Cant use more than 2 monitors at the same time

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1

Hello, im new, i dont know what info you need from me, but on KDE wayland, i just cant use multiple displays.

My PC specs are as follows:

CPU - 9950x
GPU - 7900xtx
RAM - DDR5 96GB
OS: archlinux

PC was built month ago or less.

I have multiple monitors/displays.

  1. MSI monitor (DP) 360Hz 2k
  2. DELL monitor (DP) 144Hz 2k
  3. TV (HDMI) 60Hz 4k
  4. huion kamvas 13 gen3 60Hz 1080p via thunderbolt to DP - here i have tried directly to connect pen display outside thunderbolt card via hdmi, it didnt help. The pen display has USBC connections, so thats why im using thunderbolt add-in card. THe tablet came with 2x USB 1x hdmi to USBC adapter cable.

Problem is as follows:

When i have monitors 1. and 2. online, i try to turn on my 3. TV then TV has no signal. I turn off either 1. or 2. and TV has signal.
When i have TV online, and i turn on the 4. huion tablet, everything works. Now if i turn on the monitor, huion tablet goes offline. If i turn off TV here then huion tablet has signal and monitor has signal. Now if i turn on another monitor then the second monitor has no signal. If i turn off the huion tablet then my monitor has signal.

This is very unusable situation because i want to use my monitors and tablet at the same time. I can live with TV not working when monitors are online. But not with tablet not working.

I tested this with gnome wayland, while it works with three displays (1. 2. 4.) when i turn on the TV then it has signal but all the monitors report that they have the native refresh rates in the settings GUI page but in reality they are choppy on high refreshrate displays and it feels like 60HZ like the lowest screen refresh rate which is either TV or huion pen display.

X11 has the problem not playing well with multiple resolutions and multiple refreshrates, so its unusable while all my screens have varriable refresh rates.. i dont know how to properly set it up or if it is possible.

Prior to plasma updates and mesa driver updates that happened last week or this week, the problem was more severe. While now the display that i connect shows no signal, in the past it would throw me into tty. I then turn off the last display and the session goes back to normal with kwin crash report.

Now it just goes no signal, no kwin crashes, no session to tty.

It would be nice to fix this problem. I was sold that on linux AMD “Just works” … but… you know my pain.

2 
                  -`                     kuligs2
                 .o+`                    -----------------
                `ooo/                    OS: Arch Linux x86_64
               `+oooo:                   Kernel: Linux 6.13.8-arch1-1
              `+oooooo:                  Uptime: 3 hours, 27 mins
              -+oooooo+:                 Packages: 1564 (pacman), 14 (flatpak)
            `/:-:++oooo+:                Shell: zsh 5.9
           `/++++/+++++++:               Display (MSI3CD7): 2560x1440 @ 360 Hz in 27" [External] *
          `/++++++++++++++:              Display (Dell S2417DG): 2560x1440 @ 144 Hz in 24" [Extern]
         `/+++ooooooooooooo/`            DE: KDE Plasma 6.3.4
        ./ooosssso++osssssso+`           WM: KWin (Wayland)
       .oossssso-````/ossssss+`          WM Theme: Breeze
      -osssssso.      :ssssssso.         Theme: Breeze (Dark) [Qt], Breeze-Dark [GTK2], Breeze [GT]
     :osssssss/        osssso+++.        Icons: breeze-dark [Qt], breeze-dark [GTK2/3/4]
    /ossssssss/        +ssssooo/-        Font: Noto Sans (10pt) [Qt], Noto Sans (10pt) [GTK2/3/4]
  `/ossssso+/:-        -:/+osssso+-      Cursor: breeze (24px)
 `+sso+:-`                 `.-/+oso:     Terminal: ghostty 1.1.3-arch1
`++:.                           `-/+/    Terminal Font: JetBrainsMono Nerd Font (13pt)
.`                                 `/    CPU: AMD Ryzen 9 9950X (32) @ 5.75 GHz
                                         GPU 1: AMD Radeon RX 7900 XTX [Discrete]
                                         GPU 2: AMD Radeon Graphics [Integrated]
                                         Memory: 8.34 GiB / 91.97 GiB (9%)
                                         Swap: 0 B / 4.00 GiB (0%)

IDK what logs or whatever i should add here..

3

Are you using an USB-C hub to connect all those? Sounds like underpowered thing to me.
Are those DP internal?

4

Monitors are connected to their own powersupply in the grid socket, and the signal cable is connected to my GPU 7900xtx via DP DP DP and HDMI cable.

As i explained, it works with 3 screens on gnome wayland but not on kde.

If its a power issue, then KDE is using too much power?

5

Nope. Something else.

6

Drivers are supposed to take care of this automatically, but try putting KWIN_DRM_PREFER_COLOR_DEPTH=24 into /etc/environment, and reboot

7

Wont that mess up the colors? I’ll try it later.

Also maybe anyone can point me how to fix flickering when other high refreshrate monitors are connected but not on, when TV is on (60Hz). Like if im watching a movie and it pans, it looks like very choppy (VLC). I tried to look for vsync in vlc but didnt find.. It looks smoother on x11, but x messes up the scaling on UI with 4k displays, and when i switch from TV screen to monitors, its just too much of a hassle to reset and rescale stuff.. and then i loose VRR on the monitors.

8

No. You only get 8 bits per color on Xorg at all times, which is likely why that happens to work.

9

uhm, i tried this and it didnt help my 4 monitor setup.. was that color trick supposed to help with choppyness?

im still having same issues with my tv and pendisplay working and then if i turn on 2 monitors only one of them turns on.. i cant have 2 monitors DP with a TV hdmi at the same time :frowning:

10

So i contacted the manufacturer and they tested with their setup on “windows”, they got 4x 2K screens working, tho i dont know at what refresh rate.

I did make USB windows11-to-go bootable to test. Installed adrenalin drivers, and yeah, all my 4x screens worked but with caveats.

Turns out, not all GPUs can push data as hard as your monitors request.

In my windows 11 case i could do 1x 4k @ 60Hz + 1x 1080p @ 60Hz + 2x 1440p @ 120Hz.

While this worked on windows, it does not work on KDE wayland or x11. I tried to set all monitors to 60Hz in KDE, but they still wont turn on at certain monitor combinations.

Can we not have some kind of fallback on linux like it is on windows. If GPU cant push this much data and or monitor is not getting signal then lower the refreshrate and or resolution. This is what windows was doing while i was testing to see what combination of resolution/refresh rate was working.

Any other suggestions i could try?

11

In theory, yes. In practice, driver APIs are really lacking feedback for why a given display configuration doesn’t work, do KWin just turns off one screen after another, until as many displays as possible work.

There’s another part to the story: when the driver detects that it can’t drive all displays, it’s supposed to use things like reduced bit depth and chroma subsampling to make it work… However, drivers only implement that in a very limited manner.

Sometimes displays also don’t work while the driver tells us they’re enabled, but I don’t know if that’s a problem on the display side or a driver bug.

Unfortunately I’m out of ideas. You could open a bug report at Issues · drm / amd · GitLab, but the driver developers often have issues debugging such niche issues too

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