If the dGPU LED stays lit, the option has probably taken effect but did the dmesg error go away? I mean that one:
[CODE]radeon 0000:01:00.0: failed VCE resume (-22).
[drm:r600_ring_test [radeon]] *ERROR* radeon: ring 0 test failed (scratch(0x850C)=0xCAFEDEAD)
radeon 0000:01:00.0 disabling GPU acceleration
[drm:si_dpm_set_power_state [radeon]] *ERROR* si_upload_sw_state failed[/CODE]

[QUOTE]Do you have the radeon.runpm=0 as a flag in the built kernel or as a boot option in grub?[/QUOTE]
As a boot option, and I use an official Ubuntu kernel. Even an old stock kernel from 16.04 or older should work, considering I've been using this option for more than 2 years.

[QUOTE]Do you use the AMGDPU driver at all? Or only radeon?[/QUOTE]
Only radeon, never tried AMDGPU.

[QUOTE=ecth;n949056]I get this when I try to use DRI_PRIME=1:
[code]DRI_PRIME=1 glxgears
radeon: Failed to allocate virtual address for buffer:
radeon: size : 65536 bytes
radeon: alignment : 4096 bytes
radeon: domains : 4
radeon: va : 0x0000000000800000
radeon: Failed to deallocate virtual address for buffer:
radeon: size : 65536 bytes
radeon: va : 0x800000
radeon: Failed to allocate virtual address for buffer:
radeon: size : 65536 bytes
radeon: alignment : 4096 bytes
radeon: domains : 4
radeon: va : 0x0000000000800000
radeon: Failed to deallocate virtual address for buffer:
radeon: size : 65536 bytes
radeon: va : 0x800000
radeonsi: Failed to create a context.
Speicherzugriffsfehler (Speicherabzug geschrieben)[/code][/QUOTE]
I don't recall ever seeing this error, there must be something else wrong with your setup. Could you paste the full dmesg and /var/log/Xorg.0.log somewhere?

[QUOTE]What exactly does radeon.modeset=0 or =1 or nomodeset? I had radeon.modeset=0 but I don't see much difference.... oô[/QUOTE]
All I know is that modesetting is required for graphics acceleration at least for the open source graphics stack, "nomodeset" is mostly used as a safe mode if accelerations borks the system (happened for example with i915 on my netbook with old kernels) or when using a proprietary driver.

Also a note about DRI_PRIME: if you're trying Steam games it's sometimes tricky to get it working properly. For The Talos Principle for example I have to run Steam and then the Talos executable both with STEAM_RUNTIME=0 and DRI_PRIME=1 set.