[Intel-gfx] [PATCH] drm/i915: Reduce locking in command submission

[Ben Widawsky]

On Thu, Jan 15, 2015 at 11:21:30AM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> This eliminates six needless spin lock/unlock pairs when writing out ELSP. Apart
> from tidier code main benefit is between 0.51% and 0.73% speedup on some OGL
> tests under CHV (bench_OglBatch4 bench_OglDeferred respectively).

With 95% confidence t-test on n=5

> 
> Kindly benchmarked by Ben Widawsky.

FWIW, as I mentioned on IRC, I think the reduction of the unnecessary forcewake
(someone should fix the shadow register list) is probably more beneficial than
removing the spin on an uncontested lock. I was tempted to try that myself, but
I didn't have time or much interest since your patch accomplishes the same
thing.

The sucky thing, which I actually care about since I've been doing a lot of
profiling, is the raw MMIO doesn't show up with our i915 trace functions. It's
obtainable still, but then I get a mess of other stuff I don't want.

> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Dave Gordon <david.s.gordon at intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Ben Widawsky <ben at bwidawsk.net>
[snip]