drm/i915: Provide a fastpath for waiting on vma bindings

Before we can execute a request, we must wait for all of its vma to be
bound. This is a frequent operation for which we can optimise away a
few atomic operations (notably a cmpxchg) in lieu of the RCU protection.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Thomas Hellström <thomas.hellstrom@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200731085015.32368-7-chris@chris-wilson.co.uk
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_active.h

@@ -231,4 +231,19 @@ struct i915_active *i915_active_create(void);
 struct i915_active *i915_active_get(struct i915_active *ref);
 void i915_active_put(struct i915_active *ref);
 
+static inline int __i915_request_await_exclusive(struct i915_request *rq,
+						 struct i915_active *active)
+{
+	struct dma_fence *fence;
+	int err = 0;
+
+	fence = i915_active_fence_get(&active->excl);
+	if (fence) {
+		err = i915_request_await_dma_fence(rq, fence);
+		dma_fence_put(fence);
+	}
+
+	return err;
+}
+
 #endif /* _I915_ACTIVE_H_ */

drivers/gpu/drm/i915/i915_vma.c

@@ -1167,15 +1167,20 @@ void i915_vma_revoke_mmap(struct i915_vma *vma)
 		list_del(&vma->obj->userfault_link);
 }
 
+static int
+__i915_request_await_bind(struct i915_request *rq, struct i915_vma *vma)
+{
+	return __i915_request_await_exclusive(rq, &vma->active);
+}
+
 int __i915_vma_move_to_active(struct i915_vma *vma, struct i915_request *rq)
 {
 	int err;
 
 	GEM_BUG_ON(!i915_vma_is_pinned(vma));
 
 	/* Wait for the vma to be bound before we start! */
-	err = i915_request_await_active(rq, &vma->active,
-					I915_ACTIVE_AWAIT_EXCL);
+	err = __i915_request_await_bind(rq, vma);
 	if (err)
 		return err;