diff --git a/services/audioflinger/Threads.cpp b/services/audioflinger/Threads.cpp
index f206e96..e2932f1 100644
--- a/services/audioflinger/Threads.cpp
+++ b/services/audioflinger/Threads.cpp
@@ -1134,14 +1134,14 @@
 }
 
 void AudioFlinger::ThreadBase::setEffectSuspended(
-        const effect_uuid_t *type, bool suspend, int sessionId)
+        const effect_uuid_t *type, bool suspend, audio_session_t sessionId)
 {
     Mutex::Autolock _l(mLock);
     setEffectSuspended_l(type, suspend, sessionId);
 }
 
 void AudioFlinger::ThreadBase::setEffectSuspended_l(
-        const effect_uuid_t *type, bool suspend, int sessionId)
+        const effect_uuid_t *type, bool suspend, audio_session_t sessionId)
 {
     sp<EffectChain> chain = getEffectChain_l(sessionId);
     if (chain != 0) {
@@ -1181,7 +1181,7 @@
 
 void AudioFlinger::ThreadBase::updateSuspendedSessions_l(const effect_uuid_t *type,
                                                          bool suspend,
-                                                         int sessionId)
+                                                         audio_session_t sessionId)
 {
     ssize_t index = mSuspendedSessions.indexOfKey(sessionId);
 
@@ -1242,7 +1242,7 @@
 
 void AudioFlinger::ThreadBase::checkSuspendOnEffectEnabled(const sp<EffectModule>& effect,
                                                             bool enabled,
-                                                            int sessionId)
+                                                            audio_session_t sessionId)
 {
     Mutex::Autolock _l(mLock);
     checkSuspendOnEffectEnabled_l(effect, enabled, sessionId);
@@ -1250,7 +1250,7 @@
 
 void AudioFlinger::ThreadBase::checkSuspendOnEffectEnabled_l(const sp<EffectModule>& effect,
                                                             bool enabled,
-                                                            int sessionId)
+                                                            audio_session_t sessionId)
 {
     if (mType != RECORD) {
         // suspend all effects in AUDIO_SESSION_OUTPUT_MIX when enabling any effect on
@@ -1274,7 +1274,7 @@
         const sp<AudioFlinger::Client>& client,
         const sp<IEffectClient>& effectClient,
         int32_t priority,
-        int sessionId,
+        audio_session_t sessionId,
         effect_descriptor_t *desc,
         int *enabled,
         status_t *status)
@@ -1413,13 +1413,15 @@
     return handle;
 }
 
-sp<AudioFlinger::EffectModule> AudioFlinger::ThreadBase::getEffect(int sessionId, int effectId)
+sp<AudioFlinger::EffectModule> AudioFlinger::ThreadBase::getEffect(audio_session_t sessionId,
+        int effectId)
 {
     Mutex::Autolock _l(mLock);
     return getEffect_l(sessionId, effectId);
 }
 
-sp<AudioFlinger::EffectModule> AudioFlinger::ThreadBase::getEffect_l(int sessionId, int effectId)
+sp<AudioFlinger::EffectModule> AudioFlinger::ThreadBase::getEffect_l(audio_session_t sessionId,
+        int effectId)
 {
     sp<EffectChain> chain = getEffectChain_l(sessionId);
     return chain != 0 ? chain->getEffectFromId_l(effectId) : 0;
@@ -1430,7 +1432,7 @@
 status_t AudioFlinger::ThreadBase::addEffect_l(const sp<EffectModule>& effect)
 {
     // check for existing effect chain with the requested audio session
-    int sessionId = effect->sessionId();
+    audio_session_t sessionId = effect->sessionId();
     sp<EffectChain> chain = getEffectChain_l(sessionId);
     bool chainCreated = false;
 
@@ -1507,13 +1509,14 @@
     }
 }
 
-sp<AudioFlinger::EffectChain> AudioFlinger::ThreadBase::getEffectChain(int sessionId)
+sp<AudioFlinger::EffectChain> AudioFlinger::ThreadBase::getEffectChain(audio_session_t sessionId)
 {
     Mutex::Autolock _l(mLock);
     return getEffectChain_l(sessionId);
 }
 
