Quoting Tvrtko Ursulin (2018-02-08 15:46:38)
> From: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> 
> We can implement limited RC6 counter wrap-around protection under the
> assumption that clients will be reading this value more frequently than
> the wrap period on a given platform.
> 
> With the typical wrap-around period being ~90 minutes, even with the
> exception of Baytrail which wraps every 13 seconds, this sounds like a
> reasonable assumption.
> 
> Implementation works by storing a 64-bit software copy of a hardware RC6
> counter, along with the previous HW counter snapshot. This enables it to
> detect wrap is polled frequently enough and keep the software copy
> monotonically incrementing.
> 
> v2:
>  * Missed GEN6_GT_GFX_RC6_LOCKED when considering slot sizing and
>    indexing.
>  * Fixed off-by-one in wrap-around handling. (Chris Wilson)
> 
> v3:
>  * Simplify index checking by using unsigned int. (Chris Wilson)
>  * Expand the comment to explain why indexing works.
> 
> v4:
>  * Use __int128 if supported.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94852
> Cc: Chris Wilson <ch...@chris-wilson.co.uk>
> Reviewed-by: Chris Wilson <ch...@chris-wilson.co.uk> # v3
> Cc: Ville Syrjälä <ville.syrj...@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h |  2 ++
>  drivers/gpu/drm/i915/intel_pm.c | 72 
> +++++++++++++++++++++++++++++++++++------
>  2 files changed, 64 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index ad1fc845cd1b..28a2671a26c7 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -946,6 +946,8 @@ struct intel_rps {
>  
>  struct intel_rc6 {
>         bool enabled;
> +       u64 prev_hw_residency[4];
> +       u64 cur_residency[4];
>  };
>  
>  struct intel_llc_pstate {
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 41f26ab46501..24d9423c5f9e 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -9417,15 +9417,16 @@ static u64 vlv_residency_raw(struct drm_i915_private 
> *dev_priv,
>                              const i915_reg_t reg)
>  {
>         u32 lower, upper, tmp;
> -       unsigned long flags;
>         int loop = 2;
>  
> -       /* The register accessed do not need forcewake. We borrow
> +       /*
> +        * The register accessed do not need forcewake. We borrow
>          * uncore lock to prevent concurrent access to range reg.
>          */
> -       spin_lock_irqsave(&dev_priv->uncore.lock, flags);
> +       lockdep_assert_held(&dev_priv->uncore.lock);
>  
> -       /* vlv and chv residency counters are 40 bits in width.
> +       /*
> +        * vlv and chv residency counters are 40 bits in width.
>          * With a control bit, we can choose between upper or lower
>          * 32bit window into this counter.
>          *
> @@ -9449,29 +9450,54 @@ static u64 vlv_residency_raw(struct drm_i915_private 
> *dev_priv,
>                 upper = I915_READ_FW(reg);
>         } while (upper != tmp && --loop);
>  
> -       /* Everywhere else we always use VLV_COUNTER_CONTROL with the
> +       /*
> +        * Everywhere else we always use VLV_COUNTER_CONTROL with the
>          * VLV_COUNT_RANGE_HIGH bit set - so it is safe to leave it set
>          * now.
>          */
>  
> -       spin_unlock_irqrestore(&dev_priv->uncore.lock, flags);
> -
>         return lower | (u64)upper << 8;
>  }
>  
>  u64 intel_rc6_residency_ns(struct drm_i915_private *dev_priv,
>                            const i915_reg_t reg)
>  {
> -       u64 time_hw;
> +#if IS_ENABLED(CONFIG_ARCH_SUPPORTS_INT128)
> +       unsigned __int128 acc;
> +#else
> +       u64 acc;
> +#endif
> +       u64 time_hw, prev_hw, overflow_hw;
> +       unsigned int fw_domains;
> +       unsigned long flags;
> +       unsigned int i;
>         u32 mul, div;
>  
>         if (!HAS_RC6(dev_priv))
>                 return 0;
>  
> +       /*
> +        * Store previous hw counter values for counter wrap-around handling.
> +        *
> +        * There are only four interesting registers and they live next to 
> each
> +        * other so we can use the relative address, compared to the smallest
> +        * one as the index into driver storage.
> +        */
> +       i = (i915_mmio_reg_offset(reg) -
> +            i915_mmio_reg_offset(GEN6_GT_GFX_RC6_LOCKED)) / sizeof(u32);
> +       if (WARN_ON_ONCE(i >= ARRAY_SIZE(dev_priv->gt_pm.rc6.cur_residency)))
> +               return 0;
> +
> +       fw_domains = intel_uncore_forcewake_for_reg(dev_priv, reg, 
> FW_REG_READ);
> +
> +       spin_lock_irqsave(&dev_priv->uncore.lock, flags);
> +       intel_uncore_forcewake_get__locked(dev_priv, fw_domains);
> +
>         /* On VLV and CHV, residency time is in CZ units rather than 1.28us */
>         if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv)) {
>                 mul = 1000000;
>                 div = dev_priv->czclk_freq;
> +               overflow_hw = BIT_ULL(40);
>                 time_hw = vlv_residency_raw(dev_priv, reg);
>         } else {
>                 /* 833.33ns units on Gen9LP, 1.28us elsewhere. */
> @@ -9483,10 +9509,36 @@ u64 intel_rc6_residency_ns(struct drm_i915_private 
> *dev_priv,
>                         div = 1;
>                 }
>  
> -               time_hw = I915_READ(reg);
> +               overflow_hw = BIT_ULL(32);
> +               time_hw = I915_READ_FW(reg);
>         }
>  
> -       return DIV_ROUND_UP_ULL(time_hw * mul, div);
> +       /*
> +        * Counter wrap handling.
> +        *
> +        * But relying on a sufficient frequency of queries otherwise counters
> +        * can still wrap.
> +        */
> +       prev_hw = dev_priv->gt_pm.rc6.prev_hw_residency[i];
> +       dev_priv->gt_pm.rc6.prev_hw_residency[i] = time_hw;
> +
> +       /* RC6 delta from last sample. */
> +       if (time_hw >= prev_hw)
> +               time_hw -= prev_hw;
> +       else
> +               time_hw += overflow_hw - prev_hw;
> +
> +       /* Add delta to RC6 extended raw driver copy. */
> +       time_hw += dev_priv->gt_pm.rc6.cur_residency[i];
> +       dev_priv->gt_pm.rc6.cur_residency[i] = time_hw;
> +
> +       intel_uncore_forcewake_put__locked(dev_priv, fw_domains);
> +       spin_unlock_irqrestore(&dev_priv->uncore.lock, flags);
> +
> +       acc = time_hw;
> +       acc *= mul;
> +
> +       return DIV_ROUND_UP_ULL(acc, div);

return mul_u64_u32_div(time_hw, mul, div).

Loses roundup. But meh.
-Chris
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