Re: [Intel-gfx] [PATCH 10/27] drm/i915/guc: Introduce context parent-child relationship

[John Harrison <john.c.harrison@intel.com>]

On 8/20/2021 15:44, 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.
>
> v2:
>   (Daniel Vetter)
>    - Add kernel doc, add wrapper to access parent to ensure safety
>
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_context.c       | 29 ++++++++++++++
>   drivers/gpu/drm/i915/gt/intel_context.h       | 39 +++++++++++++++++++
>   drivers/gpu/drm/i915/gt/intel_context_types.h | 23 +++++++++++
>   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 508cfe5770c0..00d1aee6d199 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -404,6 +404,8 @@ intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
>   
>   	INIT_LIST_HEAD(&ce->destroyed_link);
>   
No need for this blank line?

> +	INIT_LIST_HEAD(&ce->guc_child_list);
> +
>   	/*
>   	 * Initialize fence to be complete as this is expected to be complete
>   	 * unless there is a pending schedule disable outstanding.
> @@ -418,10 +420,17 @@ 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);
>   }
> @@ -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->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..c2985822ab74 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> @@ -44,6 +44,45 @@ 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;
> +}
> +
> +static inline bool intel_context_is_pinned(struct intel_context *ce);
No point declaring 'static inline' if there is no function body?

> +
> +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 pointer
has pointer -> 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->parent));
> +
> +                return ce->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)->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)
Do these macros not need some kind of intel_context prefix? Or at least 
be 'for_each_guc_child' given the naming of the list/link fields? But 
maybe not if the guc_ is dropped from the variable names - see below.

> +
>   /**
>    * 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 fd338a30617e..0fafc178cf2c 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context_types.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
> @@ -213,6 +213,29 @@ struct intel_context {
>   	 */
>   	struct list_head destroyed_link;
>   
> +	/** anonymous struct for parent / children only members */
> +	struct {
> +		union {
> +			/**
> +			 * @guc_child_list: parent's list of of children
> +			 * contexts, no protection as immutable after context
> +			 * creation
> +			 */
> +			struct list_head guc_child_list;
> +			/**
> +			 * @guc_child_link: child's link into parent's list of
> +			 * children
> +			 */
> +			struct list_head guc_child_link;
> +		};
> +
> +		/** @parent: pointer to parent if child */
> +		struct intel_context *parent;
> +
> +		/** @guc_number_children: number of children if parent */
> +		u8 guc_number_children;
These are not really a GuC specific fields? The parent/child thing might 
only be necessary for GuC submission (although can you say it won't be 
required by any future backend, such as the DRM scheduler?) but it is a 
context level concept. None of the files changed in this patch are GuC 
specific. So no need for 'guc_' prefix? Alternatively, if it all really 
is completely GuC specific then the 'parent' field should also have the 
prefix? Or even just name the outer struct 'guc_family' or something and 
drop the prefixes from all the inner members.

John.

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