-sp<AudioFlinger::EffectChain> AudioFlinger::ThreadBase::getEffectChain_l(int sessionId) const
+sp<AudioFlinger::EffectChain> AudioFlinger::ThreadBase::getEffectChain_l(audio_session_t sessionId)
+        const
 {
     size_t size = mEffectChains.size();
     for (size_t i = 0; i < size; i++) {
@@ -1769,7 +1772,7 @@
         audio_channel_mask_t channelMask,
         size_t *pFrameCount,
         const sp<IMemory>& sharedBuffer,
-        int sessionId,
+        audio_session_t sessionId,
         IAudioFlinger::track_flags_t *flags,
         pid_t tid,
         int uid,
@@ -2039,14 +2042,14 @@
             TrackBase::track_state state = track->mState;
             mLock.unlock();
             status = AudioSystem::startOutput(mId, track->streamType(),
-                                              (audio_session_t)track->sessionId());
+                                              track->sessionId());
             mLock.lock();
             // abort track was stopped/paused while we released the lock
             if (state != track->mState) {
                 if (status == NO_ERROR) {
                     mLock.unlock();
                     AudioSystem::stopOutput(mId, track->streamType(),
-                                            (audio_session_t)track->sessionId());
+                                            track->sessionId());
                     mLock.lock();
                 }
                 return INVALID_OPERATION;
@@ -2421,7 +2424,7 @@
     }
 }
 
-uint32_t AudioFlinger::PlaybackThread::hasAudioSession(int sessionId) const
+uint32_t AudioFlinger::PlaybackThread::hasAudioSession(audio_session_t sessionId) const
 {
     Mutex::Autolock _l(mLock);
     uint32_t result = 0;
@@ -2440,7 +2443,7 @@
     return result;
 }
 
-uint32_t AudioFlinger::PlaybackThread::getStrategyForSession_l(int sessionId)
+uint32_t AudioFlinger::PlaybackThread::getStrategyForSession_l(audio_session_t sessionId)
 {
     // session AUDIO_SESSION_OUTPUT_MIX is placed in same strategy as MUSIC stream so that
     // it is moved to correct output by audio policy manager when A2DP is connected or disconnected
@@ -2523,14 +2526,14 @@
             const sp<Track>& track = tracksToRemove.itemAt(i);
             if (track->isExternalTrack()) {
                 AudioSystem::stopOutput(mId, track->streamType(),
-                                        (audio_session_t)track->sessionId());
+                                        track->sessionId());
 #ifdef ADD_BATTERY_DATA
                 // to track the speaker usage
                 addBatteryData(IMediaPlayerService::kBatteryDataAudioFlingerStop);
 #endif
                 if (track->isTerminated()) {
                     AudioSystem::releaseOutput(mId, track->streamType(),
-                                               (audio_session_t)track->sessionId());
+                                               track->sessionId());
                 }
             }
         }
@@ -2696,13 +2699,13 @@
 
 status_t AudioFlinger::PlaybackThread::addEffectChain_l(const sp<EffectChain>& chain)
 {
-    int session = chain->sessionId();
+    audio_session_t session = chain->sessionId();
     int16_t* buffer = reinterpret_cast<int16_t*>(mEffectBufferEnabled
             ? mEffectBuffer : mSinkBuffer);
     bool ownsBuffer = false;
 
     ALOGV("addEffectChain_l() %p on thread %p for session %d", chain.get(), this, session);
-    if (session > 0) {
+    if (session > AUDIO_SESSION_OUTPUT_MIX) {
         // Only one effect chain can be present in direct output thread and it uses
         // the sink buffer as input
         if (mType != DIRECT) {
@@ -2741,15 +2744,18 @@
     chain->setOutBuffer(reinterpret_cast<int16_t*>(mEffectBufferEnabled
             ? mEffectBuffer : mSinkBuffer));
     // Effect chain for session AUDIO_SESSION_OUTPUT_STAGE is inserted at end of effect
-    // chains list in order to be processed last as it contains output stage effects
+    // chains list in order to be processed last as it contains output stage effects.
     // Effect chain for session AUDIO_SESSION_OUTPUT_MIX is inserted before
     // session AUDIO_SESSION_OUTPUT_STAGE to be processed
-    // after track specific effects and before output stage
+    // after track specific effects and before output stage.
     // It is therefore mandatory that AUDIO_SESSION_OUTPUT_MIX == 0 and
-    // that AUDIO_SESSION_OUTPUT_STAGE < AUDIO_SESSION_OUTPUT_MIX
+    // that AUDIO_SESSION_OUTPUT_STAGE < AUDIO_SESSION_OUTPUT_MIX.
     // Effect chain for other sessions are inserted at beginning of effect
     // chains list to be processed before output mix effects. Relative order between other
-    // sessions is not important
+    // sessions is not important.
+    static_assert(AUDIO_SESSION_OUTPUT_MIX == 0 &&
+            AUDIO_SESSION_OUTPUT_STAGE < AUDIO_SESSION_OUTPUT_MIX,
+            "audio_session_t constants misdefined");
     size_t size = mEffectChains.size();
     size_t i = 0;
     for (i = 0; i < size; i++) {
@@ -2765,7 +2771,7 @@
 
 size_t AudioFlinger::PlaybackThread::removeEffectChain_l(const sp<EffectChain>& chain)
 {
-    int session = chain->sessionId();
+    audio_session_t session = chain->sessionId();
 
