[Bug 90340] RADEON "PITCAIRN" displayport output breaks when monitor turned off and on

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90340

            Bug ID: 90340
           Summary: RADEON "PITCAIRN" displayport output breaks when
                    monitor turned off and on
           Product: DRI
           Version: XOrg git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Radeon
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: steve at einval.com

As initially reporte in Debian at
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=783705

I'm running Debian Jessie (3.16.x kernel) on a machine with a Pitcairn-based
video adapter and an NEC Multisync EA274WMi 27" monitor, connected via
DisplayPort.

I've also tried upgrading to the 4.0 kernel and I see exactly the same
symptoms.

If I turn the monitor off and then on again (as I was doing overnight), I get
no display at all. 

I've wiggled the mouse, hit numlock on the keybard (the numlock
led illuminates fine), etc., but no display. I've seen this kind of
thing happen in the past on some machines, so I switch to VT1 and back
to see if that helps. Still no display at all, either on console or
under X. I log in remotely and I can see that the Xorg.0.log file has
been updated with mode lines for the monitor, suggesting things have
just woken up fine. But still no display.

Here's the really weird thing: at this point, the monitor has
basically locked up. It won't respond to the power/input/menu butttons
at all, and is still showing the blue LED that says "I have signal"
rather than switching to the amber "no signal" warning. Therefore, I
can only assume there's a problem here with some weird invalid DP
signal being produced.

I've carried on playing with this. If I log in from another machine and run
xrandr, I can see the display details of the monitor just fine. If I use xrandr
to turn the display off and then on again:

xrandr --output DisplayPort-0 --off
xrandr --output DisplayPort-0 2560x1440

then things start working again.

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