Linux uevent机制

Uevent是内核通知android有状态变化的一种方法，比如USB线插入、拔出，电池电量变化等等。其本质是内核发送（可以通过socket）一个字符串，应用层（android）接收并解释该字符串，获取相应信息。

一、Kernel侧：

UEVENT的发起在Kernel端，主要是通过函数

int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,char *envp_ext[])

该函数的主要功能是根据参数组合一个字符串并发送。一个典型的字符串如下：change@/devices/platform/msm-battery/power_supply/usb纮ACTION=change纮DEVPATH=/devices/platform/msm-battery/power_supply/usb纮SUBSYSTEM=power_supply纮POWER_SUPPLY_NAME=usb纮POWER_SUPPLY_ONLINE=0纮SEQNUM=1486纮

上面这块来自网上，这段内容是否有问题，待考究。

下面看这个函数：

int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,

char *envp_ext[])

{

struct kobj_uevent_env *env;

const char *action_string = kobject_actions[action];//获取object的动作

//蓝色为为方便看代码加入的

static const char *kobject_actions[] = {

[KOBJ_ADD] = "add",

[KOBJ_REMOVE] = "remove",

[KOBJ_CHANGE] = "change",

[KOBJ_MOVE] = "move",

[KOBJ_ONLINE] = "online",

[KOBJ_OFFLINE] = "offline",

};

//以上为kobject标准的动作，调用时需要传入相应的enum值

const char *devpath = NULL;

const char *subsystem;

struct kobject *top_kobj;

struct kset *kset;

const struct kset_uevent_ops *uevent_ops;

u64 seq;

;

;

#ifdef CONFIG_NET

struct uevent_sock *ue_sk;

#endif

pr_debug("kobject: '%s' (%p): %s\n",

kobject_name(kobj), kobj, __func__);

=========================================================

这段代码用来查找该kobject所属于的kset，得到 uevent_ops

top_kobj = kobj;

while (!top_kobj->kset && top_kobj->parent)

top_kobj = top_kobj->parent;

if (!top_kobj->kset) {

pr_debug("kobject: '%s' (%p): %s: attempted to send uevent "

"without kset!\n", kobject_name(kobj), kobj,

__func__);

return -EINVAL;

}

kset = top_kobj->kset;

uevent_ops = kset->uevent_ops;

=========================================================

if (kobj->uevent_suppress) {

pr_debug("kobject: '%s' (%p): %s: uevent_suppress "

"caused the event to drop!\n",

kobject_name(kobj), kobj, __func__);

;

}

if (uevent_ops && uevent_ops->filter)

if (!uevent_ops->filter(kset, kobj)) {

pr_debug("kobject: '%s' (%p): %s: filter function "

"caused the event to drop!\n",

kobject_name(kobj), kobj, __func__);

;

}

====================================================

获取subsystem信息

if (uevent_ops && uevent_ops->name)

subsystem = uevent_ops->name(kset, kobj);

else

subsystem = kobject_name(&kset->kobj);

if (!subsystem) {

pr_debug("kobject: '%s' (%p): %s: unset subsystem caused the "

"event to drop!\n", kobject_name(kobj), kobj,

__func__);

;

}

=========================================================

env = kzalloc(sizeof(struct kobj_uevent_env), GFP_KERNEL);

if (!env)

return -ENOMEM;

devpath = kobject_get_path(kobj, GFP_KERNEL);//获取kobject的设备路径

if (!devpath) {

retval = -ENOENT;

goto exit;

}

//下面准备要传递的信息数据

retval = add_uevent_var(env, "ACTION=%s", action_string);

if (retval)

goto exit;

retval = add_uevent_var(env, "DEVPATH=%s", devpath);

if (retval)

goto exit;

retval = add_uevent_var(env, "SUBSYSTEM=%s", subsystem);

if (retval)

goto exit;

//envp_ext[i]是传进来的参数，为该event时携带的一些自定义的信息

if (envp_ext) {

; envp_ext[i]; i++) {

retval = add_uevent_var(env, "%s", envp_ext[i]);

if (retval)

goto exit;

}

}

if (uevent_ops && uevent_ops->uevent) {

retval = uevent_ops->uevent(kset, kobj, env);

if (retval) {

pr_debug("kobject: '%s' (%p): %s: uevent() returned "

"%d\n", kobject_name(kobj), kobj,

__func__, retval);

goto exit;

