[Intel-gfx] [PATCH 2/2] drm/i915: Filter out spurious execlists context-switch interrupts

[Chris Wilson]

Quoting Mika Kuoppala (2017-10-20 14:21:08)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Back in commit a4b2b01523a8 ("drm/i915: Don't mark an execlists
> > context-switch when idle") we noticed the presence of late
> > context-switch interrupts. We were able to filter those out by looking
> > at whether the ELSP remained active, but in commit beecec901790
> > ("drm/i915/execlists: Preemption!") that became problematic as we now
> > anticipate receiving a context-switch event for preemption while ELSP
> > may be empty. To restore the spurious interrupt suppression, add a
> 
> This confuses me, how can we preempt something that was never submitted.
> Could you elaborate?

The ELSP may become empty after we told the hw to switch to the preempt
context. So the preemption context-switch may appear as a separate
interrupt after !port_isset(execlists.port[0]). The joy of asynchronous
communications.
-Chris