drm/i915/guc: Introduce context parent-child relationship

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.

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)
v4:
 (John Harrison)
  - s/its'/its/g
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014172005.27155-8-matthew.brost@intel.com

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

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

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