Re: [PATCH v3] drm/i915: Handle RC6 counter wrap

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Feb 08, 2018 at 08:18:42AM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@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.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94852
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_drv.h |  2 ++
>  drivers/gpu/drm/i915/intel_pm.c | 62 +++++++++++++++++++++++++++++++++++------
>  2 files changed, 55 insertions(+), 9 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..063c885175e7 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 +9450,49 @@ 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;
> +	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. */
> @@ -9483,9 +9504,32 @@ 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);
>  	}
>  
> +	/*
> +	 * 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);
> +
>  	return DIV_ROUND_UP_ULL(time_hw * mul, div);

What happens when time_hw*mul+div-1 overflows?

>  }
>  
> -- 
> 2.14.1
> 
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-- 
Ville Syrjälä
Intel OTC
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