Does DSC disable MPO support? (Display cloning issue)

[olui]

I have an Alienware AW2725DF, which uses DSC (Display Stream Compression) over DisplayPort, but not via HDMI. Over DisplayPort it can reach 1440p 360hz, on HDMI it only reaches 144hz.

When I plug it in via DP, Special K shows no MPO support (only 1 plane). Plugging it in via HDMI shows MPO support (4x planes). Dxdiag gives the same results.

Here's my issue:
I have a dual-PC setup, with an Elgato 4k60 in the second pc, and I duplicate the displays to make it work.

Scenario A:
If I run the main monitor through DisplayPort, I can get the full 360hz, but it becomes the "slave" display.
Main display gets no tearing + added delay, and the capture card gets prioritized for latency and awful tearing.
Even if I lower the main display's refresh rate to the same as - or lower than - the Elgato's, this behavior still persists.

Scenario B:
However, if I connect the main display through HDMI, I can only get 144hz, but it becomes the "master".
The main display gets minimal delay and tearing, while the capture card gets no tearing, which is great for the videos.
This persists regardless of what display has the highest refresh rate.

Scenario C:
If I use "extend" instead of "duplicate" the game runs normally on the main monitor (checked on PresentMon), and I can get OBS to send a fullscreen preview to the capture card. This isn't ideal, since it generates a lot of skipped frames on the recording, and increases CPU usage, which sucks bc my main game is CPU-bound.

My goal I to get my main monitor as master, even when using DisplayPort. Any information can be helpful!

[RealNC]

If you only use that one monitor, and nothing else, just that one monitor directly to the GPU without a capture card in-between, and reboot, there's still no MPOs?

[Gias]

you also don't have the dsr/dldsr options in nvidia's control panel, right ?

dsc can disable MPOs just like it can make you lose the dsr/dldsr options...

according to nvidia, when a display is connected to the gpu and the display uses dsc, the gpu may use two internal heads to drive the display when the pixel rate needed to drive the display mode exceeds the gpu’s single head limit

and apparently if a display that is using DSC is taking up 2 internal display heads, you lose the dsr/dldsr options and MPOs

(lowering your display's resolution or refresh rate won't necessarily fix that btw as your display may still be taking up or reserving the 2 internal heads with the dsc integration anyway, unless you have an osd option to completely disable the dsc -- or, like you did by switching to the hdmi port, you could switch to a port that doesn't enable dsc...)

that said, an rtx 5090 gpu is known to basically fix the situation with its higher pixel rate per display head. people that were using certain displays with dsc (and didn't have the dsr/dldsr options and MPOs) have confirmed that the rtx 5090 gpu gave those features back to them etc

so the rtx 5090 or some 50 series gpu with its dp 2.1 port could give you MPOs; even if you're using a display with dp 1.4

i do have a display that uses dsc and still gives me the dsr/dldsr options and MPOs (though running a game with a dsr/dldsr resolution would also effectively disable MPOs), but that display of mine only does up to 4k144hz over hdmi 2.1 and apparently its hdmi 2.1 ports are limited to 24Gbps (rather than the full 48gbp that hdmi 2.1 could have) and then uses dsc 1.2a for anything that requires higher bandwidth. i believe my display ends up only taking up 1 internal gpu head in this configuration (and i use an rtx 4090 with my pc btw)

edit:

the displays with dsc that take up 2 internal heads are also basically "tiled dsc" displays...

and if you disable the tiled display feature from the windows registry, you can get the dsr/dldsr options and MPOs back. it may lower your display's max refresh rate and in your case it'd lower it to 240hz i believe, but you could get the dsr/dldsr options and MPOs that way (or change to an rtx 5090 gpu and you won't need to disable the tiled display feature...)

you can go in the windows registry to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4d36e968-e325-11ce-bfc1-08002be10318}\0000

and set EnableTiledDisplay to 0

so "EnableTiledDisplay"=dword:00000000

0 disables the tiled display feature basically, while 1 would enable it back (and would kill dsr/dldsr and MPOs again at least with displays that are taking up 2 internal heads with DSC...)

you may get a black screen. also you may need a pc restart after the registry change for it to take effect

[olui]

Hey! Sorry for taking so long to reply, a lot of things happened since then.

I found out, by chance, that when you are using cloned display mode, and change the refresh rate of a display (on the settings page), windows prioritizes that recently altered display as the master.

Don't know why that happens, but if I change the refresh rate of the elgato, the tearing goes to the elgato, and if I change the refresh rate of the main monitor, the tearing goes to the main monitor.

The .csv generated by PresentMon also confirms that theory. If I change elgato's refresh rate, dwm.exe starts showing frames at the monitor's refresh rate, and if I change the monitor's refresh rate, dwm.exe starts showing frames at the elgato's refresh rate.
Since dwm is always [AllowTearing = 0], it explains the lack of tearing on the slave display.

What you said about the RTX 5090 is correct! This week I got a new RTX 5090, and not only I'm able to run DSR with my DSC enabled display, but I'm also getting 7 MPO planes with only the main display connected, and 3 planes each when using cloned mode. This doesn't seem to interfere with the display duplication.

Ever since I was able to make the capture card the slave display, the tearing is gone, but I have been noticing frame duplication on my recordings, some frames stay displayed for 33.333ms.

At first, I thought it was something wrong with the streaming PC, but I then realized the PresentMon csv shows some dwm.exe frames have DisplayedTime at >33ms.

https://imgur.com/a/4qolxkQ

The pattern is consistent with the frame duplication from the recordings I manually counted in Premiere Pro. About 1 duplicated frame every 3~15 frames.

This makes my recordings look like 30fps sometimes, any idea how it can be fixed?

I bought a new Elgato 4k Pro to test against my Elgato 4k60 Pro MK.2.
Since it's a newer model, I hope it can make an impact. I'll come back after testing it out.

[olui]

The new Elgato 4k Pro just arrived, still skipping frames.
Looks like DWM is just terrible at maintaining a regular frame pace to the cloned display