[Bug 193500] New: Interrupt storm after loading i915kms module on Gen4 Intel GPU

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=193500

            Bug ID: 193500
           Summary: Interrupt storm after loading i915kms module on Gen4
                    Intel GPU
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: amd64
                OS: Any
            Status: Needs Triage
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: jan.kokemueller at gmail.com

When I test the new i915 code (r270990 snapshot iso or backported to 10-stable,
doesn't matter) on my gen4 Intel GPU (GMA 4500MHD), I'm getting reproducible
interrupt storms from "irq16: uhci0" after loading the i915kms module. It does
not matter if I start X or not. This looks very similar to the situation here:

https://lists.freebsd.org/pipermail/freebsd-hackers/2014-June/045369.html

Setting hw.drm.msi=0 in /boot/loader.conf doesn't help. One suspend/resume
cycle makes the problem go away. When I revert commit r270516 (the opregion
changes) the problem goes away, too. Then there is no irq16 at all in the
output of "vmstat -i".

I've attached the output of "pciconf -lvc" and the contents of
/var/run/dmesg.boot.

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