[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 #22 from Gary Wang <gary.c.wang at intel.com> ---
Hi Paul (reply to #21),
Totally agree on you. The title of this issue is really confusing.

Customer reported it suffered from system hung when plug-in into VGA port
originally in SKL-U platform, and didn't say how design of display in DUT.

After checking its internal display design of DUT, it uses one DP MST-enabled
Hub (Synaptics DP Hub VMM3320) and one DP-to-VGA converter to extend theirs
display port for 2 DPs and 2 VGAs.

For comment #1, I used DP-to-VGA converters (DP 1.1a & DP 1.2) to check it,
they works well in my Intel SKL-U CRB.

For comment #7, I used DP 1.2 MST-enabled HUB (Sunix IT DisplayPort to Dual
Graphics Splitter 1-input DP and 2 output DPs) to test this issue in SKL-U
Intel CRB, i.e. using CRB's miniDP port connecting to DP MST Hub for DP MST on
two DP monitors via that DP MST Hub. It suffered from the same issue in my
Intel SKL-U CRB (dp training error from
intel_mst_pre_enable_dp()/intel_dp_mst_compute_config()).

For comment #13 and #14, it seems to cause trouble between source MST link
computation among DP source/branch/sink when enabling MST display (but aux
works well). Then disable MST for SST between source and sink makes test (from
#7 with one DP 1.2 monitor & one DP 1.1a monitor) working well with
functionality of DP MST in Ubuntu 15.04 (but still not try DP daisy chain and
guess could be trouble due to lacking MST).

All of them were tested based on drm-intel-nightly-2015/10/19.

Thanks!

Gary

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