Re: [PATCH] drm/i915: Convert the forcewake worker into a timer func

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

On Fri, Feb 28, 2014 at 06:44:03PM +0000, Chris Wilson wrote:
> We don't want to suffer scheduling delay when turning off the GPU after
> waking it up to touch registers. Ideally, we only want to keep the GPU
> awake for the register access sequence, with a single forcewake dance on
> the first access and release immediately after the last. We set a timer
> on the first access so that we only dance once and on the next scheduler
> tick, we drop the forcewake again.
> 
> This moves the cleanup routine from the common i915 workqueue to a timer
> func so that we don't anger powertop, and drop the forcewake again
> quicker.
> 
> v2: Enable the deferred force_wake_put for regular register reads as
> well.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Ben Widawsky <ben@bwidawsk.net>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h     |  2 +-
>  drivers/gpu/drm/i915/intel_uncore.c | 34 +++++++++++++++-------------------
>  2 files changed, 16 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index b22765192018..8af8e0dd3943 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -508,7 +508,7 @@ struct intel_uncore {
>  	unsigned fw_rendercount;
>  	unsigned fw_mediacount;
>  
> -	struct delayed_work force_wake_work;
> +	struct timer_list force_wake_timer;
>  };
>  
>  #define DEV_INFO_FOR_EACH_FLAG(func, sep) \
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index c62841404c82..8ee171178bfe 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -289,10 +289,8 @@ void vlv_force_wake_put(struct drm_i915_private *dev_priv,
>  	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
>  }
>  
> -static void gen6_force_wake_work(struct work_struct *work)
> +static void gen6_force_wake_timer(struct drm_i915_private *dev_priv)
>  {
> -	struct drm_i915_private *dev_priv =
> -		container_of(work, typeof(*dev_priv), uncore.force_wake_work.work);
>  	unsigned long irqflags;
>  
>  	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
> @@ -405,9 +403,8 @@ void gen6_gt_force_wake_put(struct drm_i915_private *dev_priv, int fw_engine)
>  	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
>  	if (--dev_priv->uncore.forcewake_count == 0) {
>  		dev_priv->uncore.forcewake_count++;
> -		mod_delayed_work(dev_priv->wq,
> -				 &dev_priv->uncore.force_wake_work,
> -				 1);
> +		mod_timer_pinned(&dev_priv->uncore.force_wake_timer,
> +				 jiffies + 1);

This could expire more or less immediately, but it should be fine. We'd
just end up doing two forcewake_get()s instead of one, which should
still be better than >2 if the theory of the timer holds.

>  	}
>  	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
>  
> @@ -484,17 +481,15 @@ gen5_read##x(struct drm_i915_private *dev_priv, off_t reg, bool trace) { \
>  static u##x \
>  gen6_read##x(struct drm_i915_private *dev_priv, off_t reg, bool trace) { \
>  	REG_READ_HEADER(x); \
> -	if (NEEDS_FORCE_WAKE((dev_priv), (reg))) { \
> -		if (dev_priv->uncore.forcewake_count == 0) \
> -			dev_priv->uncore.funcs.force_wake_get(dev_priv, \
> -							FORCEWAKE_ALL); \
> -		val = __raw_i915_read##x(dev_priv, reg); \
> -		if (dev_priv->uncore.forcewake_count == 0) \
> -			dev_priv->uncore.funcs.force_wake_put(dev_priv, \
> -							FORCEWAKE_ALL); \
> -	} else { \
> -		val = __raw_i915_read##x(dev_priv, reg); \
> +	if (dev_priv->uncore.forcewake_count == 0 && \
> +	    NEEDS_FORCE_WAKE((dev_priv), (reg))) { \
> +		dev_priv->uncore.funcs.force_wake_get(dev_priv, \
> +						      FORCEWAKE_ALL); \
> +		dev_priv->uncore.forcewake_count++; \
> +		mod_timer_pinned(&dev_priv->uncore.force_wake_timer, \
> +				 jiffies + 1); \
>  	} \
> +	val = __raw_i915_read##x(dev_priv, reg); \
>  	REG_READ_FOOTER; \
>  }
>  
> @@ -681,8 +676,9 @@ void intel_uncore_init(struct drm_device *dev)
>  {
>  	struct drm_i915_private *dev_priv = dev->dev_private;
>  
> -	INIT_DELAYED_WORK(&dev_priv->uncore.force_wake_work,
> -			  gen6_force_wake_work);
> +	setup_timer(&dev_priv->uncore.force_wake_timer,
> +		    (void (*)(unsigned long))gen6_force_wake_timer,

I'd prefer to do the required casting in gen6_force_wake_timer(). Seems
a bit less error prone since we'd at least get type checking for the
function pointer.

> +		    (unsigned long)dev_priv);
>  
>  	if (IS_VALLEYVIEW(dev)) {
>  		dev_priv->uncore.funcs.force_wake_get = __vlv_force_wake_get;
> @@ -794,7 +790,7 @@ void intel_uncore_fini(struct drm_device *dev)
>  {
>  	struct drm_i915_private *dev_priv = dev->dev_private;
>  
> -	flush_delayed_work(&dev_priv->uncore.force_wake_work);
> +	del_timer_sync(&dev_priv->uncore.force_wake_timer);

This could leave force wake enabled.

>  
>  	/* Paranoia: make sure we have disabled everything before we exit. */
>  	intel_uncore_sanitize(dev);
> -- 
> 1.9.0

-- 
Ville Syrjälä
Intel OTC