[Intel-gfx] [PATCH 13/14] drm/i915/guc: Workaround the missing user interrupt after preemption

[Chris Wilson]

Quoting Michał Winiarski (2017-10-19 19:36:18)
> With GuC, we're scheduling tasklet on USER_INTERRUPT - since some work
> has completed, perhaps we're able to submit some more work. We're doing
> similar thing for preemption - after preemption has completed, it's time
> to schedule the tasklet and submit more work (since the engine is now
> idle). Unfortunately, we can hit the scenarios where the preemption is
> done, but the interrupt is nowhere to be seen. To work around the
> problem, let's use a delayed work that's kicking the tasklet if
> preemption is done, and queueing itself otherwise.

I'm not buying it yet. Missing USER_INTERRUPT would not be guc specific
(surely?) and so we would be getting similar missed-breadcrumb warnings
on skl+. We dedicate a lot of CI towards detecting those...

The first alternative that springs to mind is bad ordering between ggtt
write and MI_USER_INTERRUPT, but that too is common to execlists, and
also results in missed-breadcrumb warnings (which are notable by their
absence). Though having said that, the HWSP ordering issue with
intel_iommu is worrying me that we have similar issues with breadcrumbs
+ intel_iommu. Is iommu a factor?

In short, if this is a problem here, it should be a problem everywhere.
Right?
-Chris