status_t BufferQueueConsumer::setMaxAcquiredBufferCount(
        int maxAcquiredBuffers) {
    if (maxAcquiredBuffers < 1 ||
            maxAcquiredBuffers > BufferQueueCore::MAX_MAX_ACQUIRED_BUFFERS) {
        BQ_LOGE("setMaxAcquiredBufferCount: invalid count %d",
                maxAcquiredBuffers);
        return BAD_VALUE;
    }

    sp<IConsumerListener> listener;
    { // Autolock scope
        std::unique_lock<std::mutex> lock(mCore->mMutex);
        mCore->waitWhileAllocatingLocked(lock);

        if (mCore->mIsAbandoned) {
            BQ_LOGE("setMaxAcquiredBufferCount: consumer is abandoned");
            return NO_INIT;
        }

        if (maxAcquiredBuffers == mCore->mMaxAcquiredBufferCount) {
            return NO_ERROR;
        }

• 检查 maxAcquiredBuffers，它不能小于 1 且不能大于 MAX_MAX_ACQUIRED_BUFFERS。
• 调用 BufferQueueCore::waitWhileAllocatingLocked() 等待 buffer 分配完成。
• mIsAbandoned 是 true，说明消费者调用了 BufferQueueConsumer::disconnect()，无法再使用。

        // The new maxAcquiredBuffers count should not be violated by the number
        // of currently acquired buffers
        int acquiredCount = 0;
        for (int slot : mCore->mActiveBuffers) {
            if (mSlots[slot].mBufferState.isAcquired()) {
                acquiredCount++;
            }
        }
        if (acquiredCount > maxAcquiredBuffers) {
            BQ_LOGE("setMaxAcquiredBufferCount: the requested maxAcquiredBuffer"
                    "count (%d) exceeds the current acquired buffer count (%d)",
                    maxAcquiredBuffers, acquiredCount);
            return BAD_VALUE;
        }

        if ((maxAcquiredBuffers + mCore->mMaxDequeuedBufferCount +
                (mCore->mAsyncMode || mCore->mDequeueBufferCannotBlock ? 1 : 0))
                > mCore->mMaxBufferCount) {
            BQ_LOGE("setMaxAcquiredBufferCount: %d acquired buffers would "
                    "exceed the maxBufferCount (%d) (maxDequeued %d async %d)",
                    maxAcquiredBuffers, mCore->mMaxBufferCount,
                    mCore->mMaxDequeuedBufferCount, mCore->mAsyncMode ||
                    mCore->mDequeueBufferCannotBlock);
            return BAD_VALUE;
        }

• 先计算状态为 ACQUIRED 的 slot 个数 acquiredCount，它不能大于 maxAcquiredBuffers。
• 最大 ACQUIRED 个数和最大 DEQUEUED 个数之和不能大于 mMaxBufferCount，当 mAsyncMode 或 mDequeueBufferCannotBlock 是 true 时可超 1 个。mMaxBufferCount 的默认值是 NUM_BUFFER_SLOTS，即 64。

        int delta = maxAcquiredBuffers - mCore->mMaxAcquiredBufferCount;
        if (!mCore->adjustAvailableSlotsLocked(delta)) {
            return BAD_VALUE;
        }

        mCore->mMaxAcquiredBufferCount = maxAcquiredBuffers;
        if (delta < 0 && mCore->mBufferReleasedCbEnabled) {
            listener = mCore->mConsumerListener;
        }
    } // Autolock scope
    // Call back without lock held
    if (listener != nullptr) {
        listener->onBuffersReleased();
    }

    return NO_ERROR;
}

• 计算差值 delta，调用 BufferQueueCore::adjustAvailableSlotsLocked() 调整 FREE slot 数。 如果调整失败，则返回 BAD_VALUE。
• 更新 mMaxAcquiredBufferCount。如果 delta 小于零时，可能释放了 mFreeBuffers 中的 buffer，所以回调 onBuffersReleased()。