[Intel-gfx] [PATCH 2/2] drm/i915: Remove requirement for holding i915_request.lock for breadcrumbs

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-07-16 11:11:55)
> 
> 
> On 16/07/2020 10:16, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-07-16 10:01:15)
> >>
> >> On 14/07/2020 10:47, Chris Wilson wrote:
> >>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> index e0280a672f1d..aa7be7f05f8c 100644
> >>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>> @@ -1148,20 +1148,6 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
> >>>                } else {
> >>>                        struct intel_engine_cs *owner = rq->context->engine;
> >>>    
> >>> -                     /*
> >>> -                      * Decouple the virtual breadcrumb before moving it
> >>> -                      * back to the virtual engine -- we don't want the
> >>> -                      * request to complete in the background and try
> >>> -                      * and cancel the breadcrumb on the virtual engine
> >>> -                      * (instead of the old engine where it is linked)!
> >>> -                      */
> >>> -                     if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> >>> -                                  &rq->fence.flags)) {
> >>> -                             spin_lock_nested(&rq->lock,
> >>> -                                              SINGLE_DEPTH_NESTING);
> >>> -                             i915_request_cancel_breadcrumb(rq);
> >>> -                             spin_unlock(&rq->lock);
> >>> -                     }
> >>
> >> Why is this not needed any more?
> > 
> > Hindsight says that this was just paper. We have just done the same
> > cancellation step in i915_request_unsubmit, so all this was doing is
> > shrinking the window for a breadcrumb being attached [and with the
> > serialisation around testing the I915_FENCE_FLAG_ACTIVE_BIT there should
> > be no window at all] while the request is being moved back to the virtual
> > engine.
> 
> True, how did we miss it until now..
> 
> > Now the question is how do we end up attaching to the virtual engine
> > while unsubmitted, and that appears to be entirely due to the ACTIVE
> > flag being set in retire after completion. The other way is that we use
> > the local virtual engine breadcrumb list to defer the signaling on
> > submission of an old request.
> > 
> > So... If I can fix the issues around retirement that means we should be
> > able to kill off virtual_xfer_breadcrumbs... I hope so.
> 
> I read this as a slightly different series will be coming?

Yes. Testing in progress, while I write comments.
-Chris