[Bug 89453] 4k MST monitor doesn't come back from "xset dpms force off": failed link training

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89453

--- Comment #10 from Palmer Dabbelt <palmer at dabbelt.com> ---
I've been trying to play around a bit to see if I could get any more
information, as I'm afraid those logs aren't useful at all.  Here's
what I found:

* My monitor (Asus PB287Q) has two modes, listed as "DisplayPort 1.1
  Steam" and "DisplayPort 1.2 Stream".  The monitor only supports
  4k at 60Hz when on DP1.2, which is what I've been using.  I believe
  this means the monitor is not actually a MST monitor, but I'm not
  sure as I don't know much about this stuff.

* When I set the monitor to "DisplayPort 1.1 Stream" it defaults to
  1080p at 60Hz, but I still get similar behavior -- the monitor works
  fine before that MST related commit, and is broken afterwards.

* In addition to not coming back from DPMS off, the monitor doesn't
  hotplug.  There aren't any errors in dmesg when it fails to come up
  this way.

* Everything appears to work fine (both before and after the MST
  commit) when I use a Dell 2408WFPb, which I assume is a DP1.1-only
  monitor (it's manufacture date is listed as August 26th, 2009, which
  I believe predates the DP1.2 standard).

* linux-4.0-rc3 exhibits the same behavior as 3.17.

Are there are any more useful debug logs I can produce?

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