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

[Ben Widawsky]

On Thu, Jan 15, 2015 at 05:05:30PM +0000, Tvrtko Ursulin wrote:
> 
> On 01/15/2015 04:54 PM, Ben Widawsky wrote:
> >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.
> 
> I missed that IRC discussion, but I don't think it was doing forcewakes
> since the outer block in execlists_elsp_write bumps the counters which made
> I915_WRITE & co skip them.
> 
> Regards,
> 
> Tvrtko

I didn't check the locking but it looks like it could actually get decremented
once the spinlock is released. Probably never happens, but I think it's
possible.

I completely missed that block somehow. I think my eyes skipped over it because
how could getting forcewake take like 10+ lines :D