     ALOGV("removeEffectChain_l() %p from thread %p for session %d", chain.get(), this, session);
 
@@ -4427,7 +4433,7 @@
 
 // getTrackName_l() must be called with ThreadBase::mLock held
 int AudioFlinger::MixerThread::getTrackName_l(audio_channel_mask_t channelMask,
-        audio_format_t format, int sessionId)
+        audio_format_t format, audio_session_t sessionId)
 {
     return mAudioMixer->getTrackName(channelMask, format, sessionId);
 }
@@ -5003,7 +5009,7 @@
 
 // getTrackName_l() must be called with ThreadBase::mLock held
 int AudioFlinger::DirectOutputThread::getTrackName_l(audio_channel_mask_t channelMask __unused,
-        audio_format_t format __unused, int sessionId __unused)
+        audio_format_t format __unused, audio_session_t sessionId __unused)
 {
     return 0;
 }
@@ -6201,7 +6207,8 @@
                                   (activeTrack->mFramesToDrop >= 0) ? "timed out" : "cancelled",
                                   activeTrack->sessionId(),
                                   (activeTrack->mSyncStartEvent != 0) ?
-                                          activeTrack->mSyncStartEvent->triggerSession() : 0);
+                                          activeTrack->mSyncStartEvent->triggerSession() :
+                                          AUDIO_SESSION_NONE);
                             activeTrack->clearSyncStartEvent();
                         }
                     }
@@ -6307,7 +6314,7 @@
         audio_format_t format,
         audio_channel_mask_t channelMask,
         size_t *pFrameCount,
-        int sessionId,
+        audio_session_t sessionId,
         size_t *notificationFrames,
         int uid,
         IAudioFlinger::track_flags_t *flags,
@@ -6426,7 +6433,7 @@
 
 status_t AudioFlinger::RecordThread::start(RecordThread::RecordTrack* recordTrack,
                                            AudioSystem::sync_event_t event,
-                                           int triggerSession)
+                                           audio_session_t triggerSession)
 {
     ALOGV("RecordThread::start event %d, triggerSession %d", event, triggerSession);
     sp<ThreadBase> strongMe = this;
@@ -6473,7 +6480,7 @@
         status_t status = NO_ERROR;
         if (recordTrack->isExternalTrack()) {
             mLock.unlock();
-            status = AudioSystem::startInput(mId, (audio_session_t)recordTrack->sessionId());
+            status = AudioSystem::startInput(mId, recordTrack->sessionId());
             mLock.lock();
             // FIXME should verify that recordTrack is still in mActiveTracks
             if (status != NO_ERROR) {
@@ -6505,7 +6512,7 @@
 
 startError:
     if (recordTrack->isExternalTrack()) {
-        AudioSystem::stopInput(mId, (audio_session_t)recordTrack->sessionId());
+        AudioSystem::stopInput(mId, recordTrack->sessionId());
     }
     recordTrack->clearSyncStartEvent();
     // FIXME I wonder why we do not reset the state here?
@@ -6559,7 +6566,7 @@
         return BAD_VALUE;
     }
 
-    int eventSession = event->triggerSession();
+    audio_session_t eventSession = event->triggerSession();
     status_t ret = NAME_NOT_FOUND;
 
     Mutex::Autolock _l(mLock);
@@ -7225,7 +7232,7 @@
     return mInput->stream->get_input_frames_lost(mInput->stream);
 }
 
-uint32_t AudioFlinger::RecordThread::hasAudioSession(int sessionId) const
+uint32_t AudioFlinger::RecordThread::hasAudioSession(audio_session_t sessionId) const
 {
     Mutex::Autolock _l(mLock);
     uint32_t result = 0;
@@ -7243,13 +7250,13 @@
     return result;
 }
 
-KeyedVector<int, bool> AudioFlinger::RecordThread::sessionIds() const
+KeyedVector<audio_session_t, bool> AudioFlinger::RecordThread::sessionIds() const
 {
-    KeyedVector<int, bool> ids;
+    KeyedVector<audio_session_t, bool> ids;
     Mutex::Autolock _l(mLock);
     for (size_t j = 0; j < mTracks.size(); ++j) {
         sp<RecordThread::RecordTrack> track = mTracks[j];
-        int sessionId = track->sessionId();
+        audio_session_t sessionId = track->sessionId();
         if (ids.indexOfKey(sessionId) < 0) {
             ids.add(sessionId, true);
         }