Created attachment 139004 [details]
callstack from DRM/radeon

DRM/radeon fails with in drm_dp_link_rate_to_bw_code 

Driver fails to negotiate video resolution with DP-connected monitor, on any of
4 DP ports. 
Controller can drive monitor at 2048x1080 through HDMI adapter. Straight DP
cable only works in console (low-res)

Disconnecting power cable from monitor for 5 minutes trick doesn't help ;)

Video controller: FirePro v8800 ChipID 0x6888 (see hardware.txt for details, 4
DPs)
DP monitor: Dell U2717DA 
Workstation: HP xw8600
Kernel messages with backtrace: see var_log_messages_excerpt.txt
Max res supported by monitor: 1440p (see Xorg.0.log)