Further debugging today while I'm on-site with the kiosks.  The system
froze, as usual, after normal idle screen blanking.  I was able to login
via ssh and retrieve some details.

First, it appears that the basic screen blanking problem is somehow
related to the Radeon driver.  I will attach the Xorg.0.log file and a
gdb backtrace on the running but non-responsive Xorg process.  No
keyboard or mouse activity will un-blank the screen.

Here's the output from lspci | grep VGA:

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV516 
[Radeon X1300/X1550 Series]

I'll also attach the PCI ROM dump from the /sys/devices rom file for
this device.

This suggests that when we use Alt-SysRq-k to restore the system,
sometimes we trigger *another* bug in the driver which results in the
overall system crash.  See the attached apr1.full.kern.log file for what
details I could recover from the crash on April 1st; that corresponds to
the screen photo I took that day (the radeon_kernel_panic1.jpg
attachment).

The Xorg "hang" bug which precedes this appears similar, but not
identical, to what is documented in #1664979:

https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1664979

I will continue to try to study the live Xorg process to see if I can
determine where its hung, but I may not have time this evening.  Let me
know if there are specific further debugging steps I can take to examine
either the Xorg hang or the subsequent crash which (sometimes) follows
on attempting to recover with Alt-SysRq-k.  (And note that I'll copy in
the files apport-bug generated from #1764211 later tonight, using my
"cdarroch" personal account, and then I'll try to close that extra bug
report.)

Thanks very much,
Chris.

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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:
  New

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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