Re: [Intel-gfx] [PATCH v2 2/4] drm/i915: Add getter/setter for i915_gem_object->frontbuffer

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

On Tue, 30 May 2023, Jouni Högander <jouni.hogander@intel.com> wrote:
> Add getter/setter for i915_gem_object->frontbuffer and use it instead of
> directly touching i915_gem_object->frontbuffer frontbuffer pointer.

Before going into the details (which, at a glance, look fine) I think we
need to talk about the potential performance impact. I've never seen any
other reason for the static inlines here than avoiding a function call
when possible. Are there any other reasons? Is that a useless
micro-optimization or something that could have an impact? On what?

BR,
Jani.

>
> Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
> ---
>  .../gpu/drm/i915/display/intel_frontbuffer.c  | 18 ++---
>  .../gpu/drm/i915/display/intel_frontbuffer.h  | 27 -------
>  drivers/gpu/drm/i915/gem/i915_gem_object.c    | 70 ++++++++++++++++++-
>  drivers/gpu/drm/i915/gem/i915_gem_object.h    |  6 ++
>  drivers/gpu/drm/i915/i915_vma.c               |  2 +-
>  5 files changed, 81 insertions(+), 42 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_frontbuffer.c b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
> index 3ce0436a0c7d..41ac65c98720 100644
> --- a/drivers/gpu/drm/i915/display/intel_frontbuffer.c
> +++ b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
> @@ -237,7 +237,7 @@ static void frontbuffer_release(struct kref *ref)
>  	}
>  	spin_unlock(&obj->vma.lock);
>  
> -	RCU_INIT_POINTER(obj->frontbuffer, NULL);
> +	i915_gem_object_set_frontbuffer(obj, NULL);
>  	spin_unlock(&intel_bo_to_i915(obj)->display.fb_tracking.lock);
>  
>  	i915_active_fini(&front->write);
> @@ -250,9 +250,9 @@ struct intel_frontbuffer *
>  intel_frontbuffer_get(struct drm_i915_gem_object *obj)
>  {
>  	struct drm_i915_private *i915 = intel_bo_to_i915(obj);
> -	struct intel_frontbuffer *front;
> +	struct intel_frontbuffer *front, *front_ret;
>  
> -	front = __intel_frontbuffer_get(obj);
> +	front = i915_gem_object_get_frontbuffer(obj);
>  	if (front)
>  		return front;
>  
> @@ -269,16 +269,10 @@ intel_frontbuffer_get(struct drm_i915_gem_object *obj)
>  			 I915_ACTIVE_RETIRE_SLEEPS);
>  
>  	spin_lock(&i915->display.fb_tracking.lock);
> -	if (rcu_access_pointer(obj->frontbuffer)) {
> -		kfree(front);
> -		front = rcu_dereference_protected(obj->frontbuffer, true);
> -		kref_get(&front->ref);
> -	} else {
> -		i915_gem_object_get(obj);
> -		rcu_assign_pointer(obj->frontbuffer, front);
> -	}
> +	front_ret = i915_gem_object_set_frontbuffer(obj, front);
>  	spin_unlock(&i915->display.fb_tracking.lock);
> -
> +	if (front_ret != front)
> +		kfree(front);
>  	return front;
>  }
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_frontbuffer.h b/drivers/gpu/drm/i915/display/intel_frontbuffer.h
> index 3c474ed937fb..eeccc847331d 100644
> --- a/drivers/gpu/drm/i915/display/intel_frontbuffer.h
> +++ b/drivers/gpu/drm/i915/display/intel_frontbuffer.h
> @@ -75,33 +75,6 @@ void intel_frontbuffer_flip(struct drm_i915_private *i915,
>  
>  void intel_frontbuffer_put(struct intel_frontbuffer *front);
>  
> -static inline struct intel_frontbuffer *
> -__intel_frontbuffer_get(const struct drm_i915_gem_object *obj)
> -{
> -	struct intel_frontbuffer *front;
> -
> -	if (likely(!rcu_access_pointer(obj->frontbuffer)))
> -		return NULL;
> -
> -	rcu_read_lock();
> -	do {
> -		front = rcu_dereference(obj->frontbuffer);
> -		if (!front)
> -			break;
> -
> -		if (unlikely(!kref_get_unless_zero(&front->ref)))
> -			continue;
> -
> -		if (likely(front == rcu_access_pointer(obj->frontbuffer)))
> -			break;
> -
> -		intel_frontbuffer_put(front);
> -	} while (1);
> -	rcu_read_unlock();
> -
> -	return front;
> -}
> -
>  struct intel_frontbuffer *
>  intel_frontbuffer_get(struct drm_i915_gem_object *obj);
>  
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> index 46a19b099ec8..6945e903e106 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> @@ -463,7 +463,7 @@ void __i915_gem_object_flush_frontbuffer(struct drm_i915_gem_object *obj,
