When trying to turn on the "pseudo NMI" kernel feature in Linux, it
was discovered that all Mediatek-based Chromebooks that ever shipped
(at least ones with GICv3) had a firmware bug where they wouldn't save
certain GIC "GICR" registers properly. If a processor ever entered a
suspend/idle mode where the GICR registers lost state then they'd be
reset to their default state.

As a result of the bug, if you try to enable "pseudo NMIs" on the
affected devices then certain interrupts will unexpectedly get
promoted to be "pseudo NMIs" and cause crashes / freezes / general
mayhem.

ChromeOS is looking to start turning on "pseudo NMIs" in production to
make crash reports more actionable. To do so, we will release firmware
updates for at least some of the affected Mediatek Chromebooks.
However, even when we update the firmware of a Chromebook it's always
possible that a user will end up booting with old firmware. We need to
be able to detect when we're running with firmware that will crash and
burn if pseudo NMIs are enabled.

The current plan is:
* Update the device trees of all affected Chromebooks to include the
  'mediatek,broken-save-restore-fw' property. The kernel can use this
  to know not to enable certain features like "pseudo NMI". NOTE:
  device trees for Chromebooks are never baked into the firmware but
  are bundled with the kernel. A kernel will never be configured to
  use "pseudo NMIs" and be bundled with an old device tree.
* When we get a fixed firmware for one of these Chromebooks, it will
  patch the device tree to remove this property.

For some details, you can also see the public bug
<https://issuetracker.google.com/281831288>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230515131353.v2.1.Iabe67a827e206496efec6beb5616d5a3b99c1e65@changeid