Re: [Intel-gfx] [PATCH 1/7] drm/i915: Allocate power domain set wakerefs dynamically

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 02 Nov 2022, Imre Deak <imre.deak@intel.com> wrote:
> Since the intel_display_power_domain_set struct, currently its current
> size close 1kB, can be allocated on the stack, it's better to allocate
> the per-domain wakeref pointer array - used for debugging - within the
> struct dynamically, so do this.
>
> The memory freeing is guaranteed by the fact that the acquired domain
> references tracked by struct can't be leaked either.
>
> Signed-off-by: Imre Deak <imre.deak@intel.com>
> ---
>  .../drm/i915/display/intel_display_power.c    | 61 ++++++++++++++++---
>  .../drm/i915/display/intel_display_power.h    |  2 +-
>  2 files changed, 53 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_display_power.c b/drivers/gpu/drm/i915/display/intel_display_power.c
> index 4c1de91e56ff9..e2da91c2a9638 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_power.c
> +++ b/drivers/gpu/drm/i915/display/intel_display_power.c
> @@ -830,19 +830,58 @@ void intel_display_power_put_unchecked(struct drm_i915_private *dev_priv,
>  }
>  #endif
>  
> +#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_RUNTIME_PM)
> +static intel_wakeref_t *
> +get_debug_wakerefs(struct drm_i915_private *i915,
> +		   struct intel_display_power_domain_set *power_domain_set)
> +{
> +	if (power_domain_set->wakerefs)
> +		return power_domain_set->wakerefs;
> +
> +	power_domain_set->wakerefs = kcalloc(POWER_DOMAIN_NUM,
> +					     sizeof(*power_domain_set->wakerefs),
> +					     GFP_KERNEL);
> +
> +	drm_WARN_ON_ONCE(&i915->drm, !power_domain_set->wakerefs);

The rule of thumb is not to warn or log on allocation failures.

> +
> +	return power_domain_set->wakerefs;
> +}
> +
> +static void
> +free_empty_debug_wakerefs(struct intel_display_power_domain_set *power_domain_set)
> +{
> +	if (power_domain_set->wakerefs &&
> +	    bitmap_empty(power_domain_set->mask.bits, POWER_DOMAIN_NUM))
> +		kfree(fetch_and_zero(&power_domain_set->wakerefs));

FWIW, I'm really not happy about fetch_and_zero() or its use anywhere. I
kind of get the point, but the impression of any kind of atomicity the
naming gives is totally misleading. And it's our own thing in
i915_utils.h, and not a global thing like the name suggests.

> +}
> +#else
> +static intel_wakeref_t *
> +get_debug_wakerefs(struct drm_i915_private *i915,
> +		   struct intel_display_power_domain_set *power_domain_set)
> +{
> +	return NULL;
> +}
> +
> +static void
> +free_empty_debug_wakerefs(struct intel_display_power_domain_set *power_domain_set)
> +{
> +}
> +#endif

get/free is an odd pairing of names.

> +
>  void
>  intel_display_power_get_in_set(struct drm_i915_private *i915,
>  			       struct intel_display_power_domain_set *power_domain_set,
>  			       enum intel_display_power_domain domain)
>  {
>  	intel_wakeref_t __maybe_unused wf;
> +	intel_wakeref_t *debug_wakerefs = get_debug_wakerefs(i915, power_domain_set);
>  
>  	drm_WARN_ON(&i915->drm, test_bit(domain, power_domain_set->mask.bits));
>  
>  	wf = intel_display_power_get(i915, domain);
> -#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_RUNTIME_PM)
> -	power_domain_set->wakerefs[domain] = wf;
> -#endif
> +	if (debug_wakerefs)
> +		debug_wakerefs[domain] = wf;
> +

If you abstracted setting the debug wakeref for a domain, it could
handle the allocation internally without any of the local vars etc.
here. And it would only be a single line in the entire function.

>  	set_bit(domain, power_domain_set->mask.bits);
>  }
>  
> @@ -852,6 +891,7 @@ intel_display_power_get_in_set_if_enabled(struct drm_i915_private *i915,
>  					  enum intel_display_power_domain domain)
>  {
>  	intel_wakeref_t wf;
> +	intel_wakeref_t *debug_wakerefs = get_debug_wakerefs(i915, power_domain_set);
>  
>  	drm_WARN_ON(&i915->drm, test_bit(domain, power_domain_set->mask.bits));
>  
> @@ -859,9 +899,9 @@ intel_display_power_get_in_set_if_enabled(struct drm_i915_private *i915,
>  	if (!wf)
>  		return false;
>  
> -#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_RUNTIME_PM)
> -	power_domain_set->wakerefs[domain] = wf;
> -#endif
> +	if (debug_wakerefs)
> +		debug_wakerefs[domain] = wf;
> +
>  	set_bit(domain, power_domain_set->mask.bits);
>  
>  	return true;
> @@ -873,6 +913,7 @@ intel_display_power_put_mask_in_set(struct drm_i915_private *i915,
>  				    struct intel_power_domain_mask *mask)
>  {
>  	enum intel_display_power_domain domain;
> +	intel_wakeref_t *debug_wakerefs = get_debug_wakerefs(i915, power_domain_set);
>  
>  	drm_WARN_ON(&i915->drm,
>  		    !bitmap_subset(mask->bits, power_domain_set->mask.bits, POWER_DOMAIN_NUM));
> @@ -880,12 +921,14 @@ intel_display_power_put_mask_in_set(struct drm_i915_private *i915,
>  	for_each_power_domain(domain, mask) {
>  		intel_wakeref_t __maybe_unused wf = -1;
>  
> -#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_RUNTIME_PM)
> -		wf = fetch_and_zero(&power_domain_set->wakerefs[domain]);
> -#endif
> +		if (debug_wakerefs)
> +			wf = fetch_and_zero(&debug_wakerefs[domain]);
> +
>  		intel_display_power_put(i915, domain, wf);
>  		clear_bit(domain, power_domain_set->mask.bits);
>  	}
> +
> +	free_empty_debug_wakerefs(power_domain_set);
>  }
>  
>  static int
> diff --git a/drivers/gpu/drm/i915/display/intel_display_power.h b/drivers/gpu/drm/i915/display/intel_display_power.h
> index 7136ea3f233e9..c847aab7b2f88 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_power.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_power.h
> @@ -146,7 +146,7 @@ struct i915_power_domains {
>  struct intel_display_power_domain_set {
>  	struct intel_power_domain_mask mask;
>  #ifdef CONFIG_DRM_I915_DEBUG_RUNTIME_PM
> -	intel_wakeref_t wakerefs[POWER_DOMAIN_NUM];
> +	intel_wakeref_t *wakerefs;
>  #endif
>  };

-- 
Jani Nikula, Intel Open Source Graphics Center