[Intel-gfx] [PATCH] drm/i915: Cancel pending execlists irq handler upon idling

[Chris Wilson]

Quoting Tvrtko Ursulin (2017-06-28 09:59:04)
> 
> On 27/06/2017 16:25, Chris Wilson wrote:
> > Due to the slight asynchronicity in handling the execlists interrupts
> > (i.e. we defer the work to a handler that may consume more than one
> > interrupt event), when the engine is idle we may still have an irq
> > tasklet queued (especially when it has been deferred to a ksoftirqd).
> > At the beginning of the tasklet, we assert that we do hold a device
> > wakeref for the access we are about to perform. This assumes that when
> > we idle and release the GT wakeref, all execlists work has been
> > completed (since the elsp tracking says the hw is idle). However, there
> > may still be a tasklet queued, so as we mark the engine idle, also
> > cancel any pending tasklet.
> 
> We check the irq_posted bit which should correspond with a pending 
> tasklet (intel_engines_are_idle/intel_engine_is_idle), before 
> transitioning to idle so I don't understand this.

Exactly, we've processed the interrupt in the current irq handler, but
due to the ordering (which is essential to ensure that we don't miss an
interrupt, i.e. the strong ordering is via the tasklet atomic ops so
that each interrupt is always followed by a tasklet, at least if we do
have elsp[]!) we can queue a second tasklet execution despite it already
being handled concurrently.

Run long enough and this rare event will then coincide with idling.
-Chris