[Intel-gfx] [PATCH v2] drm/i915/gt: Transfer old virtual breadcrumbs to irq_worker

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-05-12 17:07:23)
> 
> On 12/05/2020 16:52, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-05-12 16:17:30)
> >>
> >> On 12/05/2020 14:22, Chris Wilson wrote:
> >>> -     spin_lock(&old->breadcrumbs.irq_lock);
> >>> -     if (!list_empty(&ve->context.signal_link)) {
> >>> -             list_del_init(&ve->context.signal_link);
> >>> -
> >>> -             /*
> >>> -              * We cannot acquire the new engine->breadcrumbs.irq_lock
> >>> -              * (as we are holding a breadcrumbs.irq_lock already),
> >>> -              * so attach this request to the signaler on submission.
> >>> -              * The queued irq_work will occur when we finally drop
> >>> -              * the engine->active.lock after dequeue.
> >>> -              */
> >>> -             set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags);
> >>> -
> >>> -             /* Also transfer the pending irq_work for the old breadcrumb. */
> >>> -             intel_engine_signal_breadcrumbs(rq->engine);
> >>> -     }
> >>> -     spin_unlock(&old->breadcrumbs.irq_lock);
> >>> +     intel_engine_transfer_breadcrumbs(ve->siblings[0], &ve->context);
> >>
> >> But isn't ve->siblings[0] the old engine at this point so new target
> >> engine would have to be explicitly passed in?
> > 
> > ve->siblings[0] is the old engine, which is holding the completed
> > requests and their signals. Since their rq->engine == ve->siblings[0]
> > and we can't update rq->engine as we can't take the required locks, we
> > need to keep the breadcrumbs relative to ve->siblings[0] and not the new
> > engine (the i915_request_cancel_breadcrumb conundrum).
> 
> Who then enables breadcrumbs on the new engine?

If we enable signaling on the request we are about to submit, that
will enable the breadcrumbs on the new engine. We've cleared out
ce->signals and removed it from the old engine, so as soon as we want to
enable a new breadcrumb, it will be attached to the [new] rq->engine.
-Chris