diff --git a/neuralnetworks/1.0/IEvent.hal b/neuralnetworks/1.0/IEvent.hal
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+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/* This HAL is a work in progress */
+
+package android.hardware.neuralnetworks@1.0;
+
+/**
+ * The IEvent interface is a callback object passed by the
+ * Neuralnetworks runtime to the vendor service. It is used as a
+ * synchronization primitive between one or more runtime threads and a
+ * single asynchronous vendor thread.  An event object is passed as an
+ * argument to a HIDL call that is expected to take a non-trivial
+ * amount of time. When the asynchronous execution thread has
+ * completed its computation, it must call "notify" on the event to
+ * indicate to the Neuralnetworks runtime whether the computation was
+ * successful or not, and that the corresponding output is ready to be
+ * consumed if the execution was successful.
+ *
+ * TODO: Mention that "notify" is also called by a runtime thread
+ * during CPU fallback execution? Depends on whether the HIDL comments
+ * are strictly for vendors or not.
+ */
+interface IEvent {
+
+    /**
+     * IEvent::notify is called by the server thread (i.e. the thread doing the
+     * work) to mark the event as completed so that any threads requiring the
+     * corresponding resources can continue executing.
+     *
+     * @param status Status of the execution associated with the Event.
+     *               Should be SUCCESS or ERROR.
+     */
+    oneway notify(Status status);
+
+};