Re: [PATCH] drm/i915: prevent gt fifo count underflow

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

On Wed, May 14, 2014 at 04:18:02PM +0300, Mika Kuoppala wrote:
> If we get the final value of zero as a count of free
> entries available, we will underflow our own fifo_count
> and then it will take a long time before we check things again.
> Admittedly we are in trouble already if we get into this situation,
> but prevent the underflow by returning early.
> 
> v2: Less convoluted control flow (Daniel Vetter)
> 
> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_uncore.c |   20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index 76dc185..bf1b661 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -154,10 +154,8 @@ static void __gen7_gt_force_wake_mt_put(struct drm_i915_private *dev_priv,
>  		gen6_gt_check_fifodbg(dev_priv);
>  }
>  
> -static int __gen6_gt_wait_for_fifo(struct drm_i915_private *dev_priv)
> +static bool __gen6_gt_wait_for_fifo(struct drm_i915_private *dev_priv)
>  {
> -	int ret = 0;
> -
>  	/* On VLV, FIFO will be shared by both SW and HW.
>  	 * So, we need to read the FREE_ENTRIES everytime */
>  	if (IS_VALLEYVIEW(dev_priv->dev))
> @@ -173,12 +171,12 @@ static int __gen6_gt_wait_for_fifo(struct drm_i915_private *dev_priv)
>  			fifo = __raw_i915_read32(dev_priv, GTFIFOCTL) & GT_FIFO_FREE_ENTRIES_MASK;
>  		}
>  		if (WARN_ON(loop < 0 && fifo <= GT_FIFO_NUM_RESERVED_ENTRIES))

Maybe kill the 'loop<0' check while at it. It's redundant and IMO just
makes things less obvious.

Also I don't get why we first check for
'fifo_count < GT_FIFO_NUM_RESERVED_ENTRIES', but then the while
loop checks for 'fifo <= GT_FIFO_NUM_RESERVED_ENTRIES'.

> -			++ret;
> +			return true;
>  		dev_priv->uncore.fifo_count = fifo;

We no longer update fifo_count on failure. Not really a problem, but
since we've already done all the work maybe we should still update it.

>  	}
>  	dev_priv->uncore.fifo_count--;
>  
> -	return ret;
> +	return false;
>  }
>  
>  static void vlv_force_wake_reset(struct drm_i915_private *dev_priv)
> @@ -642,13 +640,13 @@ gen5_write##x(struct drm_i915_private *dev_priv, off_t reg, u##x val, bool trace
>  #define __gen6_write(x) \
>  static void \
>  gen6_write##x(struct drm_i915_private *dev_priv, off_t reg, u##x val, bool trace) { \
> -	u32 __fifo_ret = 0; \
> +	bool __fifo_failed = false; \
>  	REG_WRITE_HEADER; \
>  	if (NEEDS_FORCE_WAKE((dev_priv), (reg))) { \
> -		__fifo_ret = __gen6_gt_wait_for_fifo(dev_priv); \
> +		__fifo_failed = __gen6_gt_wait_for_fifo(dev_priv); \
>  	} \
>  	__raw_i915_write##x(dev_priv, reg, val); \
> -	if (unlikely(__fifo_ret)) { \
> +	if (unlikely(__fifo_failed)) { \
>  		gen6_gt_check_fifodbg(dev_priv); \
>  	} \
>  	REG_WRITE_FOOTER; \
> @@ -657,14 +655,14 @@ gen6_write##x(struct drm_i915_private *dev_priv, off_t reg, u##x val, bool trace
>  #define __hsw_write(x) \
>  static void \
>  hsw_write##x(struct drm_i915_private *dev_priv, off_t reg, u##x val, bool trace) { \
> -	u32 __fifo_ret = 0; \
> +	bool __fifo_failed = false; \
>  	REG_WRITE_HEADER; \
>  	if (NEEDS_FORCE_WAKE((dev_priv), (reg))) { \
> -		__fifo_ret = __gen6_gt_wait_for_fifo(dev_priv); \
> +		__fifo_failed = __gen6_gt_wait_for_fifo(dev_priv); \
>  	} \
>  	hsw_unclaimed_reg_clear(dev_priv, reg); \
>  	__raw_i915_write##x(dev_priv, reg, val); \
> -	if (unlikely(__fifo_ret)) { \
> +	if (unlikely(__fifo_failed)) { \
>  		gen6_gt_check_fifodbg(dev_priv); \
>  	} \
>  	hsw_unclaimed_reg_check(dev_priv, reg); \
> -- 
> 1.7.9.5
> 
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-- 
Ville Syrjälä
Intel OTC