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

[Chris Wilson]

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.
-Chris