linux-kernel - [i915] [BUG] Cannot run 4k (3840x2160) @ 60Hz: FIFO underrun

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Date:	Thu, 23 Jul 2015 13:13:48 +0200
From:	Nico Schottelius <nico-linux-20150723@...ottelius.org>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: [i915] [BUG] Cannot run 4k (3840x2160) @ 60Hz: FIFO underrun

Good day,

I'm trying to connect a 28" monitor [0] to the Lenovo X1 Carbon [1]
with the resolutions 2560x1440 on the notebook and
3840x2160 on the external monitor. The monitor is configured to use
DisplayPort 1.2 and advertises its resolution / rate correctly.

Running with a refresh rate of 30Hz works fine, however trying
to use it with 60Hz results in a screwed up image (mostly black with some dots
spread around the whole screen when moving the mouse) and the
following message in dmesg:


[49567.582373] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun

Looking for all related messages, I see:

[13:12] wurzel:~% dmesg | grep -e drm:intel -e i915
[44943.441717] i915 0000:00:02.0: BAR 6: [??? 0x00000000 flags 0x2] has bogus alignment
[44951.816797] [drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link training
[44951.819478] [drm:intel_dp_complete_link_train [i915]] *ERROR* failed to start channel equalization
[49542.971817] [drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link training
[49542.974549] [drm:intel_dp_complete_link_train [i915]] *ERROR* failed to start channel equalization
[49567.582373] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun
[49624.669823] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun
[13:12] wurzel:~% 

Has anyone a hint on how to get this running? As far as I can see, the graphic card should be able
to run this resolution / refresh rate. The monitor is connected via DisplayPort.

Cheers,

Nico

--------------------------------------------------------------------------------
[0]
eDP1 connected primary 2560x1440+0+2160 (normal left inverted right x axis y axis) 310mm x 174mm
   2560x1440     60.00*+
   1920x1440     60.00  
   1856x1392     60.01  
   1792x1344     60.01  
   1600x1200     60.00  
   1400x1050     59.98  
   1280x1024     60.02  
   1280x960      60.00  
   1024x768      60.00  
   800x600       60.32    56.25  
   640x480       59.94  

[1]
DP1 connected 3840x2160+0+0 (normal left inverted right x axis y axis) 621mm x 341mm
   3840x2160     60.00 +  30.00* 
   1920x1080     60.00    60.00    50.00    50.00    59.94  
   1920x1080i    60.00    50.00    59.94  
   1680x1050     59.95  
   1280x1024     75.02    60.02  
   1440x900      74.98    59.89  
   1280x960      60.00  
   1280x720      59.97    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    72.81    66.67    60.00    59.94  
   720x400       70.08  


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