Updated comments

    // ConsumerListener is the interface through which the BufferQueue notifies

    // the consumer of events that the consumer may wish to react to.  Because

    // the consumer will generally have a mutex that is locked during calls from

    // teh consumer to the BufferQueue, these calls from the BufferQueue to the

    // the consumer to the BufferQueue, these calls from the BufferQueue to the

    // consumer *MUST* be called only when the BufferQueue mutex is NOT locked.

    struct ConsumerListener : public virtual RefBase {

        // onFrameAvailable is called from queueBuffer each time an additional

            const sp<IGraphicBufferAlloc>& allocator = NULL);

    virtual ~BufferQueue();

    // Query native window attributes.  The "what" values are enumerated in

    // window.h (e.g. NATIVE_WINDOW_FORMAT).

    virtual int query(int what, int* value);

    // setBufferCount updates the number of available buffer slots.  After

    // calling this all buffer slots are both unallocated and owned by the

    // BufferQueue object (i.e. they are not owned by the client).

    // setBufferCount updates the number of available buffer slots.  If this

    // method succeeds, buffer slots will be both unallocated and owned by

    // the BufferQueue object (i.e. they are not owned by the producer or

    // consumer).

    //

    // This will fail if the producer has dequeued any buffers, or if

    // bufferCount is invalid.  bufferCount must generally be a value

    // between the minimum undequeued buffer count and NUM_BUFFER_SLOTS

    // (inclusive).  It may also be set to zero (the default) to indicate

    // that the producer does not wish to set a value.  The minimum value

    // can be obtained by calling query(NATIVE_WINDOW_MIN_UNDEQUEUED_BUFFERS,

    // ...).

    //

    // This may only be called by the producer.  The consumer will be told

    // to discard buffers through the onBuffersReleased callback.

    virtual status_t setBufferCount(int bufferCount);

    // requestBuffer returns the GraphicBuffer for slot N.

    //

    // In normal operation, this is called the first time slot N is returned

    // by dequeueBuffer.  It must be called again if dequeueBuffer returns

    // flags indicating that previously-returned buffers are no longer valid.

    virtual status_t requestBuffer(int slot, sp<GraphicBuffer>* buf);

    // dequeueBuffer gets the next buffer slot index for the client to use. If a

    // buffer slot is available then that slot index is written to the location

    // pointed to by the buf argument and a status of OK is returned.  If no

    // slot is available then a status of -EBUSY is returned and buf is

    // dequeueBuffer gets the next buffer slot index for the producer to use.

    // If a buffer slot is available then that slot index is written to the

    // location pointed to by the buf argument and a status of OK is returned.

    // If no slot is available then a status of -EBUSY is returned and buf is

    // unmodified.

    //

    // The fence parameter will be updated to hold the fence associated with

    // the buffer. The contents of the buffer must not be overwritten until the

    // fence signals. If the fence is NULL, the buffer may be written

    // immediately.

    // fence signals. If the fence is Fence::NO_FENCE, the buffer may be

    // written immediately.

    //

    // The width and height parameters must be no greater than the minimum of

    // GL_MAX_VIEWPORT_DIMS and GL_MAX_TEXTURE_SIZE (see: glGetIntegerv).

    // An error due to invalid dimensions might not be reported until

    // updateTexImage() is called.

    // updateTexImage() is called.  If width and height are both zero, the

    // default values specified by setDefaultBufferSize() are used instead.

    //

    // The pixel formats are enumerated in graphics.h, e.g.

    // HAL_PIXEL_FORMAT_RGBA_8888.  If the format is 0, the default format

    // will be used.

    //

    // The usage argument specifies gralloc buffer usage flags.  The values

    // are enumerated in gralloc.h, e.g. GRALLOC_USAGE_HW_RENDER.  These

    // will be merged with the usage flags specified by setConsumerUsageBits.

    //

    // The return value may be a negative error value or a non-negative

    // collection of flags.  If the flags are set, the return values are

    // valid, but additional actions must be performed.

    //

    // If IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION is set, the

    // producer must discard cached GraphicBuffer references for the slot

    // returned in buf.

    // If IGraphicBufferProducer::RELEASE_ALL_BUFFERS is set, the producer

    // must discard cached GraphicBuffer references for all slots.

