[Bug 87917] New: MST not being enabled on Haswell host

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=87917

            Bug ID: 87917
           Summary: MST not being enabled on Haswell host
           Product: xorg
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: steven at uplinklabs.net
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Created attachment 111587
  --> https://bugs.freedesktop.org/attachment.cgi?id=111587&action=edit
dmesg 3.18.1, unhappy MST

I've got a GIGABYTE Brix Pro i7-4770R box with a Mini DisplayPort output.
Attached to that port I have a Dell UP2414Q 4K monitor, with DisplayPort 1.2
mode enabled. On Windows I'm able to use the native MST-backed 60Hz 4K
resolution without issues.

On Linux (3.18.1), I'm having troubles getting that to happen.

First of all, X11 is starting at 1024x768 resolution, and the kernel said this
about it:

[    2.631515] [drm] Cannot find any crtc or sizes - going 1024x768

Once X11 is started, I'm able to see the two halves of the display in my KDE
monitor preferences (DP1-8 and DP1-9, where DP1-9 is the left half of the
display). I can also set both of them to 1920x2160 and configure the
positioning such that the two halves are treated as separate monitor outputs
side-by-side. But ideally this should be discovered as one 3840x2160 monitor,
on a single DisplayPort output. That would also make it so that my X11 WM would
treat it as a single desktop.

I've booted my kernel with drm.debug=6, and attached it for inspection.

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