}

}

if (action == KOBJ_ADD)

kobj->state_add_uevent_sent = ;

else if (action == KOBJ_REMOVE)

kobj->state_remove_uevent_sent = ;

//加入该event的序号

spin_lock(&sequence_lock);

seq = ++uevent_seqnum;

spin_unlock(&sequence_lock);

retval = add_uevent_var(env, "SEQNUM=%llu", (unsigned long long)seq);

if (retval)

goto exit;

#if defined(CONFIG_NET)

//下面通过网络socket将数据发送出去

mutex_lock(&uevent_sock_mutex);

list_for_each_entry(ue_sk, &uevent_sock_list, list) {

struct sock *uevent_sock = ue_sk->sk;

struct sk_buff *skb;

size_t len;

len = strlen(action_string) + strlen(devpath) + ;

skb = alloc_skb(len + env->buflen, GFP_KERNEL);//申请网络skb数据

if (skb) {

char *scratch;

scratch = skb_put(skb, len);

sprintf(scratch, "%s@%s", action_string, devpath);

此时scratch中就增加了change@/devices/platform/msm-battery/power_supply/usb的

//组长网络skb数据结构

; i < env->envp_idx; i++) {

len = strlen(env->envp[i]) + ;

scratch = skb_put(skb, len);

strcpy(scratch, env->envp[i]);

}

NETLINK_CB(skb).dst_group = ;//下面开始发送数据

retval = netlink_broadcast_filtered(uevent_sock, skb,

, , GFP_KERNEL,

kobj_bcast_filter,

kobj);

if (retval == -ENOBUFS || retval == -ESRCH)

retval = ;

} else

retval = -ENOMEM;

}

mutex_unlock(&uevent_sock_mutex);

#endif

] && !kobj_usermode_filter(kobj)) {

];

argv [] = uevent_helper;

argv [] = (char *)subsystem;

argv [] = NULL;

retval = add_uevent_var(env, "HOME=/");

if (retval)

goto exit;

retval = add_uevent_var(env,

"PATH=/sbin:/bin:/usr/sbin:/usr/bin");

if (retval)

goto exit;

retval = call_usermodehelper(argv[], argv,

env->envp, UMH_WAIT_EXEC);

}

exit:

kfree(devpath);

kfree(env);

return retval;

}

二 android 侧

private final UEventObserver mUEventObserver = new UEventObserver() {

@Override

public void onUEvent(UEventObserver.UEvent event) {

if (DEBUG) Slog.v(TAG, "USB UEVENT: " + event.toString());

String state = event.get("USB_STATE");

String accessory = event.get("ACCESSORY");

//Added for USB Develpment debug, more log for more debuging help

if(DEBUG) Log.w(TAG, "mUEventObserver: onUEvent: state = " + state);

//Added for USB Develpment debug, more log for more debuging help

if (state != null) {

mHandler.updateState(state);

} else if ("START".equals(accessory)) {

if (DEBUG) Slog.d(TAG, "got accessory start");

setCurrentFunction(UsbManager.USB_FUNCTION_ACCESSORY, false);

}

}

};

在类初始化时会调用下面的动作，启动监听动作。

mUEventObserver.startObserving(USB_STATE_MATCH);

最终会调用到UEventObserver的addObserver：

private ArrayList<Object> mObservers = new ArrayList<Object>();

public void addObserver(String match, UEventObserver observer) {

synchronized(mObservers) {

mObservers.add(match);

mObservers.add(observer);

}

}

private static final String USB_STATE_MATCH =

"DEVPATH=/devices/virtual/android_usb/android0";

该函数最终会将”DEVPATH=/devices/virtual/android_usb/android0”增加到匹配序列中，当kernel发送具有该字符串的数据时，就返回匹配成功，然后调用mUEventObserver 的onUEvent函数；

UeventObserver.java

private static class UEventThread extends Thread {

private ArrayList<Object> mObservers = new ArrayList<Object>();

UEventThread() {

super("UEventObserver");

}

public void run() {

native_setup();
];

int len;
while (true) {
len = next_event(buffer);
) {
String bufferStr = , len); // easier to search a String
synchronized (mObservers) {
; i < mObservers.size(); i += ) {
) {
((UEventObserver)mObservers.))
.onUEvent(new UEvent(bufferStr));
}
}
}
}
}
}