[Intel-gfx] [PATCH 10/10] drm/i915: Flush idle barriers when waiting

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-10-14 14:08:12)
> 
> On 11/10/2019 16:11, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2019-10-11 15:56:35)
> >>
> >> On 10/10/2019 08:14, Chris Wilson wrote:
> >>> If we do find ourselves with an idle barrier inside our active while
> >>> waiting, attempt to flush it by emitting a pulse using the kernel
> >>> context.
> >>
> >> The point of this one completely escapes me at the moment. Idle barriers
> >> are kept in there to be consumed by the engine_pm parking, so if any
> >> random waiter finds some (there will always be some, as long as the
> >> engine executed some user context, right?),
> > 
> > Not any random waiter; the waiter has to be waiting on a context that
> > was active and so setup a barrier.
> > 
> >> why would it want to handle
> >> them? Again just to use the opportunity for some house keeping? But what
> >> if the system is otherwise quite busy and a low-priority client just
> >> happens to want to wait on something silly?
> > 
> > There's no guarantee that it will ever be flushed. So why wouldn't we
> > use a low priority request to give a semblance of forward progress and
> > give a guarantee that the wait will complete.
> > 
> > It's a hypothetical point, there are no waiters that need to wait upon
> > their own barriers at present. We are just completing the picture for
> > idle barrier tracking.
> 
> Hm I was mistakenly remembering things like rpcs reconfiguration would 
> wait on ce->active, but I forgot about your trick with putting kernel 
> context request on an user timeline.
> 
> I guess it is fine there, but since, and as you have said, it is 
> hypothetical, then this patch is dead code and can wait.

Why would we even bother checking against the potential invalid pointer
dereference then?... :-p
-Chris