原文链接：https://blog.csdn.net/qq_30624591/article/details/92439991

转自：https://blog.csdn.net/qq_30624591/article/details/92439991

uevent事件机制

Linux设计了一种uevent的机制。当有新的设备加入的时候，将设备的信息发送消息到用户态。而用户态有一个udev的进程监听这个信息。当收到信息后做一定的解析，根据解析到的结果和用户程序的配置做一些处理，也包括加载驱动程序。

内核部分，Uevent是Kobject的一部分，用于在Kobject状态发生改变时，例如增加、移除等，通知用户空间程序,主要涉及kobject.h和kobject_uevent.c两个文件，如下：

include/linux/kobject.h
lib/kobject_uevent.c

• 内核上报事件,调用kobject_uevent_env函数进行上报事件

• int kobject_uevent_env(struct kobjectkobj, enum kobject_action action,charenvp_ext[])

  static const char *kobject_actions[] ={
  [KOBJ_ADD] = “add”,
  [KOBJ_REMOVE] = “remove”,
  [KOBJ_CHANGE] = “change”,
  [KOBJ_MOVE] = “move”,
  [KOBJ_ONLINE] = “online”,
  [KOBJ_OFFLINE] = “offline”,
  };

比如下面的一个HDMI插拔的uevent字符串

 {SUBSYSTEM=extcon, SEQNUM=2706, ACTION=change, DEVTYPE=hdmi_audio, STATE=HDMI=1, DEVPATH=/devices/virtual/amhdmitx/amhdmitx0/hdmi_audio, NAME=hdmitx_extcon_audio}

android层面去读取uevent事件

frameworks/base/core/jni/android_os_UEventObserver.cpp
uevent_init()  //open socket for UEventObserver
uevent_next_event(buffer, sizeof(buffer) //读取uevent事件

for (;;) {
    int length = uevent_next_event(buffer, sizeof(buffer) - 1);
    if (length <= 0) {
        return NULL;
    }
    buffer[length] = '\0';

    ALOGV("Received uevent message: %s", buffer);

    if (isMatch(buffer, length)) {
        // Assume the message is ASCII.
        jchar message[length];
        for (int i = 0; i < length; i++) {
            message[i] = buffer[i];
        }
        return env->NewString(message, length);
    }
}

frameworks/base/core/java/android/os/UEventObserver.java
创建一个UEventThread线程不停的读取消息
            while (true) {
                String message = nativeWaitForNextEvent();
                if (message != null) {
                    if (DEBUG) {
                        Log.d(TAG, message);
                    }
                    sendEvent(message);
                }
            }

amlogic平台调试的例子

• 这里主要是添加了gpio按键的uevent事件上报给android,下面是一个上报的简单例子，主要是传输字符串，这里可以任意指定

  char data[64];
  char *envp[] = { data, NULL };
  snprintf(data, sizeof(data), “code=%d,state=up”, key->code);
  kobject_uevent_env(&kp->config_dev->kobj, KOBJ_CHANGE, envp);

• app里面的修改

    mObserver.startObserving("DEVPATH=/devices/virtual/gpio_keyboard/gpio_keyboard");
    mObserver.startObserving("DEVPATH=/devices/adc_keypad.13/input/input1");
    private UEventObserver mObserver = new UEventObserver() {
                @Override
                public void onUEvent(UEventObserver.UEvent event) {
                    Log.i("clei onUEvent","event:"+event +"  code = "+event.get("code"));
                }
            };
  

• 注意点startObserving这个里面传入的是设备的驱动路径，必须填写正确，onUEvent接收到的就是回调的时间信息

    09-11 00:08:59.942  7450  7474 I clei onUEvent: event:{DEVNAME=gpio_keyboard, SUBSYSTEM=gpio_keyboard, SEQNUM=4095, MAJOR=222, ACTION=change, DEVPATH=/devices/virtual/gpio_keyboard/gpio_keyboard, MINOR=0, code=473,state=down}  code = 473,state=down
    09-11 00:09:00.151  7450  7474 I clei onUEvent: event:{DEVNAME=gpio_keyboard, SUBSYSTEM=gpio_keyboard, SEQNUM=4096, MAJOR=222, ACTION=change, DEVPATH=/devices/virtual/gpio_keyboard/gpio_keyboard, MINOR=0, code=473,state=up}  code = 473,state=up
    
    09-11 00:08:55.147  7450  7474 I clei onUEvent: event:{PROP=0, SUBSYSTEM=input, PRODUCT=10/1/1/100, SEQNUM=4091, EV=100003, ACTION=change, DEVPATH=/devices/adc_keypad.13/input/input1, PHYS="adc_keypad/input0", KEY=3c000000 0 0 0 0 0 40000 0, NAME="adc_keypad", MODALIAS=input:b0010v0001p0001e0100-e0,1,14,k1DA,1DB,1DC,1DD,ramlsfw, code=477,state=down}  code = 477,state=down
    09-11 00:08:55.291  7450  7474 I clei onUEvent: event:{PROP=0, SUBSYSTEM=input, PRODUCT=10/1/1/100,