Display output only works on iGPU, and no output on dGPU on Kubuntu 25.10

[YamiYukiSenpai]

Ubuntu Support Template

Ubuntu Version:
25.10

Desktop Environment (if applicable):
KDE

Problem Description:
SDDM doesn’t output on dGPU, and only on iGPU.

Tried HDMI and DisplayPort. Motherboard only has HDMI, which worked, but not on the dGPU.

Same result with LightDM.

When logged in or even during POST, there’s output on the Radeon GPU. Even TTY works

Relevant System Information:
Intel i5-6500T
Radeon RX 570

Screenshots or Error Messages:
I couldn’t find any errors that indicate that being an issue.

$ sudo dmesg  | grep -i amd
[    0.000000] Linux version 6.17.0-8-generic (buildd@lcy02-amd64-076) (x86_64-linux-gnu-gcc (Ubuntu 15.2.0-4ubuntu4) 15.2.0, GNU ld (GNU Binutils for Ubuntu) 2.45) #8-Ubuntu SMP PREEMPT_DYNAMIC Fri Nov 14 21:44:46 UTC 2025 (Ubuntu 6.17.0-8.8-generic 6.17.2)
[    0.000000]   AMD AuthenticAMD
[    0.010547] RAMDISK: [mem 0x6884d000-0x6db6cfff]
[    9.106976] [drm] amdgpu kernel modesetting enabled.
[    9.110335] amdgpu: Virtual CRAT table created for CPU
[    9.110354] amdgpu: Topology: Add CPU node
[    9.110652] amdgpu 0000:01:00.0: amdgpu: initializing kernel modesetting (POLARIS10 0x1002:0x67DF 0x1462:0x341A 0xEF).
[    9.110672] amdgpu 0000:01:00.0: amdgpu: register mmio base: 0xDF400000
[    9.110674] amdgpu 0000:01:00.0: amdgpu: register mmio size: 262144
[    9.110800] amdgpu 0000:01:00.0: amdgpu: detected ip block number 0 <vi_common>
[    9.110804] amdgpu 0000:01:00.0: amdgpu: detected ip block number 1 <gmc_v8_0>
[    9.110806] amdgpu 0000:01:00.0: amdgpu: detected ip block number 2 <tonga_ih>
[    9.110808] amdgpu 0000:01:00.0: amdgpu: detected ip block number 3 <gfx_v8_0>
[    9.110810] amdgpu 0000:01:00.0: amdgpu: detected ip block number 4 <sdma_v3_0>
[    9.110812] amdgpu 0000:01:00.0: amdgpu: detected ip block number 5 <powerplay>
[    9.110814] amdgpu 0000:01:00.0: amdgpu: detected ip block number 6 <dm>
[    9.110816] amdgpu 0000:01:00.0: amdgpu: detected ip block number 7 <uvd_v6_0>
[    9.110818] amdgpu 0000:01:00.0: amdgpu: detected ip block number 8 <vce_v3_0>
[    9.111003] amdgpu 0000:01:00.0: No more image in the PCI ROM
[    9.111019] amdgpu 0000:01:00.0: amdgpu: Fetched VBIOS from ROM BAR
[    9.111023] amdgpu: ATOM BIOS: 113-V34121-X3
[    9.203554] amdgpu 0000:01:00.0: vgaarb: deactivate vga console
[    9.203561] amdgpu 0000:01:00.0: amdgpu: Trusted Memory Zone (TMZ) feature not supported
[    9.203730] amdgpu 0000:01:00.0: amdgpu: vm size is 128 GB, 2 levels, block size is 10-bit, fragment size is 9-bit
[    9.206734] amdgpu 0000:01:00.0: amdgpu: VRAM: 4096M 0x000000F400000000 - 0x000000F4FFFFFFFF (4096M used)
[    9.206740] amdgpu 0000:01:00.0: amdgpu: GART: 256M 0x000000FF00000000 - 0x000000FF0FFFFFFF
[    9.208628] amdgpu 0000:01:00.0: amdgpu: amdgpu: 4096M of VRAM memory ready
[    9.208633] amdgpu 0000:01:00.0: amdgpu: amdgpu: 15988M of GTT memory ready.
[    9.221961] amdgpu: hwmgr_sw_init smu backed is polaris10_smu
[    9.294007] amdgpu 0000:01:00.0: amdgpu: [drm] Display Core v3.2.340 initialized on DCE 11.2
[    9.305643] snd_hda_intel 0000:01:00.1: bound 0000:01:00.0 (ops amdgpu_dm_audio_component_bind_ops [amdgpu])
[    9.721960] kfd kfd: amdgpu: Allocated 3969056 bytes on gart
[    9.721988] kfd kfd: amdgpu: Total number of KFD nodes to be created: 1
[    9.722080] amdgpu: Virtual CRAT table created for GPU
[    9.732521] amdgpu: Topology: Add dGPU node [0x67df:0x1002]
[    9.732527] kfd kfd: amdgpu: added device 1002:67df
[    9.732540] amdgpu 0000:01:00.0: amdgpu: SE 4, SH per SE 1, CU per SH 9, active_cu_number 32
[    9.736210] amdgpu 0000:01:00.0: amdgpu: Using BOCO for runtime pm
[    9.782950] amdgpu 0000:01:00.0: [drm] Registered 6 planes with drm panic
[    9.782955] [drm] Initialized amdgpu 3.64.0 for 0000:01:00.0 on minor 2
[    9.836877] fbcon: amdgpudrmfb (fb0) is primary device

It was working fine on 24.04

What I’ve Tried:

• Different DisplayPort cable
• HDMI cable

Before Posting:
Please check if similar issues have already been reported and resolved.

See the “Start here” guide:

Welcome To Support And Help

Support and Help You are welcome here. Before asking a question... Tips that will get you the best and fastest help... Are you having trouble installing Ubuntu? Is your hardware working properly after install? You installed Ubuntu successfully, and your hardware works. What's next to setup and start using your new system? Ready to learn more about about Ubuntu Discourse and the Ubuntu Community? Your Ubuntu system has worked well for some time, and now... Advanced help …

[celticwarrior]

If it’s wired like that there’s nothing you can do about it.

Usually it’s the other way around but this statement is based on experience and reports about Intel+Nvidia.

[YamiYukiSenpai]

Workaround

Created /etc/modprobe.d/disable-i915.conf with the following contents:

options i915 modeset=0

Afterwards

sudo update-initramfs -c -k all

[guiverc]

I have no helpful advice here … but this box has 5 monitors connected; but when it boots I see system firmware boot messages only on some, GRUB on one only, it changes when the boot gets to plymouth (all 5 wake up if on though usually I only have 4 powered), but when it gets to sddm it drops to 3 monitors only (2 go to power-save mode). Only after login do the sleeping monitors wake up again.

The same [Ubuntu/OS] configuration on a different box, with different hardware gives a different result - ie. hardware specific.

FYI: The three monitors on this box that work with sddm are all connected to the same graphics, the two that go dark are on a different card.

I’ve never tried to change this, but I believe your answer will be hardware specific… I’ve even changed graphics card in a box at one point & had the lightdm/sddm monitor used change, which was unexpected since I’d made no software change on the actual disk (just swapped cards in box).