[Intel-gfx] [PATCH] drm/i915: Replace some more busy waits with normal ones

[Chris Wilson]

On Wed, Mar 23, 2016 at 04:24:48PM +0000, Tvrtko Ursulin wrote:
> Biggest thing to make sure is that you don't add a lot of cycles to
> the forcewake loops since for example fw_domains_get can be the
> hottest i915 function on some benchmarks.
> 
> (This area slightly annoys me anyway with redundant looping over
> forcewake domains and we could also potentially optimize the ack
> waiting by first requesting all we want, and then doing the waits.
> That would be one additional loop, but if removed the other one,
> code would stay at the same number of domain loops.)

I hear you. I just end up weeping in the corner when I see fw_domain_get
on the profile.

We already do have a mitigation scheme to hold onto the forcewake for an
extra jiffie every time. I don't like it, but without it fw_domains_get
becomes a real hog.

Note that one thing we can actually do is restrict the domains we wakeup
for the engines (engine->fw_domain) in execlists_submit, that should
help chv/skl+ a small amount.

I don't have a good idea for how to keep rc6 residency high but avoid
forcewake when those darn elsp require forcewake. As does gen6+ legacy
RING_TAIL writes. And even then that spinlock causes quite a bit of
traffic when it shouldn't be contended. I've been thinking of whether we
can have multiple locks (hashed by register) but we would then still
need some cross-communication for the common forcewake.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre