[PATCH] drm/i915/pmu: Fix sleep under atomic in RC6 readout

[Imre Deak]

On Tue, Feb 06, 2018 at 04:04:10PM +0000, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-02-06 14:31:07)
> > +static u64 read_rc6_residency(struct drm_i915_private *i915)
> > +{
> > +       u64 val;
> > +
> > +       val = intel_rc6_residency_ns(i915, IS_VALLEYVIEW(i915) ?
> > +                                          VLV_GT_RENDER_RC6 : GEN6_GT_GFX_RC6);
> > +       if (HAS_RC6p(i915))
> > +               val += intel_rc6_residency_ns(i915, GEN6_GT_GFX_RC6p);
> > +       if (HAS_RC6pp(i915))
> > +               val += intel_rc6_residency_ns(i915, GEN6_GT_GFX_RC6pp);
> 
> We really should mention that these may produce interesting results
> every 53 minutes. Switching to a timer will allow us to notice the
> wraparound in each counter.
> 
> > +
> > +       return val;
> > +}
> > +
> > +static void
> > +update_rc6_sample(struct drm_i915_private *i915, u64 val, bool locked)
> > +{
> > +       unsigned long flags;
> > +
> > +       if (!locked)
> > +               spin_lock_irqsave(&i915->pmu.lock, flags);
> > +
> > +       /*
> > +        * Update stored RC6 counter only if it is greater than the current
> > +        * value. This deals with periods of runtime suspend during which we are
> > +        * estimating the RC6 residency, so do not want to overshoot the real
> > +        * value read once the device is woken up.
> > +        */
> > +       if (val > i915->pmu.sample[__I915_SAMPLE_RC6].cur)
> > +               i915->pmu.sample[__I915_SAMPLE_RC6].cur = val;
> 
> 64b wraparound? Maybe not today, maybe not tomorrow... ;)
> 
> > +
> > +       /* We don't need to sample RC6 from the timer any more. */
> > +       i915->pmu.timer_enabled =
> > +               __pmu_needs_timer(i915,
> > +                                 i915->pmu.enable & ~config_enabled_mask(I915_PMU_RC6_RESIDENCY),
> > +                                 READ_ONCE(i915->gt.awake));
> 
> But we do... :)
> One thing I had in mind was to hook into runtime suspend/resume to read
> the counters there and compensate, but the more I think about it, we may
> as well solve the lack of resolution in the rc6 counters whilst we are
> here. https://bugs.freedesktop.org/show_bug.cgi?id=94852.

How about using dev->power.suspended_jiffies?

--Imre