// 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// flags indicating that previously-returned buffers are no longer valid.
status_t BufferQueueProducer::requestBuffer(int slot, sp<GraphicBuffer>* buf) {
    ATRACE_CALL();
    BQ_LOGV("requestBuffer: slot %d", slot);
    std::lock_guard<std::mutex> lock(mCore->mMutex);

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

    if (mCore->mConnectedApi == BufferQueueCore::NO_CONNECTED_API) {
        BQ_LOGE("requestBuffer: BufferQueue has no connected producer");
        return NO_INIT;
    }

    if (slot < 0 || slot >= BufferQueueDefs::NUM_BUFFER_SLOTS) {
        BQ_LOGE("requestBuffer: slot index %d out of range [0, %d)",
                slot, BufferQueueDefs::NUM_BUFFER_SLOTS);
        return BAD_VALUE;
    } else if (!mSlots[slot].mBufferState.isDequeued()) {
        BQ_LOGE("requestBuffer: slot %d is not owned by the producer "
                "(state = %s)", slot, mSlots[slot].mBufferState.string());
        return BAD_VALUE;
    }

    mSlots[slot].mRequestBufferCalled = true;
    *buf = mSlots[slot].mGraphicBuffer;
    return NO_ERROR;
}

• 返回指定 slot 的 GraphicBuffer。一般情况下，第一次调用 BufferQueueProducer::dequeueBuffer() 后调用 requestBuffer()。
• BufferQueueProducer::dequeueBuffer() 中只要是 buffer 新分配或再分配了都会返回 BUFFER_NEEDS_REALLOCATION，这时需要调用 requestBuffer() 获取 GraphicBuffer 指针并缓存。之后使用缓存可以减少 GraphicBuffer 的跨进程传递。