class BufferQueueCore : public virtual RefBase {

    friend class BufferQueueProducer;
    friend class BufferQueueConsumer;

public:
    // Used as a placeholder slot number when the value isn't pointing to an
    // existing buffer.
    enum { INVALID_BUFFER_SLOT = BufferItem::INVALID_BUFFER_SLOT };

    // We reserve two slots in order to guarantee that the producer and
    // consumer can run asynchronously.
    enum { MAX_MAX_ACQUIRED_BUFFERS = BufferQueueDefs::NUM_BUFFER_SLOTS - 2 };

    enum {
        // The API number used to indicate the currently connected producer
        CURRENTLY_CONNECTED_API = -1,

        // The API number used to indicate that no producer is connected
        NO_CONNECTED_API        = 0,
    };

    typedef Vector<BufferItem> Fifo;

• INVALID_BUFFER_SLOT 是 slot 的初始值，是 -1。
• MAX_MAX_ACQUIRED_BUFFERS 在 BufferQueueConsumer::setMaxAcquiredBufferCount() 中用于检查参数。
• Fifo 是 mQueue 的类型。

private:
    ...
    // mMutex is the mutex used to prevent concurrent access to the member
    // variables of BufferQueueCore objects. It must be locked whenever any
    // member variable is accessed.
    mutable std::mutex mMutex;

    // mIsAbandoned indicates that the BufferQueue will no longer be used to
    // 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 is abandoned will return
    // the NO_INIT error from all IGraphicBufferProducer methods capable of
    // returning an error.
    bool mIsAbandoned;

    // mConsumerControlledByApp indicates whether the connected consumer is
    // controlled by the application.
    bool mConsumerControlledByApp;

• 因为生产者和消费者是并发访问 BufferQueueCore，所以访问任何 BufferQueueCore 成员都需要用 mMutex加锁。
• mIsAbandoned 表示 BufferQueue 不再消费 buffer。它初始化值是 false，在 BufferQueueConsumer::disconnect() 中设置为 true。
• mConsumerControlledByApp 表示消费者是否是应用，默认情况下 SurfaceFlinger 是消费者。

    // mConsumerName is a string used to identify the BufferQueue in log
    // messages. It is set by the IGraphicBufferConsumer::setConsumerName
    // method.
    String8 mConsumerName;

    // mConsumerListener is used to notify the connected consumer of
    // asynchronous events that it may wish to react to. It is initially
    // set to NULL and is written by consumerConnect and consumerDisconnect.
    sp<IConsumerListener> mConsumerListener;

    // mConsumerUsageBits contains flags that the consumer wants for
    // GraphicBuffers.
    uint64_t mConsumerUsageBits;

• mConsumerName 通过 setConsumerName() 更新。
• mConsumerListener 在 BufferQueueConsumer::connect() 设置，参考 ConsumerListener。
• mConsumerUsageBits 是消费者期望的 GraphicBuffer usage，通过 setConsumerUsageBits() 更新。

    // mConsumerIsProtected indicates the consumer is ready to handle protected
    // buffer.
    bool mConsumerIsProtected;

    // mConnectedApi indicates the producer API that is currently connected
    // to this BufferQueue. It defaults to NO_CONNECTED_API, and gets updated
    // by the connect and disconnect methods.
    int mConnectedApi;
    // PID of the process which last successfully called connect(...)
    pid_t mConnectedPid;

• mConsumerIsProtected 表示消费者是否可以处理受保护的 buffer。没看到使用的地方。
• mConnectedApi 在 BufferQueueProducer::connect() 设置的，但不同的值处理方式都一样，不知道有什么区别。
• mConnectedPid 在 BufferQueueProducer::connect() 设置的，它是通过 getCallingPid() 获取的调用者进程号。