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

[Mika Kuoppala]

Chris Wilson <chris at chris-wilson.co.uk> writes:

> 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.

Let me check if I got this right: as we dont use port[0] to
switch to preempt context and thus we might get 2 interrupts in a
a row, first one clearing port[0], we can't assume any idleness
by port[0] status alone?

-Mika