    //

    // In both cases, the producer will need to call requestBuffer to get a

    // GraphicBuffer handle for the returned slot.

    virtual status_t dequeueBuffer(int *buf, sp<Fence>* fence,

            uint32_t width, uint32_t height, uint32_t format, uint32_t usage);

    // queueBuffer returns a filled buffer to the BufferQueue. In addition, a

    // timestamp must be provided for the buffer. The timestamp is in

    // queueBuffer returns a filled buffer to the BufferQueue.

    //

    // Additional data is provided in the QueueBufferInput struct.  Notably,

    // a timestamp must be provided for the buffer. The timestamp is in

    // nanoseconds, and must be monotonically increasing. Its other semantics

    // (zero point, etc) are client-dependent and should be documented by the

    // client.

    // (zero point, etc) are producer-specific and should be documented by the

    // producer.

    //

    // The caller may provide a fence that signals when all rendering

    // operations have completed.  Alternatively, NO_FENCE may be used,

    // indicating that the buffer is ready immediately.

    //

    // Some values are returned in the output struct: the current settings

    // for default width and height, the current transform hint, and the

    // number of queued buffers.

    virtual status_t queueBuffer(int buf,

            const QueueBufferInput& input, QueueBufferOutput* output);

    // cancelBuffer returns a dequeued buffer to the BufferQueue, but doesn't

    // queue it for use by the consumer.

    //

    // The buffer will not be overwritten until the fence signals.  The fence

    // will usually be the one obtained from dequeueBuffer.

    virtual void cancelBuffer(int buf, const sp<Fence>& fence);

    // setSynchronousMode set whether dequeueBuffer is synchronous or

    // setSynchronousMode sets whether dequeueBuffer is synchronous or

    // asynchronous. In synchronous mode, dequeueBuffer blocks until

    // a buffer is available, the currently bound buffer can be dequeued and

    // queued buffers will be retired in order.

    // queued buffers will be acquired in order.  In asynchronous mode,

    // a queued buffer may be replaced by a subsequently queued buffer.

    //

    // The default mode is asynchronous.

    virtual status_t setSynchronousMode(bool enabled);

    // connect attempts to connect a producer client API to the BufferQueue.

    // This must be called before any other IGraphicBufferProducer methods are called

    // except for getAllocator.

    // connect attempts to connect a producer API to the BufferQueue.  This

    // must be called before any other IGraphicBufferProducer methods are

    // called except for getAllocator.  A consumer must already be connected.

    //

    // This method will fail if connect was previously called on the

    // BufferQueue and no corresponding disconnect call was made (i.e. if

    // it's still connected to a producer).

    //

    // This method will fail if the connect was previously called on the

    // BufferQueue and no corresponding disconnect call was made.

    // APIs are enumerated in window.h (e.g. NATIVE_WINDOW_API_CPU).

    virtual status_t connect(int api, QueueBufferOutput* output);

    // disconnect attempts to disconnect a producer client API from the

    // BufferQueue. Calling this method will cause any subsequent calls to other

    // disconnect attempts to disconnect a producer API from the BufferQueue.

    // Calling this method will cause any subsequent calls to other

    // IGraphicBufferProducer methods to fail except for getAllocator and connect.

    // Successfully calling connect after this will allow the other methods to

    // succeed again.

    //

    // This method will fail if the the BufferQueue is not currently

    // connected to the specified client API.

    // connected to the specified producer API.

    virtual status_t disconnect(int api);

    // dump our state in a String

        sp<Fence> mFence;

    };

    // The following public functions is the consumer facing interface

    // The following public functions are the consumer-facing interface

    // acquireBuffer attempts to acquire ownership of the next pending buffer in

    // the BufferQueue.  If no buffer is pending then it returns -EINVAL.  If a

    status_t acquireBuffer(BufferItem *buffer);

    // releaseBuffer releases a buffer slot from the consumer back to the

    // BufferQueue pending a fence sync.

    // BufferQueue.  This may be done while the buffer's contents are still

    // being accessed.  The fence will signal when the buffer is no longer

    // in use.

    //

    // If releaseBuffer returns STALE_BUFFER_SLOT, then the consumer must free

    // any references to the just-released buffer that it might have, as if it

    // consumer may be connected, and when that consumer disconnects the

    // BufferQueue is placed into the "abandoned" state, causing most

    // interactions with the BufferQueue by the producer to fail.

    //

    // consumer may not be NULL.

    status_t consumerConnect(const sp<ConsumerListener>& consumer);

    // consumerDisconnect disconnects a consumer from the BufferQueue. All

    status_t consumerDisconnect();

    // getReleasedBuffers sets the value pointed to by slotMask to a bit mask

    // indicating which buffer slots the have been released by the BufferQueue

    // indicating which buffer slots have been released by the BufferQueue

    // but have not yet been released by the consumer.

