Regression: RX 470 fails to boot with amdgpu.dpm=1 on kernel 6.7+

[Durmuş]

Hello,

I'm experiencing a critical issue on my system with an AMD RX 470 GPU.
When booting with recent kernel versions (6.7.x or newer), the system
fails to boot properly unless I explicitly disable Dynamic Power
Management (DPM) via the `amdgpu.dpm=0` kernel parameter.

When DPM is enabled (`amdgpu.dpm=1` or omitted, since it's the
default), the system either freezes during early boot or fails to
initialize the display. However, using the LTS kernel (6.6.x),
everything works as expected with DPM enabled.

This seems to be a regression introduced in kernel 6.7 or later, and
it specifically affects older GCN4 (Polaris) GPUs like the RX 470.
Disabling DPM allows the system to boot, but significantly reduces GPU
performance.

Things I’ve tried:
- Confirmed that the latest `linux-firmware` is installed.
- Verified correct firmware files exist under `/lib/firmware/amdgpu/`.
- Tested multiple kernels (mainline and LTS).
- Using Mesa with ACO (Radeon open driver stack).
- System boots fine with LTS kernel (6.6.x) + DPM enabled.

System info:
- GPU: AMD RX 470 (GCN 4 / Polaris)
- Distro: Arch Linux
- Kernel (working): linux-lts 6.6.x
- Kernel (broken): 6.7.x and newer (currently tested on 6.14.6)

Thanks in advance,
Durmus Ozaltay