If a Asus monitor PQ321 is used in MST mode it gets no signal.
This is because the function drm_dp_calc_pbn_mode() in drm_dp_mst_topology.c returns a bad pixel bandwith number for the mode
Modeline 131:"1920x2160" 60 297000 1920 2008 2052 2200 2160 2168 2178 2250 0x48 0x5
The function return 8 and not the correct value of 1063.
Using simple u32 arithmetic in drm_dp_calc_pbn_mode() the monitor works fine.
I'm not sure if unsigned 32 bit would overflow in some cases.
Presumed fixed by
Author: Harry Wentland <email@example.com>
Date: Fri Jan 22 17:07:26 2016 -0500
drm/dp/mst: Calculate MST PBN with 31.32 fixed point