[Intel-gfx] [PATCH 15/46] drm/i915/guc: Introduce context parent-child relationship

[Daniel Vetter]

On Tue, Aug 03, 2021 at 03:29:12PM -0700, Matthew Brost wrote:
> Introduce context parent-child relationship. Once this relationship is
> created all pinning / unpinning operations are directed to the parent
> context. The parent context is responsible for pinning all of its'
> children and itself.
> 
> This is a precursor to the full GuC multi-lrc implementation but aligns
> to how GuC mutli-lrc interface is defined - a single H2G is used
> register / deregister all of the contexts simultaneously.
> 
> Subsequent patches in the series will implement the pinning / unpinning
> operations for parent / child contexts.
> 
> Signed-off-by: Matthew Brost <matthew.brost at intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_context.c       | 29 +++++++++++++++++++
>  drivers/gpu/drm/i915/gt/intel_context.h       | 18 ++++++++++++
>  drivers/gpu/drm/i915/gt/intel_context_types.h | 12 ++++++++
>  3 files changed, 59 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> index 745e84c72c90..8cb92b10b547 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -395,6 +395,8 @@ intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
>  	spin_lock_init(&ce->guc_state.lock);
>  	INIT_LIST_HEAD(&ce->guc_state.fences);
>  
> +	INIT_LIST_HEAD(&ce->guc_child_list);
> +
>  	spin_lock_init(&ce->guc_active.lock);
>  	INIT_LIST_HEAD(&ce->guc_active.requests);
>  
> @@ -414,10 +416,17 @@ intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
>  
>  void intel_context_fini(struct intel_context *ce)
>  {
> +	struct intel_context *child, *next;
> +
>  	if (ce->timeline)
>  		intel_timeline_put(ce->timeline);
>  	i915_vm_put(ce->vm);
>  
> +	/* Need to put the creation ref for the children */
> +	if (intel_context_is_parent(ce))
> +		for_each_child_safe(ce, child, next)
> +			intel_context_put(child);
> +
>  	mutex_destroy(&ce->pin_mutex);
>  	i915_active_fini(&ce->active);
>  }
> @@ -533,6 +542,26 @@ struct i915_request *intel_context_find_active_request(struct intel_context *ce)
>  	return active;
>  }
>  
> +void intel_context_bind_parent_child(struct intel_context *parent,
> +				     struct intel_context *child)
> +{
> +	/*
> +	 * Callers responsibility to validate that this function is used
> +	 * correctly but we use GEM_BUG_ON here ensure that they do.
> +	 */
> +	GEM_BUG_ON(!intel_engine_uses_guc(parent->engine));
> +	GEM_BUG_ON(intel_context_is_pinned(parent));
> +	GEM_BUG_ON(intel_context_is_child(parent));
> +	GEM_BUG_ON(intel_context_is_pinned(child));
> +	GEM_BUG_ON(intel_context_is_child(child));
> +	GEM_BUG_ON(intel_context_is_parent(child));
> +
> +	parent->guc_number_children++;
> +	list_add_tail(&child->guc_child_link,
> +		      &parent->guc_child_list);
> +	child->parent = parent;
> +}
> +
>  #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
>  #include "selftest_context.c"
>  #endif
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.h b/drivers/gpu/drm/i915/gt/intel_context.h
> index c41098950746..ad6ce5ac4824 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> @@ -44,6 +44,24 @@ void intel_context_free(struct intel_context *ce);
>  int intel_context_reconfigure_sseu(struct intel_context *ce,
>  				   const struct intel_sseu sseu);
>  
> +static inline bool intel_context_is_child(struct intel_context *ce)
> +{
> +	return !!ce->parent;
> +}
> +
> +static inline bool intel_context_is_parent(struct intel_context *ce)
> +{
> +	return !!ce->guc_number_children;
> +}
> +
> +void intel_context_bind_parent_child(struct intel_context *parent,
> +				     struct intel_context *child);
> +
> +#define for_each_child(parent, ce)\
> +	list_for_each_entry(ce, &(parent)->guc_child_list, guc_child_link)
> +#define for_each_child_safe(parent, ce, cn)\
> +	list_for_each_entry_safe(ce, cn, &(parent)->guc_child_list, guc_child_link)
> +
>  /**
>   * intel_context_lock_pinned - Stablises the 'pinned' status of the HW context
>   * @ce - the context
> diff --git a/drivers/gpu/drm/i915/gt/intel_context_types.h b/drivers/gpu/drm/i915/gt/intel_context_types.h
> index 2df79ba39867..66b22b370a72 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context_types.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
> @@ -202,6 +202,18 @@ struct intel_context {
>  	/* GuC context blocked fence */
>  	struct i915_sw_fence guc_blocked;
>  
> +	/* Head of children list or link in parent's children list */

Kerneldoc layout would be nice, plus explaining when exactly this is
set or the list empty (e.g. guch_child_list is empty if and only if
guc_number_children > 0 and parent == NULL).

Also mentionting that these are invariant over the lifetime of the object
would be nice.

Finally some words on refcounting (like who holds a reference on whom and
how we guarantee that use-after-free doesn't go boom since you have links
both ways). It looks like parent holds a reference on the child, so how do
you make sure the child looking at the parent doesn't go boom?
-Daniel

> +	union {
> +		struct list_head guc_child_list;	/* parent */
> +		struct list_head guc_child_link;	/* child */
> +	};
> +
> +	/* Pointer to parent */
> +	struct intel_context *parent;
> +
> +	/* Number of children if parent */
> +	u8 guc_number_children;
> +
>  	/*
>  	 * GuC priority management
>  	 */
> -- 
> 2.28.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch