[Intel-gfx] [PATCH] drm/i915/selftests: Exercise potential false lite-restore

[Chris Wilson]

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