drm/i915: paper over missed irq issues with force wake voodoo
Two things seem to do the trick on my ivb machine here:
- prevent the gt from powering down while waiting for seqno
notification interrupts by grabbing the force_wake in get_irq (and
dropping it in put_irq again).
- ordering writes from the ring's CS by reading a CS register, ACTHD
seems to work.
Only the blt&bsd ring on ivb seem to be massively affected by this,
but for paranoia do this dance also on the render ring and on snb
(i.e. all gpus with forcewake).
Tested with Eric's glCopyPixels loop which without this patch scores a
missed irq every few seconds.
This patch needs my forcewake rework to use a spinlock instead of
dev->struct_mutex.
After crawling through docs a lot I've found the following nugget: