diff --git a/soundtrigger/2.1/ISoundTriggerHwCallback.hal b/soundtrigger/2.1/ISoundTriggerHwCallback.hal
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+++ b/soundtrigger/2.1/ISoundTriggerHwCallback.hal
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+/*
+ * Copyright 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.
+ */
+
+package android.hardware.soundtrigger@2.1;
+
+import @2.0::ISoundTriggerHwCallback;
+import @2.0::PhraseRecognitionExtra;
+
+/**
+ * SoundTrigger HAL Callback interface. Obtained during SoundTrigger setup.
+ */
+@hidl_callback
+interface ISoundTriggerHwCallback extends @2.0::ISoundTriggerHwCallback {
+
+    /**
+     * Generic recognition event sent via recognition callback.
+     */
+    struct RecognitionEvent {
+        /** Event header. Any data contained in the 'header.data' field
+         * is ignored */
+        @2.0::ISoundTriggerHwCallback.RecognitionEvent header;
+        /** Opaque event data */
+        memory data;
+    };
+
+    /**
+     * Specialized recognition event for key phrase recognitions.
+     */
+    struct PhraseRecognitionEvent {
+        /** Common part of the recognition event */
+        RecognitionEvent common;
+        /** List of descriptors for each recognized key phrase */
+        vec<PhraseRecognitionExtra> phraseExtras;
+    };
+
+    /**
+     * Event sent via load sound model callback.
+     */
+    struct ModelEvent {
+        /** Event header. Any data contained in the 'header.data' field
+         * is ignored */
+        @2.0::ISoundTriggerHwCallback.ModelEvent header;
+        /** Opaque event data, passed transparently by the framework */
+        memory data;
+    };
+
+    /**
+     * Callback method called by the HAL when the sound recognition triggers.
+     *
+     * @param event A RecognitionEvent structure containing detailed results
+     *     of the recognition triggered
+     * @param cookie The cookie passed by the framework when recognition was
+     *     started (see ISoundtriggerHw.startRecognition*())
+     */
+    recognitionCallback_2_1(RecognitionEvent event, CallbackCookie cookie);
+
+    /**
+     * Callback method called by the HAL when the sound recognition triggers
+     * for a key phrase sound model.
+     *
+     * @param event A RecognitionEvent structure containing detailed results
+     *     of the recognition triggered
+     * @param cookie The cookie passed by the framework when recognition was
+     *     started (see ISoundtriggerHw.startRecognition*())
+     */
+    phraseRecognitionCallback_2_1(PhraseRecognitionEvent event,
+                                  CallbackCookie cookie);
+
+    /**
+     * Callback method called by the HAL when the sound model loading completes.
+     *
+     * @param event A ModelEvent structure containing detailed results of the
+     *     model loading operation
+     * @param cookie The cookie passed by the framework when loading was
+     *     initiated (see ISoundtriggerHw.loadSoundModel*())
+     */
+    soundModelCallback_2_1(ModelEvent event, CallbackCookie cookie);
+};