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

Daniel Vetter
2021-08-09T14:38:02Z

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@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