One further historical detail: when these machines were running 14.04.3
and we made the mistake of accepting the HardWare Enablement (HWE)
upgrade:

https://wiki.ubuntu.com/1404_HWE_EOL

which replaced the old fglrx driver with the radeon one, that's when we
first encountered this unable-to-unblank problem.

Fortunately, at that time we were able to roll back Xorg to 1.16 and
keep things sort-of working:

https://askubuntu.com/questions/815591/ubuntu-14-04-5-16-04-and-newer-on-amd-graphics
https://askubuntu.com/questions/676216/downgrade-xorg-server

When we performed a clean, full re-install with 16.04 we were hoping
this problem would be resolved, but as I've documented here it continues
to affect the systems.

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https://bugs.launchpad.net/bugs/1763273

Title:
  kernel panic in Radeon driver while screen blank

Status in linux package in Ubuntu:
  Incomplete
Status in xorg package in Ubuntu:
  Confirmed

Bug description:
  I maintain a set of older kiosks running a mix of stock Ubuntu 14.04
  LTS and 16.04.  As we have gradually transitioned them to 16.04, we
  have noticed that the machines running 16.04.1 now regularly exhibit
  problems restoring from the blank screen which appears after a period
  of inactivity.

  This only occurs on the machines where we've installed 16.04.  All of
  the 16.04.1 installations are "fresh", that is, complete re-installs
  from scratch, and we install all security and other updates on a
  regular basis.  This problem has persisted right from the beginning
  when we started using 16.04.

  Following a period of user inactivity, the screen goes blank; this is,
  of course, expected.  When the user tries to restore the session by
  typing a key or moving the mouse, one of three outcomes occurs.

  Sometimes, the session restores normally.  Other times, the screen
  remains blank regardless of all normal keyboard input, until a magic
  SysRq  sequence is performed.  After Alt-SysRq-k, either the virtual
  console resets to a login screen (as expected after this SysRq
  sequence), or we get a kernel panic text screen (see attached screen
  photo) and can only restore via a hard power-cycle reboot.

  I will continue to try to capture a better trace output from one of
  these conditions, perhaps from /var/log/kern.log or by installing
  linux-crashdump (although these old machines may not have enough
  memory for the stock package).  But in the meantime, I'm attaching a
  photo of the kernel panic we see, which suggests a problem may reside
  in the Radeon driver ... which would seem possible, given the general
  blank-screen no-resume problem on these systems.

  Any advice on how to further capture any other needed details would be
  appreciated.  Thanks very much!

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