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[Simon <eierfrucht@xxxxxxxxx>]

After enabling the hidden BIOS options for Native ASPM and Native PCIE
Runtime PM and disabling the C-states, I got rid of the issue (a hard
CMOS reset was necessary for the BIOS modification to fully kick in)

The C-states continued to work in Ubuntu, though, due to the nature of
the intel_idle built-in driver.

I am currently investigating which one of the above mentioned three BIOS
settings triggers on the issue.

Even if they aren't part of the problem, Native ASPM and PCIE PM gave me
much better CPU temps (seem to require the pcie_aspm=force and
pcie_aspm.policy=powersave kernel boot flags to work, otherwise neither
BIOS nor the OS perform power management)

Modifying a Zenbook BIOS turned out to be very simple -- you simply
download the latest official BIOS, open it in an AMI APTIO BIOS
modification utility, look for the hidden options you want to enable and
finally change their access level from Default to User. Then you flash
the modified BIOS file using either AFUDOS with /X flag or ASUS
Emergency BIOS Restore (rename UX21EAS.214 to UX21E.BIN, put it on a
flash stick formatted as FAT16 with a partition size <1Gb, insert it in
a USB 2.0 slot and power on the ultrabook while pressing Ctrl + Fn +
Home, release the said buttons after the ASUS logo appears -- the device
will force-reflash its BIOS with the file from the flash stick)

-- 
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https://bugs.launchpad.net/bugs/1446027

Title:
  SNA acceleration causes sudden shutdowns and corrupts CMOS data on the
  the Asus UX21E ultrabook on a wide variety of kernels since at least
  Ubuntu 14.04

Status in xorg package in Ubuntu:
  Incomplete

Bug description:
  SUMMARY: On the ASUS UX21E ultrabook, enabling SNA graphics
  acceleration leads to spontaneous shutdowns while watching Flash video
  in full screen AND using the battery as the power source, the problem
  starts surfacing once the battery charge level drops below 33%. The
  issue affects both x86 and x64 platforms, affects at least the 14.04
  and 14.10 Ubuntu versions, affects at least the regular Ubuntu and its
  Kubuntu and Lubuntu flavors, affects at least kernels 3.13, 3.14,
  3.15, 3.16, 3.17, 3.18, 3.19 and 4.0. The issue can't be fixed by
  changing the various i915 driver options like rc6 and semaphores,
  neither does PCIE_ASPM play a role. Repetitive triggering of the bug
  runs the machine into an even weirder state where it would chaotically
  reboot every time a higher-than-moderate strain is put on the GPU, and
  the only way to fix this is to detach both battery connectors from the
  motherboard for a while (CMOS battery and the main power source
  battery) The issue surprisingly disappears after a hard CMOS reset AND
  setting the acceleration method to UXA. The BIOS version of my UX21E
  is 214.  I've also taken my UX21 to the official ASUS service center
  and after running the tests they replied that the unit has no hardware
  issues, so it is a Linux issue. A vaguely similar issue is reported to
  affect UX21E running Windows 8, so it may be a crippled DSDT after
  all.

  THE LONG STORY:

  =========================================================================================

  IMPORTANT UPDATE! As of 14.04 and 14.10 using the default SNA graphics
  acceleration instead of UXA leads to unexpected shutdowns and/or CMOS
  settings corruption. To activate the bug, one must be:

  -- Using the battery as the power source

  -- Having the battery charge below 33%

  -- Watching a FULLSCREEN video in Flash Player (e.g. youtube)

  -- Both Pepper Flash in Chrome/Chromium and the regular Adobe Flash in
  Firefox/Chromium are affected

  This is the most typical scenario for triggering an unexpected
  shutdown, but other variants do exist. Certain Flash-heavy sites like
  speedtest.net can trigger this, as well as KDE's KWin window
  compositor with certain GLX-accelerated eye candy options turned on.
  The bug also affects at least Kubuntu and Lubuntu, but that's just
  what I was able to confirm. Most likely it affects all other Ubuntu
  flavors and even other distributions that use the latest Intel video
  drivers AND employ SNA as the default acceleration method.

