Quoting Chris Wilson (2018-04-11 11:39:29)
> We can refine our current execlists->queue_priority if we inspect
> ELSP[1] rather than the head of the unsubmitted queue. Currently, we use
> the unsubmitted queue and say that if a subsequent request is more
> important than the current queue, we will rerun the submission tasklet
> to evaluate the need for preemption. However, we only want to preempt if
> we need to jump ahead of a currently executing request in ELSP. The
> second reason for running the submission tasklet is amalgamate requests
> into the active context on ELSP[0] to avoid a stall when ELSP[0] drains.
> (Though repeatedly amalgamating requests into the active context and
> triggering many lite-restore is off question gain, the goal really is to
> put a context into ELSP[1] to cover the interrupt.) So if instead of
> looking at the head of the queue, we look at the context in ELSP[1] we
> can answer both of the questions more accurately -- we don't need to
> rerun the submission tasklet unless our new request is important enough
> to feed into, at least, ELSP[1].
> 
> v2: Add some comments from the discussion with Tvrtko.
> v3: More commentary to cross-reference queue_request()
> 
> References: f6322eddaff7 ("drm/i915/preemption: Allow preemption between 
> submission ports")
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Michał Winiarski <michal.winiar...@intel.com>
> Cc: Michel Thierry <michel.thie...@intel.com>
> Cc: Mika Kuoppala <mika.kuopp...@linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>

From the discussion thread,

Reviewed-by: Tvrtko Ursulin <tvrtko.ursu...@intel.com>

I assume Tvrtko is happy with the comments added based on that
conversation.
-Chris
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