[Bug 221110] New: amdgpu DM skips backlight registration on ASUS TUF A14 (DCN 3.5) due to ATPX detection regardless of active GPU mode

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=221110

            Bug ID: 221110
           Summary: amdgpu DM skips backlight registration on ASUS TUF A14
                    (DCN 3.5) due to ATPX detection regardless of active
                    GPU mode
           Product: Drivers
           Version: 2.5
          Hardware: AMD
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: maurotibanez@gmail.com
        Regression: No

Hardware:

Laptop: ASUS TUF A14
Panel: BOE NE140QDM-NX7
iGPU: AMD (DCN 3.5)
dGPU: NVIDIA (disabled via vga_switcheroo / supergfxctl, Integrated mode
active)
Kernel: 6.17.9-76061709-generic

Problem:
No backlight device is registered in /sys/class/backlight/ when booting in
Integrated GPU mode. Brightness control is completely non-functional.
Kernel log showing root cause:

[3.831962] amdgpu: vga_switcheroo: detected switching method
\_SB_.PCI0.GPPA.VGA_.ATPX handle
[3.832035] amdgpu: ATPX version 1, functions 0x00000801
[3.832077] amdgpu: ATPX Hybrid Graphics
[4.320982] amdgpu 0000:65:00.0: amdgpu: [drm] Skipping amdgpu DM backlight
registration

The amdgpu driver detects ATPX at 3.83s and skips backlight registration at
4.32s. The skip occurs unconditionally based on ATPX presence, without checking
whether vga_switcheroo has actually handed display control to the iGPU. When in
Integrated mode, amdgpu owns the display pipeline (DCN 3.5 active, eDP-1
connected to card0) but does not register a backlight device.
Confirmed not caused by:

asus_wmi / asus-nb-wmi (loads at 6.08s, after the skip at 4.32s)
Missing ACPI backlight method (DSDT contains no backlight method)
Panel quirk (BOE NE140QDM-NX7 has no known quirk entry)

Kernel parameters tested with no effect:

amdgpu.backlight=0 / amdgpu.backlight=1
acpi_backlight=native
acpi_backlight=video
acpi_backlight=vendor

ATPX functions value: 0x00000801
Expected behavior:
When vga_switcheroo/supergfxctl has placed the system in Integrated mode and
the AMD iGPU owns the display pipeline, amdgpu DM should register a backlight
device normally regardless of ATPX presence.
Additional info:

/proc/acpi/video/ does not exist
card0-eDP-1 is the active internal panel path
NVIDIA modules are fully unloaded in Integrated mode
supergfxctl -g confirms Integrated

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