必读： Android 12(S) 图像显示系统 - 开篇 一、前言 前面的文章中讲解Android BufferQueue的机制时，有遇到过Fence，但没有具体讲解。这篇文章，就针对Fence这种同步机制，做一些介绍。 Fenc
必读：
Android 12(S) 图像显示系统 - 开篇一、前言
前面的文章中讲解Android BufferQueue的机制时，有遇到过Fence，但没有具体讲解。这篇文章，就针对Fence这种同步机制，做一些介绍。
Fence在Android图像显示系统中用于GraphicBuffer的同步。我们不禁有疑问：那它和其它的同步机制相比有什么特点呢？
Fence主要被用来处理跨硬件的情况，在我们关注的Android图像显示系统中即处理CPU，GPU和HWC之间的数据同步。
二、Fence的基本作用
在Android BufferQueue的机制中，GraphicBuffer在生产者--图形缓冲队列--消费者之间流转。每一个GraphicBuffer都有一个对应的BufferState标记其状态，详细可以参考：Android 12(S) 图像显示系统 - BufferQueue的工作流程（八）
本文作者@二的次方 2022-05-20发布于自由互联
通常这里的BufferState代表的仅仅是GraphicBuffer的所有权，即此刻它归谁所拥有；但这块缓存是否可以被所有者使用还需要等待同步信号到来，即使用权要等到相关的Fence信号发出才可获得。
在BufferState的各种状态的定义的解释中可以看到：
[/frameworks/native/libs/gui/include/gui/BufferSlot.h]
// DEQUEUED indicates that the buffer has been dequeued by the producer, but
// has not yet been queued or canceled. The producer may modify the
// buffer's contents as soon as the associated release fence is signaled.
// The slot is "owned" by the producer.
// 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.
// 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
// acquire fence is signaled. The slot is "owned" by the consumer.