i915kms, Xorg, and "Power management discrepency" error

[Bigby James]

Ever since rebuilding world from commit r208369 (merging the newer i915kms from
HEAD) I've continually received this error in my stdout/logs:

> kernel: error: [drm:pid1228:gen6_sanitize_pm] *ERROR* Power management discrepancy: GEN6_RP_INTERRUPT_LIMITS expected 1a0d0000, was 1a000000

The only thing that ever changes is the PID of the process, which always points
to Xorg. Under most typical production use this doesn't seem to have any effect,
but anytime I watch any sort of video on my production laptop the CPU/GPU temp
in my laptop very quickly climbs above 80 degrees celsius. On my multi-purpose
server/HTPC, starting an XBMC/Kodi session has the same effect, even if the
system is idling (just sitting at the main menu with nothing playing at all).
This doesn't stop until I cease video playback on my laptop or kill the Kodi
session on the server. Note that the hardware in both machines is identical
(yes, the "server" is a laptop), and they're base systems and kernels are
synchronized, so whatever is affectng one should be what's affecting the other.

I came across an older mailing list thread from February[1] on this same basic
subject that didn't seem to go anywhere, but it had the implication that the
i915kms module was the cause. That precisely this same error message is present
on various Linux forums bolsters  the notion that the video driver might be to
blame. If anyone has any insight into how to begin debugging this, I'd be
grateful. Thanks in advance.

- Bigby

[1]: http://freebsd.1045724.n5.nabble.com/Weird-ACPI-DRM-messages-on-current-td5986712.html

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