[Intel-gfx] [PATCH v3] drm/i915/pmu: Reconstruct active state on starting busy-stats

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-01-12 09:40:40)
> So submit side doesn't work in either case, unless I am missing 
> something. Would need the pair of port manipulation and context_in to be 
> atomic.

Sure, there is a small window with the result that we either never turn
off the stats, or turn them off one too early (and then recover if the
my understanding of the underflow protection holds). The same problem as
existed before the reconstruction, except now the window is much
smaller. I'm not that scared by this (it should not explode, nor result
in hopelessly wrong stats) so it can wait until stats enabling doesn't
need to peek at execlists. I think you will need to postpone enabling
until the next context-switch if we wanted to avoid the atomics; except
that poses a problem with the igt expecting busy-start to be accurate. A
dilemma for later.
-Chris