I've had good enough experiences with mainline 4.8.15 at this point that I would consider this bug fixed in linux 4.8.x; it's still not perfect (have to typically play around with xrandr and maybe undock/dock), but external 4k DP monitor via a ThinkPad dock works fairly reliably now.
OTOH I was unable to get any DP video output at all with mainline Linux 4.9...
I've had good enough experiences with mainline 4.8.15 at this point that I would consider this bug fixed in linux 4.8.x; it's still not perfect (have to typically play around with xrandr and maybe undock/dock), but external 4k DP monitor via a ThinkPad dock works fairly reliably now.
OTOH I was unable to get any DP video output at all with mainline Linux 4.9...