at.

 

[Edhawk]

Displays are set to extended mode in all three OS's. When I take a screenshot of my desktop I get two separate images:

left-hand display

right-hand display

Bios and OpenCore GUI screens appear on both displays, but slightly faster on the right-hand display for all three OS's.

Windows 10 login screen only appears on the right-hand display, second display only shows extended Windows 10 desktop after login completes.

I have a single instance of OC booting Catalina and Big Sur, from two separate drives (2 x Samsung SM951 NVMe 256GB), default OS is Catalina, but I can select Big Sur icon with the keyboard arrows or mouse.

I use the F12 key when I want to boot to Windows 10 (happening less and less often).