Created attachment 137979 [details]
dmesg with 4.17-wip-d1eeebbd78fd and dc=1

OK, I've gotten around to doing some more testing and playing around with some
more kcl setting, in order to better debug my crashing issues. Here's what I
found.

1) I believe that the crashing issues I kept having with force_atpx=1 and dc=1
after repeated or longer lasting invocations of the dGPU might have been
because of the shitty ACPI/UEFI implementation of my laptop vendor. I have now
tried setting acpi.osi=!* acpi.osi='Windos 2015' and voilá, the crashing has
vanished. So that seems to be a problem on that end, unfortunately improbable
to ever get properly fixed... But at least there is a workaround and the
problem does not appear to be with your code!

2) The inconsistend dpm behaviour between dc=1 and dc=0 is still a thing.
force_atpx=1 plus dc=1 gives me wonderful dpm with the dGPU fully powering down
between uses. force_atpx=1 plus dc=0 does not. No idea what's up with that.

3) There is a new bug, probably introduced by this commit:
https://cgit.freedesktop.org/~agd5f/linux/patch/?id=7c8c32854566dd9fb2ad4108029670604bc77b19.
On hybrid gpu laptops amdgpu does not actually control the brightness of the
internal screen, because that is controlled by the iGPU. So amdgpu should
probably not try. Maybe put a check in there? There is a crash referenced
related to this issue in the attached dmesg output, look starting at
[10.234751].

I hope this helps and isn't too late for a last minute pull to drm-next for
4.17, because I'd really like to see the force_atpx option in there!