android screen睡眠唤醒机制

 在android中，屏幕(framebuffer)被看成为一个device，文件/sys/power/wait_for_fb_sleep和/sys/power/wait_for_fb_wakeup被kernel用来通知framework屏幕挂起或恢复了。在framework中有两个线程，一个是画图的主线程，在循环中它会首先检查kernel的事件通知信号并设置framebuffer是否可画，下面是示意性的framework代码：

 

1. 1. surface flinger thread:  
2. while(true) {  
3.     checkEventAndSendSignal();  
4.     doSomeThing();  
5.     if (canDraw) {  
6.         swapBuffer();  
7.     }  
8. }  
9.  event function  
10. checkEventAndSendSignal() {  
11.     if (wait_for_sleep  returned) {  
12.         canDraw = false;  
13.         ();  
14.     }  
15.     if (wait_for_wakeup returned) {  
16.         canDraw = true;  
17.     }  
18. }  
19.  function  
20. swapBuffer() {  
21.     .....  
22.     ioctl(egl_native_window_t::fd, FBIOPUT_VSCREENINFO, &mInfo)  
23.      ....  
24. }  
25.  thread  
26. while(true) {  
27.     wait_for_sleep();  
28.     sendSleepReturnEvent();  
29.     ();  
30.    wait_for_wakeup();  
31.   sendWakeupReturnEvent();  
32. }  

1. surface flinger thread: while(true) { checkEventAndSendSignal(); doSomeThing(); if (canDraw) { swapBuffer(); } } event function checkEventAndSendSignal() { if (wait_for_sleep returned) { canDraw = false; (); } if (wait_for_wakeup returned) { canDraw = true; } } function swapBuffer() { ..... ioctl(egl_native_window_t::fd, FBIOPUT_VSCREENINFO, &mInfo) .... } thread while(true) { wait_for_sleep(); sendSleepReturnEvent(); (); wait_for_wakeup(); sendWakeupReturnEvent(); }

当屏幕可用时，kernel会block getEvent线程在wait_for_sleep(read(wait_for_fb_sleep file))调用上，直到屏幕不可用(用户关闭屏幕，变黑)；当屏幕不可用（挂起），kernel会block getEvent线程在wait_for_wakeup(read(wait_for_fb_wakeup file))调用上，直到屏幕可用。

在挂起屏幕前（wait_for_wakeup）mBarrier用来同步surfaceflinger线程画完上次循环未完成的framebuffer