[Bug 91791] [SKL-U] System hang when plug in a VGA monitor

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91791

--- Comment #24 from Gary Wang <gary.c.wang at intel.com> ---
Created attachment 119332
  --> https://bugs.freedesktop.org/attachment.cgi?id=119332&action=edit
Picture: suffers issue when plug-in DP MST-enabled with modeset=1

Hi Paul,

I used following information to this issue for QA's reference,

Intel CRB: Skylake Client platform/Skylake U DDR3L RVP7
CPU: i3-6100U CPU @ 2.30GHz (family: 0x6, model: 0x4e, stepping: 0x3)
BIOS version: BIOS SKLSE2R1.R00.X097.B02.1509020030 09/02/2015
Display: CRB's miniDP with DP MST-enabled Hub (Sunix IT DisplayPort,
http://www.amazon.com/IT-DisplayPort-Graphics-Splitter-DPD2001/dp/B00JATFVXI)
Linux Gfx kernel Driver: drm-intel-nightly-2015/10/19

Reproduce steps:
1. Boot into Ubuntu 15.04 with kernel drm-intel-nightly-2015/10/19 successfully
without plug-in mini-DP port
2. Prepare DP MST environment by miniDP cable-->DP MST-enabled HUB-->Connect to
both/either DP 1.2 MST-supported monitor and/or DP 1.1a monitor
3. Plug-in miniDP cable into CRB's miniPort (it will encounter it immediately)

Warning: f booting with DP MST environment (i.e. step2/3), it suffered system
hung immediately (please refer the picture).

BTW, from my analysis for booting by kernel drm-intel-nightly-2015/10/19 + x11
only, it seems to no problem on DP MST connecting (sometime MSTM_CTRL failed to
write) but failed in its enabling.
...
Feb 17 22:22:38 test kernel: [  546.442146] [drm:intel_dp_check_mst_status]
channel EQ not ok, retraining
Feb 17 22:22:38 test kernel: [  546.443339] [drm:intel_dp_set_signal_levels]
Using signal levels 00000000
Feb 17 22:22:38 test kernel: [  546.443340] [drm:intel_dp_set_signal_levels]
Using vswing level 0
Feb 17 22:22:38 test kernel: [  546.443341] [drm:intel_dp_set_signal_levels]
Using pre-emphasis level 0
...
Feb 17 22:22:38 test kernel: [  547.039900]
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too many voltage
retries, give up
...

Thanks for your help!

Gary

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