drm/i915: Add comments to explain the BSD tail write workaround
Having had to dive into the bspec to understand what each stage of the workaround meant, and how that the ring broadcasting IDLE corresponded with the GT powering down the ring (i.e. rc6) add comments to aide the next reader. And since the register "is used to control all aspects of PSMI and power saving functions" that makes it quite interesting to inspect with regards to RC6 hangs, so add it to the error-state. v2: Rediscover the piece of magic, set the RNCID to 0 before waiting for the ring to wake up. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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