[Intel-gfx] [PATCH 6/8] drm/i915: reprogram NOA muxes on context switch when using perf

[Chris Wilson]

Quoting Chris Wilson (2018-04-25 12:57:24)
> If they are not context saved, userspace isn't going to be allowed to
> modify them. So why do we need this? If they not context saved, then the
> kernel needs to control them and doesn't need to reset around every
> batch, just the one's that want non-default (fancier if we handle
> preemption)?

Preemption is the killer here. Jumping from one preempted batch back to
inside another and you skip over all ring preambles. We could use the
preemption ring itself to set state though.

Still I would rather see an alternative to wa_bb than incur the cost
where it is never needed :|
-Chris