[Bug 90909] [HSW DP] Can't get 4k at 60Hz (pipe underruns)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90909

--- Comment #3 from Tiago Madeira <tmadeira at gmail.com> ---
Created attachment 116396
  --> https://bugs.freedesktop.org/attachment.cgi?id=116396&action=edit
dmesg with monitor MST configuration set to secundary

Sorry, it turns out I had MST disabled in my monitor. I enabled MST-secondary
and now the output is different (dmesg attached). To start with, xrandr shows
me a DP1-8 instead of DP1:

Screen 0: minimum 8 x 8, current 5206 x 2160, maximum 32767 x 32767
eDP1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 277mm x
156mm
   1366x768      60.00*+
   1024x768      60.00  
   800x600       60.32    56.25  
   640x480       59.94  
DP1 disconnected (normal left inverted right x axis y axis)
DP1-1 disconnected (normal left inverted right x axis y axis)
DP1-8 connected 3840x2160+1366+0 (normal left inverted right x axis y axis)
527mm x 296mm
   3840x2160     29.98*+
   2560x1440     59.95  
   2048x1280     59.99  
   1920x1200     59.88  
   1920x1080     60.00    60.00    50.00    59.94    30.00    25.00    24.00   
29.97    23.98  
   1600x1200     60.00  
   1600x900      59.98  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.08    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08  
DP2 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

However, I still can't get 60Hz. If I change to MST-primary, X crashes.
According to the monitor user guide:

  MST
  “Off”: Default mode - 4k2k 60Hz with MST function Disabled.
  “Primary”: Set as primary mode at 4K2K 30Hz with MST (DP out) enabled.
  “Secondary”: Set as secondary mode at 4K2K 30Hz with MST (DP out) disabled.
  NOTE: Secondary mode can be set using hot key to switch to DP 1.1 when there
is no display. See Troubleshooting.

Not sure what configuration the driver expects me to use here. I believe HSW is
supposed to support 4k at 60Hz with MST enabled, right?

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