    //

    // This should be called from the onBuffersReleased() callback.

    status_t getReleasedBuffers(uint32_t* slotMask);

    // setDefaultBufferSize is used to set the size of buffers returned by

    // requestBuffers when a with and height of zero is requested.

    // dequeueBuffer when a width and height of zero is requested.  Default

    // is 1x1.

    status_t setDefaultBufferSize(uint32_t w, uint32_t h);

    // setDefaultBufferCount set the buffer count. If the client has requested

    // a buffer count using setBufferCount, the server-buffer count will

    // take effect once the client sets the count back to zero.

    // setDefaultMaxBufferCount sets the default value for the maximum buffer

    // count (the initial default is 2). If the producer has requested a

    // buffer count using setBufferCount, the default buffer count will only

    // take effect if the producer sets the count back to zero.

    //

    // The count must be between 2 and NUM_BUFFER_SLOTS, inclusive.

    status_t setDefaultMaxBufferCount(int bufferCount);

    // setMaxAcquiredBufferCount sets the maximum number of buffers that can

    // be acquired by the consumer at one time.  This call will fail if a

    // producer is connected to the BufferQueue.

    // be acquired by the consumer at one time (default 1).  This call will

    // fail if a producer is connected to the BufferQueue.

    status_t setMaxAcquiredBufferCount(int maxAcquiredBuffers);

    // isSynchronousMode returns whether the BufferQueue is currently in

    // setDefaultBufferFormat allows the BufferQueue to create

    // GraphicBuffers of a defaultFormat if no format is specified

    // in dequeueBuffer

    // in dequeueBuffer.  Formats are enumerated in graphics.h; the

    // initial default is HAL_PIXEL_FORMAT_RGBA_8888.

    status_t setDefaultBufferFormat(uint32_t defaultFormat);

    // setConsumerUsageBits will turn on additional usage bits for dequeueBuffer

    // setConsumerUsageBits will turn on additional usage bits for dequeueBuffer.

    // These are merged with the bits passed to dequeueBuffer.  The values are

    // enumerated in gralloc.h, e.g. GRALLOC_USAGE_HW_RENDER; the default is 0.

    status_t setConsumerUsageBits(uint32_t usage);

    // setTransformHint bakes in rotation to buffers so overlays can be used

    // setTransformHint bakes in rotation to buffers so overlays can be used.

    // The values are enumerated in window.h, e.g.

    // NATIVE_WINDOW_TRANSFORM_ROT_90.  The default is 0 (no transform).

    status_t setTransformHint(uint32_t hint);

private:

    // freeBufferLocked frees the resources (both GraphicBuffer and EGLImage)

    // for the given slot.

    // freeBufferLocked frees the GraphicBuffer and sync resources for the

    // given slot.

    void freeBufferLocked(int index);

    // freeAllBuffersLocked frees the resources (both GraphicBuffer and

    // EGLImage) for all slots.

    // freeAllBuffersLocked frees the GraphicBuffer and sync resources for

    // all slots.

    void freeAllBuffersLocked();

    // freeAllBuffersExceptHeadLocked frees the resources (both GraphicBuffer

    // and EGLImage) for all slots except the head of mQueue

    // freeAllBuffersExceptHeadLocked frees the GraphicBuffer and sync

    // resources for all slots except the head of mQueue.

    void freeAllBuffersExceptHeadLocked();

    // drainQueueLocked drains the buffer queue if we're in synchronous mode

    // returns immediately otherwise. It returns NO_INIT if the BufferQueue

    // became abandoned or disconnected during this call.

    // drainQueueLocked waits for the buffer queue to empty if we're in

    // synchronous mode, or returns immediately otherwise. It returns NO_INIT

    // if the BufferQueue is abandoned (consumer disconnected) or disconnected

    // (producer disconnected) during the call.

    status_t drainQueueLocked();

    // drainQueueAndFreeBuffersLocked drains the buffer queue if we're in

    // synchronous mode and free all buffers. In asynchronous mode, all buffers

    // are freed except the current buffer.

    // are freed except the currently queued buffer (if it exists).

    status_t drainQueueAndFreeBuffersLocked();

    // setDefaultMaxBufferCountLocked sets the maximum number of buffer slots

    // that will be used if the producer does not override the buffer slot

    // count.

    // count.  The count must be between 2 and NUM_BUFFER_SLOTS, inclusive.

