Re: [PATCH] drm/i915: Replace obj->pin_global with obj->frontbuffer

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On Fri, Aug 23, 2019 at 05:20:41PM +0100, Chris Wilson wrote:
> obj->pin_global was original used as a means to keep the shrinker off
> the active scanout, but we use the vma->pin_count itself for that and
> the obj->frontbuffer to delay shrinking active framebuffers. The other
> role that obj->pin_global gained was for spotting display objects inside
> GEM and working harder to keep those coherent; for which we can again
> simply inspect obj->frontbuffer directly.
> 
> Coming up next, we will want to manipulate the pin_global counter
> outside of the principle locks, so would need to make pin_global atomic.
> However, since obj->frontbuffer is already managed atomically, it makes
> sense to use that the primary key for display objects instead of having
> pin_global.

The difference being that pin_global was only incremented while active
scanout was happening, but obj->frontbuffer simply indicates that the
obj is attached to at least one framebuffer regardless of whether it's
ever scanned out or not.

> 
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> ---
>  .../gpu/drm/i915/display/intel_frontbuffer.c  | 13 +++++--
>  drivers/gpu/drm/i915/gem/i915_gem_domain.c    | 34 ++++++-------------
>  drivers/gpu/drm/i915/gem/i915_gem_object.h    |  3 +-
>  .../gpu/drm/i915/gem/i915_gem_object_types.h  |  2 --
>  drivers/gpu/drm/i915/gem/i915_gem_shrinker.c  | 15 +++-----
>  drivers/gpu/drm/i915/i915_debugfs.c           | 12 ++-----
>  6 files changed, 29 insertions(+), 50 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_frontbuffer.c b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
> index 719379774fa5..43047b676b7b 100644
> --- a/drivers/gpu/drm/i915/display/intel_frontbuffer.c
> +++ b/drivers/gpu/drm/i915/display/intel_frontbuffer.c
> @@ -220,11 +220,18 @@ static void frontbuffer_release(struct kref *ref)
>  {
>  	struct intel_frontbuffer *front =
>  		container_of(ref, typeof(*front), ref);
> +	struct drm_i915_gem_object *obj = front->obj;
> +	struct i915_vma *vma;
>  
> -	front->obj->frontbuffer = NULL;
> -	spin_unlock(&to_i915(front->obj->base.dev)->fb_tracking.lock);
> +	spin_lock(&obj->vma.lock);
> +	for_each_ggtt_vma(vma, obj)
> +		vma->display_alignment = I915_GTT_PAGE_SIZE;
> +	spin_unlock(&obj->vma.lock);
>  
> -	i915_gem_object_put(front->obj);
> +	obj->frontbuffer = NULL;
> +	spin_unlock(&to_i915(obj->base.dev)->fb_tracking.lock);
> +
> +	i915_gem_object_put(obj);
>  	kfree(front);
>  }
>  
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_domain.c b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> index 9c58e8fac1d9..0341b3da930a 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_domain.c
> @@ -27,7 +27,7 @@ static void __i915_gem_object_flush_for_display(struct drm_i915_gem_object *obj)
>  
>  void i915_gem_object_flush_if_display(struct drm_i915_gem_object *obj)
>  {
> -	if (!READ_ONCE(obj->pin_global))
> +	if (!READ_ONCE(obj->frontbuffer))
>  		return;

So we maybe get a few more flushes now?

