[Intel-gfx] [PATCH v3 12/13] drm/i915/execlists: Preemption!

[Chris Wilson]

Quoting Mika Kuoppala (2017-09-29 07:55:45)
> > @@ -533,7 +555,45 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >       spin_lock_irq(&engine->timeline->lock);
> >       rb = execlists->first;
> >       GEM_BUG_ON(rb_first(&execlists->queue) != rb);
> > -     while (rb) {
> > +     if (!rb)
> > +             goto unlock;
> > +
> > +     if (last) {
> > +             /*
> > +              * Don't resubmit or switch until all outstanding
> > +              * preemptions (lite-restore) are seen. Then we
> > +              * know the next preemption status we see corresponds
> > +              * to this ELSP update.
> > +              */
> > +             if (port_count(&port[0]) > 1)
> > +                     goto unlock;
> > +
> > +             if (can_preempt(engine) &&
> > +                 rb_entry(rb, struct i915_priolist, node)->priority >
> > +                 max(last->priotree.priority, 0)) {
> > +                     /*
> > +                      * Switch to our empty preempt context so
> > +                      * the state of the GPU is known (idle).
> > +                      */
> > +                     inject_preempt_context(engine);
> > +                     execlists->preempt = true;
> > +                     goto unlock;
> > +             } else {
> > +                     if (port_count(&port[1]))
> > +                             goto unlock;
> 
> I am assuming that this is check for hw
> availability and nothing else? 

Technically this check is that we use last = port[0], and only check
port[0] for preemption. In theory, it would be possible to coalesce new
requests onto the second port, but it's complicated. (We would need to
track the possible lite-restore on the second port, i.e. whilst
submitting there was a context switch, but we may never see that
preemption event. Then we have the complication with priorities, where
we might be coalescing a high priority request onto the second port,
losing track of it.)
-Chris