[Bug 107546] Screen is frozen on second connection of DP MST dock

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107546

--- Comment #2 from Stanislav Lisovskiy <stanislav.lisovskiy at intel.com> ---
(In reply to Sergey Menshikov from comment #0)
> Kernel 4.15.0.30 Ubuntu 18.04.1 Xorg/Gnome Lenovo 4th gen X1 20FBxxx Skylake
> i7 Intel built-in video, Lenovo OneLink+ DP MST dock with dual dell 24"
> displays
> 
> Error messages upon dock disconnect:
> 
> [drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link training
> [drm:intel_mst_pre_enable_dp [i915]] *ERROR* failed to allocate vcpi
> 
> Next connection leads to frozen screen on laptop, no change on exterenal
> monitors. 
> 
> [drm:intel_wait_ddi_buf_idle [i915]] *ERROR* Timeout waiting for DDI BUF C
> idle bit
> 
> Disconnecting the dock restores laptop screen functionality, but subsequent
> connection of the dock has the same symptoms repeating. Reboot fixes the
> issue until first dock disconnect. 
> 

Does it happen only to Laptop integrated screen? This might be a duplicate of
https://bugs.freedesktop.org/show_bug.cgi?id=106250, which in fact consists of 
two different bugs: one is related to watermark calculation algorithm, 
basically exceeding available resources for Y Tiled framebuffer for 4K
screenmodes - which manifests as black laptop integrated screen and if you
enable drm.debug=0x14, you should see some complaints regarding exceeding
available ddb_allocation.
Another bug, which it might be related to is that sometimes we don't get
RRSetCrtcConfig request from X Server client, upon receiving the hotplug event,
which results in not receiving drm_mode_setcrtc call for one of the screeens.
This one seems to affect external displays and happens every second time you
plug/unplug external display.
So please attach dmesg traces with drm.debug=0x14 enabled, so that we could
differentiate if its a duplicate or something else.
Also you can try to manually enable the screen by using xrandr --output (output
name) --crtc (number) - try different crtc numbers here, if it helps it might
also mean that we simply don't get the modesetting call.

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