status_t BufferQueueProducer::connect(const sp<IProducerListener>& listener,
        int api, bool producerControlledByApp, QueueBufferOutput *output) {
    ATRACE_CALL();
    std::lock_guard<std::mutex> lock(mCore->mMutex);
    mConsumerName = mCore->mConsumerName;
    BQ_LOGV("connect: api=%d producerControlledByApp=%s", api,
            producerControlledByApp ? "true" : "false");

    if (mCore->mIsAbandoned) {
        BQ_LOGE("connect: BufferQueue has been abandoned");
        return NO_INIT;
    }

    if (mCore->mConsumerListener == nullptr) {
        BQ_LOGE("connect: BufferQueue has no consumer");
        return NO_INIT;
    }

    if (output == nullptr) {
        BQ_LOGE("connect: output was NULL");
        return BAD_VALUE;
    }

    if (mCore->mConnectedApi != BufferQueueCore::NO_CONNECTED_API) {
        BQ_LOGE("connect: already connected (cur=%d req=%d)",
                mCore->mConnectedApi, api);
        return BAD_VALUE;
    }

检查一些状态和参数。

    int delta = mCore->getMaxBufferCountLocked(mCore->mAsyncMode,
            mDequeueTimeout < 0 ?
            mCore->mConsumerControlledByApp && producerControlledByApp : false,
            mCore->mMaxBufferCount) -
            mCore->getMaxBufferCountLocked();
    if (!mCore->adjustAvailableSlotsLocked(delta)) {
        BQ_LOGE("connect: BufferQueue failed to adjust the number of available "
                "slots. Delta = %d", delta);
        return BAD_VALUE;
    }

从 BufferQueueCore::getMaxBufferCountLocked() 两个重载函数的实现看，delta 的值取决于第二个参数 dequeueBufferCannotBlock 的计算：

mDequeueTimeout < 0 ? mCore->mConsumerControlledByApp && producerControlledByApp : false

其他参数都是一样的，所以 delta 的值是 -1 或 0 或 1。

计算 delta 之后调用 BufferQueueCore::adjustAvailableSlotsLocked() 调整 mUnusedSlots、mFreeSlots 和 mFreeBuffers。

    int status = NO_ERROR;
    switch (api) {
        case NATIVE_WINDOW_API_EGL:
        case NATIVE_WINDOW_API_CPU:
        case NATIVE_WINDOW_API_MEDIA:
        case NATIVE_WINDOW_API_CAMERA:
            mCore->mConnectedApi = api;

            output->width = mCore->mDefaultWidth;
            output->height = mCore->mDefaultHeight;
            output->transformHint = mCore->mTransformHintInUse = mCore->mTransformHint;
            output->numPendingBuffers =
                    static_cast<uint32_t>(mCore->mQueue.size());
            output->nextFrameNumber = mCore->mFrameCounter + 1;
            output->bufferReplaced = false;
            output->maxBufferCount = mCore->mMaxBufferCount;

            if (listener != nullptr) {
                // Set up a death notification so that we can disconnect
                // automatically if the remote producer dies
#ifndef NO_BINDER
                if (IInterface::asBinder(listener)->remoteBinder() != nullptr) {
                    status = IInterface::asBinder(listener)->linkToDeath(
                            static_cast<IBinder::DeathRecipient*>(this));
                    if (status != NO_ERROR) {
                        BQ_LOGE("connect: linkToDeath failed: %s (%d)",
                                strerror(-status), status);
                    }
                    mCore->mLinkedToDeath = listener;
                }
#endif
                mCore->mConnectedProducerListener = listener;
                mCore->mBufferReleasedCbEnabled = listener->needsReleaseNotify();
            }
            break;
        default:
            BQ_LOGE("connect: unknown API %d", api);
            status = BAD_VALUE;
            break;
    }

所有 api 流程处理一样。填充 QueueBufferOutput 成员，有 numPendingBuffers、nextFrameNumber、maxBufferCount。

如果生产者不在同进程，监听死亡并设置 listener。从现在的代码看 mLinkedToDeath 有点重复了，其实只需要用一个 boolean 保存是否监听了死亡即可。

if (listener != NULL) {
    if (IInterface::asBinder(listener)->remoteBinder() != NULL) {
        status = IInterface::asBinder(listener)->linkToDeath(
                static_cast<IBinder::DeathRecipient*>(this));
        ...
        mCore->mLinkedToDeath = listener;
    }
    if (listener->needsReleaseNotify()) {
        mCore->mConnectedProducerListener = listener;
    }
}

上面是 mLinkedToDeath 引入时的代码。从这里可以知道，因为当时 mConnectedProducerListener 只有在需要通知的时候才设置，所以需要 mLinkedToDeath。

    mCore->mConnectedPid = BufferQueueThreadState::getCallingPid();
    mCore->mBufferHasBeenQueued = false;
    mCore->mDequeueBufferCannotBlock = false;
    mCore->mQueueBufferCanDrop = false;
    mCore->mLegacyBufferDrop = true;
    if (mCore->mConsumerControlledByApp && producerControlledByApp) {
        mCore->mDequeueBufferCannotBlock = mDequeueTimeout < 0;
        mCore->mQueueBufferCanDrop = mDequeueTimeout <= 0;
    }

    mCore->mAllowAllocation = true;
    return status;
}