Issue is reproducible on Dell Chromebook 13(lulu board) - kernel 3.14

To recover from freeze it is required to unplug DisplayLink device which is quite strange. Maybe the switch to VT mode is not finished hence we see that hang.

So in nutshell, the problem is that after switching to VT2 you can't go back to VT1... unless you unplug the docking station first, then try to switch, and reconnect the dock when the graphical interface is back. After these steps, both built-in and docked screens work again.

This is reproducible with both UDL and evdi.

Dominik, is this something in your jurisdiction?

In general this is probably a nice to fix, not a release blocker, since VT2 is a dev mode only concept and the primary concern is normal mode. So if this turns out to be complex to fix it may be left as is indefinitely.

The easy work around would be to use the ctrl+alt+t terminal, running `shell` there gives you a similar effect to VT2. 

Yep, found the problem in frecon. We were leaking UDL devices, fix upcoming.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/frecon/+/45896023df222fd0cc1e075a9392b414bd42330b

commit 45896023df222fd0cc1e075a9392b414bd42330b
Author: Dominik Behr <dbehr@chromium.org>
Date: Thu Nov 10 22:12:59 2016

frecon: fix a leak of  DRM device

When we scan DRM devices we fail to destroy DRM device that we are not going
to use. Because of how devices are ordered we did not hit this problem with
VGEM, now we do with UDL. It also prevented using UDL immediately after boot.

BUG= chromium:663707 
TEST=boot Chrome OS with UDL connected, switch to frecon and back to Chrome.

Change-Id: Ic1d76d93557af0f02a40dcc68b6ad61c565a5fe4
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/410221
Reviewed-by: Haixia Shi <hshi@chromium.org>

[modify] https://crrev.com/45896023df222fd0cc1e075a9392b414bd42330b/drm.c

This only matters in dev mode, why would we need to merge this into 55?

Assuming rejected for now.

Actually, it fixes more than just dev mode.
If you boot with UDL connected, frecon creates UDL DRM device which has master and because it leaks it, it keeps master and Chrome cannot get it. So Chrome cannot initalize UDL display when it starts and you will see blank screen on UDL.
You will have to disconnect and connect it back to see something.

I suspect this change may fix some complaints that people have about USB docks with DisplayLink displays.

Ok, that is reasonable, thanks for the clarification, please feel free to merge at your convenience.

Thanks Bernie, Dominik.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/frecon/+/c2f9d1522e7a8df2aa4396f692c0f1cf93cf3e02

commit c2f9d1522e7a8df2aa4396f692c0f1cf93cf3e02
Author: Dominik Behr <dbehr@chromium.org>
Date: Thu Nov 10 22:12:59 2016

frecon: fix a leak of  DRM device

When we scan DRM devices we fail to destroy DRM device that we are not going
to use. Because of how devices are ordered we did not hit this problem with
VGEM, now we do with UDL. It also prevented using UDL immediately after boot.

BUG= chromium:663707 
TEST=boot Chrome OS with UDL connected, switch to frecon and back to Chrome.

Change-Id: Ic1d76d93557af0f02a40dcc68b6ad61c565a5fe4
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/410221
Reviewed-by: Haixia Shi <hshi@chromium.org>
(cherry picked from commit 45896023df222fd0cc1e075a9392b414bd42330b)
Reviewed-on: https://chromium-review.googlesource.com/410268
Reviewed-by: YH Lin <yueherngl@chromium.org>
Commit-Queue: YH Lin <yueherngl@chromium.org>
Tested-by: YH Lin <yueherngl@chromium.org>

[modify] https://crrev.com/c2f9d1522e7a8df2aa4396f692c0f1cf93cf3e02/drm.c

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