>  {
>  	struct intel_frontbuffer *front;
>  
> -	front = __intel_frontbuffer_get(obj);
> +	front = i915_gem_object_get_frontbuffer(obj);
>  	if (front) {
>  		intel_frontbuffer_flush(front, origin);
>  		intel_frontbuffer_put(front);
> @@ -475,7 +475,7 @@ void __i915_gem_object_invalidate_frontbuffer(struct drm_i915_gem_object *obj,
>  {
>  	struct intel_frontbuffer *front;
>  
> -	front = __intel_frontbuffer_get(obj);
> +	front = i915_gem_object_get_frontbuffer(obj);
>  	if (front) {
>  		intel_frontbuffer_invalidate(front, origin);
>  		intel_frontbuffer_put(front);
> @@ -952,6 +952,72 @@ bool i915_gem_object_has_unknown_state(struct drm_i915_gem_object *obj)
>  	return obj->mm.unknown_state;
>  }
>  
> +/**
> + * i915_gem_object_get_frontbuffer - Get the object's frontbuffer
> + * @obj: The object whose frontbuffer to get.
> + *
> + * Get pointer to object's frontbuffer if such exists. Please note that RCU
> + * mechanism is used to handle e.g. ongoing removal of frontbuffer pointer.
> + *
> + * Return: pointer to object's frontbuffer is such exists or NULL
> + */
> +struct intel_frontbuffer *
> +i915_gem_object_get_frontbuffer(const struct drm_i915_gem_object *obj)
> +{
> +	struct intel_frontbuffer *front;
> +
> +	if (likely(!rcu_access_pointer(obj->frontbuffer)))
> +		return NULL;
> +
> +	rcu_read_lock();
> +	do {
> +		front = rcu_dereference(obj->frontbuffer);
> +		if (!front)
> +			break;
> +
> +		if (unlikely(!kref_get_unless_zero(&front->ref)))
> +			continue;
> +
> +		if (likely(front == rcu_access_pointer(obj->frontbuffer)))
> +			break;
> +
> +		intel_frontbuffer_put(front);
> +	} while (1);
> +	rcu_read_unlock();
> +
> +	return front;
> +}
> +
> +/**
> + * i915_gem_object_set_frontbuffer - Set the object's frontbuffer
> + * @obj: The object whose frontbuffer to set.
> + * @front: The frontbuffer to set
> + *
> + * Set object's frontbuffer pointer. If frontbuffer is already set for the
> + * object keep it and return it's pointer to the caller. Please note that RCU
> + * mechanism is used to handle e.g. ongoing removal of frontbuffer pointer.
> + *
> + * Return: pointer to frontbuffer which was set.
> + */
> +struct intel_frontbuffer *
> +i915_gem_object_set_frontbuffer(struct drm_i915_gem_object *obj,
> +				struct intel_frontbuffer *front)
> +{
> +	struct intel_frontbuffer *front_ret = front;
> +
> +	if (!front) {
> +		RCU_INIT_POINTER(obj->frontbuffer, NULL);
> +	} else if (rcu_access_pointer(obj->frontbuffer)) {
> +		front_ret = rcu_dereference_protected(obj->frontbuffer, true);
> +		kref_get(&front_ret->ref);
> +	} else {
> +		drm_gem_object_get(&intel_bo_to_drm_bo(obj));
> +		rcu_assign_pointer(obj->frontbuffer, front);
> +	}
> +
> +	return front_ret;
> +}
> +
>  #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
>  #include "selftests/huge_gem_object.c"
>  #include "selftests/huge_pages.c"
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
> index 884a17275b3a..69c5fa91152a 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
> @@ -891,4 +891,10 @@ static inline int i915_gem_object_userptr_validate(struct drm_i915_gem_object *o
>  
>  #endif
>  
> +struct intel_frontbuffer *
> +i915_gem_object_get_frontbuffer(const struct drm_i915_gem_object *obj);
> +struct intel_frontbuffer *
> +i915_gem_object_set_frontbuffer(struct drm_i915_gem_object *obj,
> +				struct intel_frontbuffer *front);
> +
>  #endif
> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> index ffb425ba591c..c66ff2157f6a 100644
> --- a/drivers/gpu/drm/i915/i915_vma.c
> +++ b/drivers/gpu/drm/i915/i915_vma.c
> @@ -1908,7 +1908,7 @@ int _i915_vma_move_to_active(struct i915_vma *vma,
>  	if (flags & EXEC_OBJECT_WRITE) {
>  		struct intel_frontbuffer *front;
>  
> -		front = __intel_frontbuffer_get(obj);
> +		front = i915_gem_object_get_frontbuffer(obj);
>  		if (unlikely(front)) {
>  			if (intel_frontbuffer_invalidate(front, ORIGIN_CS))
>  				i915_active_add_request(&front->write, rq);

-- 
Jani Nikula, Intel Open Source Graphics Center