threaded handlers.
It's not easy to see from the diffs of this change exactly how it
accomplishes the above. The arm_mask_irq() and arm_unmask_irq() functions
are, respectively, the pre_thread and post_thread hooks. Not seen in
these diffs, the arm_post_filter() routine also EOIs. The post_filter
routine runs after filter handlers if there will be no threaded handlers,
so it just EOIs. The pre_thread routine masks the interrupt (at the
controller, not the source) and EOIs. So one way or another, the EOI
happens at the point where filter handlers are done.
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