    // The initial default is 2.

    status_t setDefaultMaxBufferCountLocked(int count);

    // getMinBufferCountLocked returns the minimum number of buffers allowed

        // if no buffer has been allocated.

        sp<GraphicBuffer> mGraphicBuffer;

        // mEglDisplay is the EGLDisplay used to create mEglImage.

        // mEglDisplay is the EGLDisplay used to create EGLSyncKHR objects.

        EGLDisplay mEglDisplay;

        // BufferState represents the different states in which a buffer slot

        // can be.

        // can be.  All slots are initially FREE.

        enum BufferState {

            // FREE indicates that the buffer is not currently being used and

            // will not be used in the future until it gets dequeued and

            // subsequently queued by the client.

            // aka "owned by BufferQueue, ready to be dequeued"

            // FREE indicates that the buffer is available to be dequeued

            // by the producer.  The buffer may be in use by the consumer for

            // a finite time, so the buffer must not be modified until the

            // associated fence is signaled.

            //

            // The slot is "owned" by BufferQueue.  It transitions to DEQUEUED

            // when dequeueBuffer is called.

            FREE = 0,

            // DEQUEUED indicates that the buffer has been dequeued by the

            // client, but has not yet been queued or canceled. The buffer is

            // considered 'owned' by the client, and the server should not use

            // it for anything.

            //

            // Note that when in synchronous-mode (mSynchronousMode == true),

            // the buffer that's currently attached to the texture may be

            // dequeued by the client.  That means that the current buffer can

            // be in either the DEQUEUED or QUEUED state.  In asynchronous mode,

            // however, the current buffer is always in the QUEUED state.

            // aka "owned by producer, ready to be queued"

            // producer, but has not yet been queued or canceled.  The

            // producer may modify the buffer's contents as soon as the

            // associated ready fence is signaled.

            //

            // The slot is "owned" by the producer.  It can transition to

            // QUEUED (via queueBuffer) or back to FREE (via cancelBuffer).

            DEQUEUED = 1,

            // QUEUED indicates that the buffer has been queued by the client,

            // and has not since been made available for the client to dequeue.

            // Attaching the buffer to the texture does NOT transition the

            // buffer away from the QUEUED state. However, in Synchronous mode

            // the current buffer may be dequeued by the client under some

            // circumstances. See the note about the current buffer in the

            // documentation for DEQUEUED.

            // aka "owned by BufferQueue, ready to be acquired"

            // QUEUED indicates that the buffer has been filled by the

            // producer and queued for use by the consumer.  The buffer

            // contents may continue to be modified for a finite time, so

            // the contents must not be accessed until the associated fence

            // is signaled.

            //

            // The slot is "owned" by BufferQueue.  It can transition to

            // ACQUIRED (via acquireBuffer) or to FREE (if another buffer is

            // queued in asynchronous mode).

            QUEUED = 2,

            // aka "owned by consumer, ready to be released"

            // ACQUIRED indicates that the buffer has been acquired by the

            // consumer.  As with QUEUED, the contents must not be accessed

            // by the consumer until the fence is signaled.

            //

            // The slot is "owned" by the consumer.  It transitions to FREE

            // when releaseBuffer is called.

            ACQUIRED = 3

        };

        // mBufferState is the current state of this buffer slot.

        BufferState mBufferState;

        // mRequestBufferCalled is used for validating that the client did

        // mRequestBufferCalled is used for validating that the producer did

        // call requestBuffer() when told to do so. Technically this is not

        // needed but useful for debugging and catching client bugs.

        // needed but useful for debugging and catching producer bugs.

        bool mRequestBufferCalled;

        // mCrop is the current crop rectangle for this buffer slot.

        // to set by queueBuffer each time this slot is queued.

        int64_t mTimestamp;

        // mFrameNumber is the number of the queued frame for this slot.

        // mFrameNumber is the number of the queued frame for this slot.  This

        // is used to dequeue buffers in LRU order (useful because buffers

        // may be released before their release fence is signaled).

        uint64_t mFrameNumber;

        // mEglFence is the EGL sync object that must signal before the buffer

        // associated with this buffer slot may be dequeued. It is initialized

        // to EGL_NO_SYNC_KHR when the buffer is created and (optionally, based

        // on a compile-time option) set to a new sync object in updateTexImage.

        // to EGL_NO_SYNC_KHR when the buffer is created and may be set to a

        // new sync object in releaseBuffer.  (This is deprecated in favor of

        // mFence, below.)