>  
>  	i915_gem_object_lock(obj);
> @@ -422,12 +422,8 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
>  
>  	assert_object_held(obj);
>  
> -	/* Mark the global pin early so that we account for the
> -	 * display coherency whilst setting up the cache domains.
> -	 */
> -	obj->pin_global++;
> -
> -	/* The display engine is not coherent with the LLC cache on gen6.  As
> +	/*
> +	 * The display engine is not coherent with the LLC cache on gen6.  As
>  	 * a result, we make sure that the pinning that is about to occur is
>  	 * done with uncached PTEs. This is lowest common denominator for all
>  	 * chipsets.
> @@ -439,12 +435,11 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
>  	ret = i915_gem_object_set_cache_level(obj,
>  					      HAS_WT(to_i915(obj->base.dev)) ?
>  					      I915_CACHE_WT : I915_CACHE_NONE);
> -	if (ret) {
> -		vma = ERR_PTR(ret);
> -		goto err_unpin_global;
> -	}
> +	if (ret)
> +		return ERR_PTR(ret);
>  
> -	/* As the user may map the buffer once pinned in the display plane
> +	/*
> +	 * As the user may map the buffer once pinned in the display plane
>  	 * (e.g. libkms for the bootup splash), we have to ensure that we
>  	 * always use map_and_fenceable for all scanout buffers. However,
>  	 * it may simply be too big to fit into mappable, in which case
> @@ -461,22 +456,19 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
>  	if (IS_ERR(vma))
>  		vma = i915_gem_object_ggtt_pin(obj, view, 0, alignment, flags);
>  	if (IS_ERR(vma))
> -		goto err_unpin_global;
> +		return vma;
>  
>  	vma->display_alignment = max_t(u64, vma->display_alignment, alignment);
>  
>  	__i915_gem_object_flush_for_display(obj);
>  
> -	/* It should now be out of any other write domains, and we can update
> +	/*
> +	 * It should now be out of any other write domains, and we can update
>  	 * the domain values for our changes.
>  	 */
>  	obj->read_domains |= I915_GEM_DOMAIN_GTT;
>  
>  	return vma;
> -
> -err_unpin_global:
> -	obj->pin_global--;
> -	return vma;
>  }
>  
>  static void i915_gem_object_bump_inactive_ggtt(struct drm_i915_gem_object *obj)
> @@ -514,12 +506,6 @@ i915_gem_object_unpin_from_display_plane(struct i915_vma *vma)
>  
>  	assert_object_held(obj);
>  
> -	if (WARN_ON(obj->pin_global == 0))
> -		return;
> -
> -	if (--obj->pin_global == 0)
> -		vma->display_alignment = I915_GTT_MIN_ALIGNMENT;
> -
>  	/* Bump the LRU to try and avoid premature eviction whilst flipping  */
>  	i915_gem_object_bump_inactive_ggtt(obj);
>  
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
> index 5efb9936e05b..d6005cad9d5c 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
> @@ -406,7 +406,8 @@ static inline bool cpu_write_needs_clflush(struct drm_i915_gem_object *obj)
>  	if (!(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_WRITE))
>  		return true;
>  
> -	return obj->pin_global; /* currently in use by HW, keep flushed */
> +	/* Currently in use by HW (display engine)? Keep flushed. */
> +	return READ_ONCE(obj->frontbuffer);
>  }
>  
>  static inline void __start_cpu_write(struct drm_i915_gem_object *obj)
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> index ede0eb4218a8..13b9dc0e1a89 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
> @@ -152,8 +152,6 @@ struct drm_i915_gem_object {
>  
>  	/** Count of VMA actually bound by this object */
>  	atomic_t bind_count;
> -	/** Count of how many global VMA are currently pinned for use by HW */
> -	unsigned int pin_global;
>  
>  	struct {
>  		struct mutex lock; /* protects the pages and their use */
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c b/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
> index edd21d14e64f..4e55cfc2b0dc 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
> @@ -61,7 +61,8 @@ static bool can_release_pages(struct drm_i915_gem_object *obj)
>  	if (!i915_gem_object_is_shrinkable(obj))
>  		return false;
>  
> -	/* Only report true if by unbinding the object and putting its pages
> +	/*
> +	 * Only report true if by unbinding the object and putting its pages
>  	 * we can actually make forward progress towards freeing physical
>  	 * pages.
>  	 *
> @@ -72,16 +73,8 @@ static bool can_release_pages(struct drm_i915_gem_object *obj)
>  	if (atomic_read(&obj->mm.pages_pin_count) > atomic_read(&obj->bind_count))
>  		return false;
>  
> -	/* If any vma are "permanently" pinned, it will prevent us from
> -	 * reclaiming the obj->mm.pages. We only allow scanout objects to claim
> -	 * a permanent pin, along with a few others like the context objects.
> -	 * To simplify the scan, and to avoid walking the list of vma under the
> -	 * object, we just check the count of its permanently pinned.
> -	 */
> -	if (READ_ONCE(obj->pin_global))
> -		return false;

Are we giving the shrinker false hope here when it comes to the actual
frontbuffer?

> -
> -	/* We can only return physical pages to the system if we can either
> +	/*
> +	 * We can only return physical pages to the system if we can either
>  	 * discard the contents (because the user has marked them as being
>  	 * purgeable) or if we can move their contents out to swap.
>  	 */
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index e103fcba6435..b90371844689 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -77,11 +77,6 @@ static int i915_capabilities(struct seq_file *m, void *data)
>  	return 0;
>  }
>  
> -static char get_pin_flag(struct drm_i915_gem_object *obj)
> -{
> -	return obj->pin_global ? 'p' : ' ';
> -}
> -
>  static char get_tiling_flag(struct drm_i915_gem_object *obj)
>  {
>  	switch (i915_gem_object_get_tiling(obj)) {
> @@ -140,9 +135,8 @@ describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
>  	struct i915_vma *vma;
>  	int pin_count = 0;
>  
> -	seq_printf(m, "%pK: %c%c%c%c %8zdKiB %02x %02x %s%s%s",
> +	seq_printf(m, "%pK: %c%c%c %8zdKiB %02x %02x %s%s%s",
>  		   &obj->base,
> -		   get_pin_flag(obj),
>  		   get_tiling_flag(obj),
>  		   get_global_flag(obj),
>  		   get_pin_mapped_flag(obj),
> @@ -221,8 +215,8 @@ describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
>  	seq_printf(m, " (pinned x %d)", pin_count);
>  	if (obj->stolen)
>  		seq_printf(m, " (stolen: %08llx)", obj->stolen->start);
> -	if (obj->pin_global)
> -		seq_printf(m, " (global)");
> +	if (READ_ONCE(obj->frontbuffer))
> +		seq_printf(m, " (front)");

A bit confusing to say it's "front" when in fact it can be any random backbuffer.
Maybe should be just "fb" or something?

>  
>  	engine = i915_gem_object_last_write_engine(obj);
>  	if (engine)
> -- 
> 2.23.0
> 
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> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
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-- 
Ville Syrjälä
Intel
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