Linux的uevent事件机制

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, 
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