        EGLSyncKHR mEglFence;

        // mFence is a fence which will signal when work initiated by the

        // QUEUED, it indicates when the producer has finished filling the

        // buffer. When the buffer is DEQUEUED or ACQUIRED, the fence has been

        // passed to the consumer or producer along with ownership of the

        // buffer, and mFence is empty.

        // buffer, and mFence is set to NO_FENCE.

        sp<Fence> mFence;

        // Indicates whether this buffer has been seen by a consumer yet

        bool mAcquireCalled;

        // Indicates whether this buffer needs to be cleaned up by consumer

        // Indicates whether this buffer needs to be cleaned up by the

        // consumer.  This is set when a buffer in ACQUIRED state is freed.

        // It causes releaseBuffer to return STALE_BUFFER_SLOT.

        bool mNeedsCleanupOnRelease;

    };

    // mSlots is the array of buffer slots that must be mirrored on the client

    // side. This allows buffer ownership to be transferred between the client

    // and server without sending a GraphicBuffer over binder. The entire array

    // is initialized to NULL at construction time, and buffers are allocated

    // for a slot when requestBuffer is called with that slot's index.

    // mSlots is the array of buffer slots that must be mirrored on the

    // producer side. This allows buffer ownership to be transferred between

    // the producer and consumer without sending a GraphicBuffer over binder.

    // The entire array is initialized to NULL at construction time, and

    // buffers are allocated for a slot when requestBuffer is called with

    // that slot's index.

    BufferSlot mSlots[NUM_BUFFER_SLOTS];

    // mDefaultWidth holds the default width of allocated buffers. It is used

    // in requestBuffers() if a width and height of zero is specified.

    // in dequeueBuffer() if a width and height of zero is specified.

    uint32_t mDefaultWidth;

    // mDefaultHeight holds the default height of allocated buffers. It is used

    // in requestBuffers() if a width and height of zero is specified.

    // in dequeueBuffer() if a width and height of zero is specified.

    uint32_t mDefaultHeight;

    // mMaxAcquiredBufferCount is the number of buffers that the consumer may

    // mSynchronousMode whether we're in synchronous mode or not

    bool mSynchronousMode;

    // mAllowSynchronousMode whether we allow synchronous mode or not

    // mAllowSynchronousMode whether we allow synchronous mode or not.  Set

    // when the BufferQueue is created (by the consumer).

    const bool mAllowSynchronousMode;

    // mConnectedApi indicates the API that is currently connected to this

    // BufferQueue.  It defaults to NO_CONNECTED_API (= 0), and gets updated

    // by the connect and disconnect methods.

    // mConnectedApi indicates the producer API that is currently connected

    // to this BufferQueue.  It defaults to NO_CONNECTED_API (= 0), and gets

    // updated by the connect and disconnect methods.

    int mConnectedApi;

    // mDequeueCondition condition used for dequeueBuffer in synchronous mode

    Fifo mQueue;

    // mAbandoned indicates that the BufferQueue will no longer be used to

    // consume images buffers pushed to it using the IGraphicBufferProducer interface.

    // It is initialized to false, and set to true in the abandon method.  A

    // BufferQueue that has been abandoned will return the NO_INIT error from

    // all IGraphicBufferProducer methods capable of returning an error.

    // consume image buffers pushed to it using the IGraphicBufferProducer

    // interface.  It is initialized to false, and set to true in the

    // consumerDisconnect method.  A BufferQueue that has been abandoned will

    // return the NO_INIT error from all IGraphicBufferProducer methods

    // capable of returning an error.

    bool mAbandoned;

    // mName is a string used to identify the BufferQueue in log messages.

    // It is set by the setName method.

    // mConsumerName is a string used to identify the BufferQueue in log

    // messages.  It is set by the setConsumerName method.

    String8 mConsumerName;

    // mMutex is the mutex used to prevent concurrent access to the member

    // member variables are accessed.

    mutable Mutex mMutex;

    // mFrameCounter is the free running counter, incremented for every buffer queued

    // with the surface Texture.

    // mFrameCounter is the free running counter, incremented on every

    // successful queueBuffer call.

    uint64_t mFrameCounter;

    // mBufferHasBeenQueued is true once a buffer has been queued.  It is reset

    // by changing the buffer count.

    // mBufferHasBeenQueued is true once a buffer has been queued.  It is

    // reset when something causes all buffers to be freed (e.g. changing the

    // buffer count).

    bool mBufferHasBeenQueued;

    // mDefaultBufferFormat can be set so it will override