[PATCH 07/26] drm/i915/guc: Introduce context parent-child relationship

[John Harrison]

On 10/4/2021 15:06, 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'
No need for an apostrophe.

> 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.
>
> v2:
>   (Daniel Vetter)
>    - Add kernel doc, add wrapper to access parent to ensure safety
> v3:
>   (John Harrison)
>    - Fix comment explaing GEM_BUG_ON in to_parent()
>    - Make variable names generic (non-GuC specific)
>
> 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       | 41 +++++++++++++++++++
>   drivers/gpu/drm/i915/gt/intel_context_types.h | 21 ++++++++++
>   3 files changed, 91 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> index f601323b939f..c5bb7ccfb3f8 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -403,6 +403,8 @@ intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
>   
>   	INIT_LIST_HEAD(&ce->destroyed_link);
>   
> +	INIT_LIST_HEAD(&ce->parallel.child_list);
> +
>   	/*
>   	 * Initialize fence to be complete as this is expected to be complete
>   	 * unless there is a pending schedule disable outstanding.
> @@ -417,10 +419,17 @@ 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);
>   	i915_sw_fence_fini(&ce->guc_state.blocked);
> @@ -537,6 +546,26 @@ 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->parallel.number_children++;
> +	list_add_tail(&child->parallel.child_link,
> +		      &parent->parallel.child_list);
> +	child->parallel.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..b63c10a144af 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> @@ -44,6 +44,47 @@ 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->parallel.parent;
> +}
> +
> +static inline bool intel_context_is_parent(struct intel_context *ce)
> +{
> +	return !!ce->parallel.number_children;
> +}
> +
> +static inline bool intel_context_is_pinned(struct intel_context *ce);
> +
> +static inline struct intel_context *
> +intel_context_to_parent(struct intel_context *ce)
> +{
> +	if (intel_context_is_child(ce)) {
> +		/*
> +		 * The parent holds ref count to the child so it is always safe
> +		 * for the parent to access the child, but the child has a
> +		 * pointer to the parent without a ref. To ensure this is safe
> +		 * the child should only access the parent pointer while the
> +		 * parent is pinned.
> +		 */
> +		GEM_BUG_ON(!intel_context_is_pinned(ce->parallel.parent));
> +
> +		return ce->parallel.parent;
> +	} else {
> +		return ce;
> +	}
> +}
> +
> +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)->parallel.child_list,\
> +			    parallel.child_link)
> +#define for_each_child_safe(parent, ce, cn)\
> +	list_for_each_entry_safe(ce, cn, &(parent)->parallel.child_list,\
> +				 parallel.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 4613d027cbc3..76dfca57cb45 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context_types.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
> @@ -220,6 +220,27 @@ struct intel_context {
>   	 */
>   	struct list_head destroyed_link;
>   
> +	/** @parallel: sub-structure for parallel submission members */
> +	struct {
> +		union {
> +			/**
> +			 * @child_list: parent's list of children
> +			 * contexts, no protection as immutable after context
> +			 * creation
> +			 */
> +			struct list_head child_list;
> +			/**
> +			 * @child_link: child's link into parent's list of
> +			 * children
> +			 */
> +			struct list_head child_link;
> +		};
> +		/** @parent: pointer to parent if child */
> +		struct intel_context *parent;
> +		/** @number_children: number of children if parent */
> +		u8 number_children;
Is there any particular reason for using 'u8'? A simple 'int' can be 
much more efficient depending upon the host CPU architecture.

Not a blocker though. So with the typo above fixed:
Reviewed-by: John Harrison <John.C.Harrison at Intel.com>

> +	} parallel;
> +
>   #ifdef CONFIG_DRM_I915_SELFTEST
>   	/**
>   	 * @drop_schedule_enable: Force drop of schedule enable G2H for selftest