Gaming Mode black screen on iGPU + dGPU desktop: gamescope selects the iGPU (no connected outputs) and exits in a loop — regression 3.8.16 → 3.8.28

[adammathes]

Your system information

• Steam client version: 1788652215
• SteamOS version: 3.8.28 (Stable). Last good: 3.8.16
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

I have a weird setup -- the GPU's output goes to an ASUS ProArt board's DP-in, then over USB4 to a CalDigit Thunderbolt dock, and then to an Apple Studio Display (the older 5k one.) I'm grateful it works on Linux and SteamOS at all!

Based on the logs and Claude analysis (below) the failure itself may be independent of my bizarro display path and hardware -- gamescope seems to have picked the iGPU over the dGPU, so this may affect people with other setups. Or maybe it only triggers in my bizarro setup. I have more logs if anyone digs in, mostly posting so it's a KI. The workaround is pretty simple (disable iGPU in BIOS) so may prove helpful to anyone that experiences a similar issue in the short term.

Details below from logs + Claude analysis with light editing --

Additional details:

Kernel	6.18.50-valve2-1-neptune-618-gc7289a96b14d (3.8.16: 6.16.12-valve24.5-1-neptune-616-gb2f7cfe85e45)
gamescope	3.16.23.6
CPU / iGPU	AMD Ryzen 7 9800X3D — Raphael iGPU, 0000:7b:00.0, 1002:13c0
dGPU	Sapphire Nitro+ RX 9070 XT — 0000:03:00.0, 1002:7550 (Navi 48, rev c0)
Motherboard	ASUS ProArt X870E-Creator WiFi
Display path	RX 9070 XT DisplayPort → motherboard DP-in → USB4 → CalDigit Thunderbolt dock → Apple Studio Display (5K). Link: 4× HBR2 with DSC on dGPU DP-2

Please describe your issue in as much detail as possible:

Expected: Gaming Mode starts on the dGPU, which drives the only connected display, as it did on 3.8.16 with identical hardware and BIOS settings.

Actual: After updating from 3.8.16 to 3.8.28, Gaming Mode boots to a black screen. gamescope opens the iGPU's DRM node (/dev/dri/card1) and selects Vulkan device RADV RAPHAEL_MENDOCINO. All of the iGPU's connectors are disconnected, so it fails with cannot find any connected connector! / Failed to find a primary plane and exits with status 1. sddm relaunches the session and it fails the same way, in a loop.

• Desktop Mode (KWin) works on the same boot and the same display, so the kernel, amdgpu, and the display link are fine. The problem seems to be gamescope's device selection.
• Suggested behavior: prefer a DRM device with a connected connector, or the discrete GPU over iGPU.

Workaround: disable the iGPU in BIOS (Advanced → NB Configuration → Integrated Graphics = Disabled, Primary Video Device = PCIe). Gaming Mode then starts on the 9070 XT and games run normally.

Logs (failed boot, journalctl -b -1):

gamescope picks the iGPU, finds no connected connectors, and exits:

gamescope-session[6562]: [gamescope] [Info]  console: gamescope version 3.16.23.6 (gcc 15.1.1)
gamescope-session[6562]: [gamescope] [Info]  vulkan: selecting physical device 'AMD Ryzen 7 9800X3D 8-Core Processor (RADV RAPHAEL_MENDOCINO)': queue family 1 (general queue family 0)
gamescope-session[6562]: [gamescope] [Info]  drm: opening DRM node '/dev/dri/card1'
gamescope-session[6562]: [gamescope] [Info]  drm: Connectors:
gamescope-session[6562]: [gamescope] [Info]  drm:   DP-4 (disconnected)
gamescope-session[6562]: [gamescope] [Info]  drm:   DP-3 (disconnected)
gamescope-session[6562]: [gamescope] [Info]  drm:   HDMI-A-3 (disconnected)
gamescope-session[6562]: [gamescope] [Info]  drm: cannot find any connected connector!
gamescope-session[6562]: [gamescope] [Error] drm: Failed to find a primary plane
gamescope-session[6562]: Failed to create backend.
systemd[2412]: gamescope-session.service: Main process exited, code=exited, status=1/FAILURE
systemd[2412]: Failed to start Gamescope Session.
sddm[2399]: Session "/usr/share/wayland-sessions/gamescope-wayland.desktop" selected, command: "start-gamescope-session" for VT 1

Kernel: card1 / minor 1 is the iGPU (0000:7b:00.0), with nothing attached. The Studio Display enumerates over Thunderbolt:

kernel: [drm] Initialized amdgpu 3.65.0 for 0000:7b:00.0 on minor 1
kernel: amdgpu 0000:7b:00.0: [drm] Cannot find any crtc or sizes
kernel: thunderbolt 0-303: new device found, vendor=0x1 device=0x801f
kernel: thunderbolt 0-303: Apple Inc. Studio Display

dGPU link state (debugfs, captured on the working 3.8.16 boot):

== /sys/kernel/debug/dri/0000:03:00.0/DP-2
link_settings: Current:  4  0x14  0  Verified:  4  0x14  16  Reported:  4  0x14  16  Preferred:  0  0x0  0
dsc_clock_en: 1
dsc_slice_width: 1280
dsc_bits_per_pixel: 256

Other warnings on 3.8.28, possibly unrelated:

amdgpu 0000:03:00.0: [drm] REG_WAIT timeout 1us * 150000 tries - optc401_disable_crtc line:235
amdgpu 0000:03:00.0: [drm] CRB Config Warning: DET size (3,14,8,0) + Compbuf size (1) >  CRB segments (21)
amdgpu 0000:7b:00.0: [drm] Mode Validation Warning: Validation OK failed validation.   (repeated every ~1–2s)

Steps for reproducing this issue:

1. On a desktop with an AMD Ryzen CPU, leave the iGPU enabled in BIOS (Integrated Graphics = Auto/Enabled) and install an AMD discrete GPU.
2. Connect the only display to the discrete GPU, with nothing on the iGPU/motherboard outputs, and boot SteamOS 3.8.28 into Gaming Mode.
3. Observe a black screen and a gamescope-session restart loop (journalctl -b | grep gamescope shows opening DRM node '/dev/dri/card1' → cannot find any connected connector!). Switch to Desktop Mode, or disable the iGPU in BIOS, and the display works.