[Intel-gfx] [PATCH 1/3] drm/i915: Drop stray clearing of rps->last_adj

[Chris Wilson]

Quoting Mika Kuoppala (2018-08-01 10:38:55)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > We used to reset last_adj to 0 on crossing a power domain boundary, to
> > slow down our rate of change. However, commit 60548c554be2 ("drm/i915:
> > Interactive RPS mode") accidentally caused it to be reset on every
> > frequency update, nerfing the fast response granted by the slow start
> > algorithm.
> >
> > Fixes: 60548c554be2 ("drm/i915: Interactive RPS mode")
> > Testcase: igt/pm_rps/mix-max-config-loaded
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_pm.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> > index 2531eb75bdce..f90a3c7f1c40 100644
> > --- a/drivers/gpu/drm/i915/intel_pm.c
> > +++ b/drivers/gpu/drm/i915/intel_pm.c
> > @@ -6371,7 +6371,6 @@ static void gen6_set_rps_thresholds(struct drm_i915_private *dev_priv, u8 val)
> >               new_power = HIGH_POWER;
> >       rps_set_power(dev_priv, new_power);
> >       mutex_unlock(&rps->power.mutex);
> > -     rps->last_adj = 0;
> 
> To follow the old logic, you should zero it in rps_set_power ?

I was opting for a new logic, no more nerf on domain crossing. The
domain crossing is just a rate change, so shouldn't really impact on the
slow start algorithm; what I really want is to detect a null EI and use
that as the backoff. That would help with the overshoot, but the effect
of dampening is likely to be firmly in the noise.
-Chris