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

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

--- Comment #11 from jan.kokemueller at gmail.com ---
Created attachment 147358
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=147358&action=edit
Fix/workaround for interrupt storm on GM45 when loading i915kms

This seems to be a known hardware quirk with the GM45 GPU where the gmbus can
generate both MSI and non-MSI interrupts:
http://lists.freedesktop.org/archives/dri-devel/2013-March/036295.html

I've managed to stop the interrupts with the attached patch. The first part
restarts the IRQ stuff before calling intel_opregion_init. The second part just
makes sure that intel_opregion_enable_asle called from the irq_postinstall
handler in drm_irq_install has no effect, as this is called again in
intel_opregion_init.

I've tested this on 10-stable with the drm stuff backported and will now check
if it also works with a recent snapshot of -current.

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