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("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 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 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 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 onUEvent: event:{PROP=0, SUBSYSTEM=input, PRODUCT=10/1/1/100, SEQNUM=4092, 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=up} code = 477,state=up