[RFC 4/4] drm/i915: Expose RPS thresholds in sysfs

[Prahlad Kilambi]

> One question is are we able to find a "one size fits all" values.

> However regardless of that, given we already expose frequency controls in sysfs
> with the same reasoning of allowing system owners explicit control if so wanted,
> I think exposing the thresholds can be equally justified.

Exposing these RPS thresholds via sysfs allows for dynamic tuning of these values at runtime. Common scenarios where we can benefit from variable frequency ramping include plugged in vs battery where differing thresholds allow to weight more for performance vs power. Data from testing on ChromeOS Gen12 platforms where GuC isn't enabled indicates gains > 10% across several games. See https://gitlab.freedesktop.org/drm/intel/-/issues/8389#note_1890428 for details.