(In reply to Alex Deucher from comment #88)
> Created attachment 115774 [details] [review] [review]
> add some debugging output
> 
> Can you attach your dmesg output with this patch attached?  It should help
> narrow down where the dpcd info is getting corrupted.

Here it is (only the relevant section):
[    5.534907] [drm] radeon_dp_getdpcd
[    5.534913] [drm] DPCD: 11 0a 84 01 00 0b 01 01 02 00 00 00 00 00 00
[    5.565816] [drm] fb mappable at 0xD047B000
[    5.565823] [drm] vram apper at 0xD0000000
[    5.565825] [drm] size 8294400
[    5.565828] [drm] fb depth is 24
[    5.565830] [drm]    pitch is 7680
[    5.566022] fbcon: radeondrmfb (fb0) is primary device
[    5.566093] [drm] radeon_dp_set_link_config, 270000, 2
[    5.566095] [drm] DPCD: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    6.706063] [drm] radeon_dp_set_rx_power_state, 0
[    6.706065] [drm] DPCD: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    7.843201] [drm] radeon_dp_link_train
[    7.843203] [drm] DPCD: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    7.843204] [drm] dp_info dpcd: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    7.843206] [drm] radeon_dp_set_rx_power_state, 0
[    7.843206] [drm] DPCD: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00