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

[Chris Wilson]

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?

Adding an untested unsafe modparam is a big no. If you want us to
support such a parameter, we at least need it not to taint the kernel
and come with some tests that prove it functions as intended. Better
still might be a I915_SETPARAM (or hooking up the write func of the
modparam) to support changing the value on the fly.
-Chris