[Bug 97244] [SKL] 5k tiled monitor DP sync issues

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97244

--- Comment #3 from Tomas Bzatek <bugs at bzatek.net> ---
After number of experiments I've managed to get proper image in native
resolution. Hit rate is very low though, it takes about 20 tries fiddling with
xrandr trying to get the right sync.

Here is my theory: the monitor is one physical panel taking data from two DP
inputs making a final composited image from both, side by side. It probably
takes one output as a reference and tries to synchronize scanlines from the
other. It also probably expects equal timings and particular frames sent at the
same moment on both outputs.

When I see tearing (e.g. scrolling a large image) it's consistent across whole
image, on both outputs.

This was normally not the case when I had two monitors connected to each DP
port in a clone mode - tearing was at different positions (perhaps not
relevant, but worth to mention).


Related xf86-video-intel options:

  Option  "TearFree"           "off"
  Option  "VSync"              "on"
  Option  "DRI"                "3"
  Option  "Present"            "on"
  Option  "ReprobeOutputs"     "off"
  Option  "HotPlug"            "off"
  Option  "HWRotation"         "off"
  Option  "DebugFlushCaches"   "off"
  Option  "DebugWait"          "on"
  Option  "DebugFlushBatches"  "off"
  Option  "FallbackDebug"      "off"

(basically random options but seem to help a little)

This is with 4.7.0 kernel (zen patchset), from drm point of view it's basically
vanilla with the following patches:

  drm/i915/skl: Add support for the SAGV, fix underrun hangs
  drm/i915/skl: Fix redundant cursor update, fix cursor underruns 

So yes, it is possible to drive the monitor with current SW, just the drivers
need some tweaking. Hope this helps...

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