https://bugzilla.kernel.org/show_bug.cgi?id=200695
--- Comment #14 from Claude Heiland-Allen (claude@mathr.co.uk) --- I checked out linux v4.19-rc4 from git, then reverted that commit - no change, display goes blank about 5 seconds into boot.
I noticed something else in the dmesg (it was there in 4.19-rc1,rc2,rc3,rc4,rc4 with reverted commit, but not earlier versions):
[ 5.109572] amdgpu: [powerplay] Failed to retrieve minimum clocks. [ 5.109577] amdgpu: [powerplay] Error in phm_get_clock_info [ 5.109627] [drm] DM_PPLIB: values for Engine clock [ 5.109629] [drm] DM_PPLIB: 300000 [ 5.109631] [drm] DM_PPLIB: 600000 [ 5.109632] [drm] DM_PPLIB: 900000 [ 5.109633] [drm] DM_PPLIB: 1145000 [ 5.109634] [drm] DM_PPLIB: 1215000 [ 5.109636] [drm] DM_PPLIB: 1257000 [ 5.109637] [drm] DM_PPLIB: 1300000 [ 5.109638] [drm] DM_PPLIB: 1366000 [ 5.109640] [drm] DM_PPLIB: Validation clocks: [ 5.109641] [drm] DM_PPLIB: engine_max_clock: 136600 [ 5.109642] [drm] DM_PPLIB: memory_max_clock: 200000 [ 5.109644] [drm] DM_PPLIB: level : 8 [ 5.109646] [drm] DM_PPLIB: values for Memory clock [ 5.109647] [drm] DM_PPLIB: 300000 [ 5.109648] [drm] DM_PPLIB: 1000000 [ 5.109649] [drm] DM_PPLIB: 2000000 [ 5.109651] [drm] DM_PPLIB: Validation clocks: [ 5.109652] [drm] DM_PPLIB: engine_max_clock: 136600 [ 5.109653] [drm] DM_PPLIB: memory_max_clock: 200000 [ 5.109655] [drm] DM_PPLIB: level : 8 [ 5.124083] [drm] Display Core initialized with v3.1.59!
The last (largest) value for "engine clock" and "memory clock" are 10x the validation values for "engine clock max" and "memory clock max". I see in the amd/powerplay sources some values are in units of 10kHz, some in units of 1kHz(?) - maybe a conversion was missed somewhere? Or maybe the printout is totally normal and I know nothing :)
The error message common to all kernels with amdgpu.dc=1 since 4.17 is:
[ 5.256378] [drm] Cannot find any crtc or sizes