  The issue persists with various Ubuntu versions (at least 14.04 &
  14.10), various kernel versions (I tried almost every kernel from 3.13
  to 4.0) and even using the cutting-edge drivers from the xorg-edgers
  PPA does nothing to fix this. Both x86 and x64 platforms are affected.
  Flash Player version plays no role, either. The bug apparently happens
  because the ultrabook's ACPI-based power management mechanisms just
  can't get on terms with the latest implementation of SNA. Something
  happens when the battery charge drops below 1/3 -- and once SNA draws
  too much power from the GPU, a shutdown happens. The various i915.*
  driver options like rc6 and semaphores do nothing to fix this!

  The worst thing is that after you get several unexpected shutdowns,
  your UX21E may start rebooting chaotically whenever any program tries
  to use graphics acceleration, be it Chromium's accelerated canvas
  rendering or simply the window manager using transparency. This
  further leads to file system corruption up to the point when you can
  no longer boot into your user profile. You can still create another
  user profile via the terminal, but this does not get rid of the reboot
  glitch.

  If you happen to run into this, you must perform a hard CMOS reset. To
  do this, one must unscrew the twelve torx screws and remove the bottom
  part of the casing, then detach *BOTH* the main battery connector and
  the CMOS battery connector from the motherboard, then wait for a few
  minutes and attach them back. Flushing CMOS programmatically with a
  command-line utility like CmosPwd does not fix this issue! If you are
  unsure how to open your UX21E or detach the battery connectors, search
  youtube for an UX21E disassemby guide -- detaching the battery on this
  model used to be a popular trick a few years ago when a certain
  Windows glitch caused similar behavior and required hard CMOS reset as
  well, so there are a number of videos and forum threads on this issue
  across the web.

  Once you have made sure your CMOS is okay (that is, you don't get
  frequent shutdowns shortly after logging into the system while working
  on battery with a charge below 33%) you can proceed to fix the
  ultimate cause of this issue. If it exists, delete the
  /usr/share/X11/xorg.conf.d/20-intel.conf file then create a new one
  containing the following lines:

  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  Section "Device"

  Identifier "Intel Graphics"

  Driver "intel"

  Option "AccelMethod" "uxa"

  EndSection

  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  Reboot and the issue is fixed!

  I also recommend using the following kernel boot arguments with this
  model to provide maximum power saving and stability in 14.04 and
  14.10:

  intel_pstate=enable pcie_aspm=force pcie_aspm.policy=default
  ath9k.ps_enable=1 i915.semaphores=1 i915.enable_rc6=7 i915.powersave=1
  i915.lvds_downclock=1 drm.vblankoffdelay=1 i915.enable_fbc=1

  Enabling 'i915.enable_fbc' and 'i915.lvds_downclock' can lead to minor
  graphical glitches with SNA acceleration, but since SNA is a no go due
  to the above mentioned bug, you can safely turn them on with UXA.
  'pcie_aspm' absolutely needs being set to 'force' and
  'pcie_aspm.policy' to 'default' -- the UX21E is very picky about this
  as of the latest official kernels in 14.04 and 14.10.
  'ath9k.ps_enable' is required to enable wireless power saving in power
  managing software like TLP

  I hope someone re-formats my remarks to fit into the article in a more
  befitting manner and/or files a bug on the Ubuntu bug tracker because
  I'm almost exhausted and broken while typing this. Tracking this
  sucker down cost me almost two months of my life because once you run
  into the chaotic reboot issue, you can't fix anything without hard
  resetting the CMOS, and it took me too long to realize.

  Thanks for understanding.

  P.S. This may also affect other Asus ultrabooks with integrated Intel
  graphics like UX31E. A Windows bug with very similar symptoms is known
  to affect the whole model line, especially with Windows 8.1

  =========================================================================================

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