Created attachment 123000 [details]
dmesg from drm-fixes-4.6

HW is R9 380X in primary PCIe 3 slot on ASUS P8B75-V with three monitors
attached (2x DVI and 1 HDMI).

First observed in 4.5 when Powerplay was enabled by default. Trace of SMC
messages around failure from drm-fixes-4.6 kernel:

[    9.666775] tonga_send_msg_to_smc(..., 0x145)
[    9.666799] tonga_send_msg_to_smc(..., 0x146)
[    9.666875] tonga_send_msg_to_smc(..., 0x5E)           
[    9.666895] tonga_send_msg_to_smc(..., 0x148)
[    9.667775] tonga_send_msg_to_smc(..., 0x145)
[    9.667800] tonga_send_msg_to_smc(..., 0x146)
[    9.774079] [drm] fb mappable at 0xE0BAA000
[    9.774083] [drm] vram apper at 0xE0000000
[    9.774085] [drm] size 9216000
[    9.774087] [drm] fb depth is 24
[    9.774089] [drm]    pitch is 7680
[    9.774213] fbcon: amdgpudrmfb (fb0) is primary device
[    9.774345] tonga_send_msg_to_smc(..., 0x5E)
[    9.943267] Failed to send Message.
[   10.112239] tonga_send_msg_to_smc(..., 0x145)

Other Powerplay functionality works correctly with pcie_dpm_key_disabled=1.