[Intel-gfx] [RFC] drm/i915: Add a new modparam for customized ring multiplier

[Chris Wilson]

Quoting Chris Wilson (2017-12-18 21:47:25)
> Quoting Jackie Li (2017-12-18 21:22:08)
> > From: Zhipeng Gong <zhipeng.gong at intel.com>
> > 
> > SKL platforms requires a higher ring multiplier when there's massive
> > GPU load. Current driver doesn't provide a way to override the ring
> > multiplier.
> > 
> > This patch adds a new module parameter to allow the overriding of
> > ring multiplier for Gen9 platforms.
> 
> So the default ring-scaling is not good enough, the first thing we do is
> to try and ensure the defaults work for nearly all use cases. My
> impression is that you want a nonlinear scalefactor, low power workloads
> don't try and ramp up the ring frequencies as aggressively, high power
> workloads try hard for higher frequencies, and then get throttled back
> harder as well. How well can we autotune it? What events tells us if the
> ratio is too high or too low?

One thing that came to mind is that we don't know the min/max ring
frequencies and just program them blindly. Is it the case that at max
gpu freq, there is still headroom on the ring freq, or do you require a
steeper ramp so that you hit the max ringfreq earlier for your workload
(which then presumably can run at less than max gpufreq, so pushing
power elsewhere).
-Chris