Quoting Tvrtko Ursulin (2019-10-01 13:16:19)
>
> On 01/10/2019 10:51, Chris Wilson wrote:
> > +
> > + /*
> > + * Setup the pair of contexts such that if we
> > + * lite-restore using the RING_TAIL from ce[1] it
> > + * will execute garbage from ce[0]->ring.
> > + */
> > + memset(ce[n]->ring->vaddr,
> > + POISON_INUSE,
> > + ce[n]->ring->vma->size);
> > + }
> > + intel_ring_reset(ce[1]->ring, ce[1]->ring->vma->size / 2);
> > + __execlists_update_reg_state(ce[1], engine);
> > +
> > + rq[0] = igt_spinner_create_request(&spin, ce[0], MI_ARB_CHECK);
> > + if (IS_ERR(rq[0])) {
> > + err = PTR_ERR(rq[0]);
> > + goto err_ce;
> > + }
> > +
> > + GEM_BUG_ON(rq[0]->tail > ce[1]->ring->emit);
> > + i915_request_get(rq[0]);
> > + i915_request_add(rq[0]);
> > +
> > + if (!igt_wait_for_spinner(&spin, rq[0])) {
> > + i915_request_put(rq[0]);
> > + goto err_ce;
> > + }
> > +
> > + rq[1] = i915_request_create(ce[1]);
> > + if (IS_ERR(rq[1])) {
> > + err = PTR_ERR(rq[1]);
> > + i915_request_put(rq[0]);
> > + goto err_ce;
> > + }
> > + GEM_BUG_ON(rq[1]->tail <= rq[0]->tail);
> > +
> > + /* Ensure we do a completion switch from ce[0] to ce[1] */
> > + i915_request_await_dma_fence(rq[1], &rq[0]->fence);
>
> What do you mean by completion switch? You are setting up a dependency
> so rq[1] (and rq[2]) won't be put into the elsp until spinner is ended
> so it may not even be a context switch. Wouldn't you actually need the
> opposite? I was expecting you would let the spinner run, make sure rq[1]
> is in elsp and then count on time slicing to trigger a context switch.
The test I had in mind was to have
ELSP[0] = ce[0]
ELSP[1] = ce[1]
and so chose to prevent any timeslicing reordering that. Same engine, so
it will add a wait-on-submit-fence (i.e. a no-op) but would install the
dependency link to prevent any reordering.
A second test to have the spinner running then using priority to preempt
it, seems like a good addition.
-Chris
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