Quoting Daniele Ceraolo Spurio (2020-02-12 01:08:30)
>
>
> On 2/10/20 12:57 PM, Chris Wilson wrote:
> > @@ -1934,6 +2002,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >
> > return;
> > }
> > +
> > + last = skip_lite_restore(engine, last, &submit);
> > }
> > }
> >
> > @@ -2155,10 +2225,11 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> > WRITE_ONCE(execlists->yield, -1);
> > execlists_submit_ports(engine);
> > set_preempt_timeout(engine);
> > - } else {
> > + }
> > +
> > + if (intel_engine_has_tail_lrm(engine) || !submit)
>
> Why do we skip here if intel_engine_has_tail_lrm is true? I see that if
> we have work pending we either take the skip_lite_restore() or the
> submit path above, but I can't see why we need to explicitly skip
> re-starting the ring.
You mean if !has_lrm. We have to delay letting the RING_TAIL move past
the end of the request until the HW has acknowledged the preemption
request. This is required to avoid the ELSP submission from trying to
move the RING_TAIL backwards.
As it turns out, I can't special case has_lrm here since if we read the
new RING_TAIL before the ELSP event, we end up submitting the same
RING_TAIL again and trigging the HW bug.
>
> > skip_submit:
> > ring_set_paused(engine, 0);
> > - }
> > }
> >
> > static void
> > @@ -2325,7 +2396,8 @@ static void process_csb(struct intel_engine_cs *engine)
> >
> > GEM_BUG_ON(!assert_pending_valid(execlists, "promote"));
> >
> > - ring_set_paused(engine, 0);
> > + if (!intel_engine_has_tail_lrm(engine))
> > + ring_set_paused(engine, 0);
> >
>
> here as well, although I'm assuming it has the same explanation as the
> one above.
For has_lrm, it will have already seen the new RING_TAIL at the end of
the request regardless of the preempting ELSP.
However, as noted without only setting wa_tail in the context-image and
LRM normal tail, we end up hitting WaIdleLiteRestore and killing the HW.
Bleugh.
-Chris
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