1. Uevent的功能

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。用户空间程序收到这样的事件后，会做相应的处理。

该机制通常是用来支持热拔插设备的，例如U盘插入后，USB相关的驱动软件会动态创建用于表示该U盘的device结构（相应的也包括其中的kobject），并告知用户空间程序，为该U盘动态的创建/dev/目录下的设备节点，更进一步，可以通知其它的应用程序，将该U盘设备mount到系统中，从而动态的支持该设备。

2. Uevent在kernel中的位置

下面图片描述了Uevent模块在内核中的位置：

由此可知，Uevent的机制是比较简单的，设备模型中任何设备有事件需要上报时，会触发Uevent提供的接口。Uevent模块准备好上报事件的格式后，可以通过两个途径把事件上报到用户空间：一种是通过kmod模块，直接调用用户空间的可执行文件；另一种是通过netlink通信机制，将事件从内核空间传递给用户空间。

3，数据结构描述

  • kobject_action

/** The actions here must match the index to the string array* in lib/kobject_uevent.c** Do not add new actions here without checking with the driver-core* maintainers. Action strings are not meant to express subsystem* or device specific properties. In most cases you want to send a* kobject_uevent_env(kobj, KOBJ_CHANGE, env) with additional event* specific variables added to the event environment.*/enum kobject_action {KOBJ_ADD, KOBJ_REMOVE, //Kobject（或上层数据结构）的添加/移除事件KOBJ_CHANGE, //Kobject（或上层数据结构）的状态或者内容发生改变; 如果设备驱动需要上报的事件不再上面事件的范围内，或者是自定义的事件，可以使用该event，并携带相应的参数。KOBJ_MOVE, //Kobject（或上层数据结构）更改名称或者更改Parent（意味着在sysfs中更改了目录结构）KOBJ_ONLINE,KOBJ_OFFLINE, //Kobject（或上层数据结构）的上线/下线事件，其实是是否使能KOBJ_BIND,KOBJ_UNBIND,};
  • kobj_uevent_env
#define UEVENT_NUM_ENVP64/* number of env pointers */#define UEVENT_BUFFER_SIZE2048/* buffer for the variables *//* environment buffer */struct kobj_uevent_env {char *argv[3];char *envp[UEVENT_NUM_ENVP]; //环境变量的指针数组，envp指向每一个环境变量int envp_idx; //环境变量的索引char buf[UEVENT_BUFFER_SIZE]; //存储所有环境变量的bufferint buflen; //环境变量的buffer的长度};
  • kset_uevent_ops
struct kset {struct list_head list;spinlock_t list_lock;struct kobject kobj;const struct kset_uevent_ops *uevent_ops;ANDROID_KABI_RESERVE(1);ANDROID_KABI_RESERVE(2);ANDROID_KABI_RESERVE(3);ANDROID_KABI_RESERVE(4);} __randomize_layout;kset_uevent_ops 是为kset量身订做的一个数据结构，里面包含filter和uevent两个回调函数* @uevent_ops: the set of uevent operations for this kset.These are* called whenever a kobject has something happen to it so that the kset* can add new environment variables, or filter out the uevents if so* desired.struct kset_uevent_ops {int (* const filter)(struct kset *kset, struct kobject *kobj); //当任何Kobject需要上报uevent时，它所属的kset可以通过该接口过滤，阻止不希望上报的event，从而达到从整体上管理的目的const char *(* const name)(struct kset *kset, struct kobject *kobj); //接口可以返回kset的名称。如果一个kset没有合法的名称，则其下的所有Kobject将不允许上报uventint (* const uevent)(struct kset *kset, struct kobject *kobj, struct kobj_uevent_env *env); //当任何Kobject需要上报uevent时，它所属的kset可以通过该接口统一为这些event添加环境变量};eg：static const struct kset_uevent_ops bus_uevent_ops = {.filter = bus_uevent_filter,};bus_kset = kset_create_and_add("bus", &bus_uevent_ops, NULL);kset = kset_create(name, uevent_ops, parent_kobj);kset->uevent_ops = uevent_ops;kset_register(kset);kobject_uevent(&k->kobj, KOBJ_ADD);

4，常用API

4.1kobject_uevent_env

以envp为环境变量，上报一个指定action的uevent。环境变量的作用是为执行用户空间程序指定运行环境。

/*** kobject_uevent_env - send an uevent with environmental data** @kobj: struct kobject that the action is happening to* @action: action that is happening* @envp_ext: pointer to environmental data** Returns 0 if kobject_uevent_env() is completed with success or the* corresponding error when it fails.*/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];const char *devpath = NULL;const char *subsystem;struct kobject *top_kobj;struct kset *kset;const struct kset_uevent_ops *uevent_ops;int i = 0;int retval = 0;/* * Mark "remove" event done regardless of result, for some subsystems * do not want to re-trigger "remove" event via automatic cleanup. */if (action == KOBJ_REMOVE)kobj->state_remove_uevent_sent = 1;pr_debug("kobject: '%s' (%p): %s\n", kobject_name(kobj), kobj, __func__);/* search the kset we belong to */top_kobj = kobj;while (!top_kobj->kset && top_kobj->parent)top_kobj = top_kobj->parent;//查找kobj本身或者其parent是否从属于某个kset，如果不是，则报错返回（由此可以说明，如果一个kobject没有加入kset，是不允许上报uevent的）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;//查看kobj->uevent_suppress是否设置，如果设置，则忽略所有的uevent上报并返回（注3：由此可知，可以通过Kobject的uevent_suppress标志，管控Kobject的uevent的上报）/* skip the event, if uevent_suppress is set*/if (kobj->uevent_suppress) {pr_debug("kobject: '%s' (%p): %s: uevent_suppress " "caused the event to drop!\n", kobject_name(kobj), kobj, __func__);return 0;}//如果所属的kset有uevent_ops->filter函数，则调用该函数，过滤此次上报（注4：这佐证了3.2小节有关filter接口的说明，kset可以通过filter接口过滤不希望上报的event，从而达到整体的管理效果）/* skip the event, if the filter returns zero. */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__);return 0;}//判断所属的kset是否有合法的名称（称作subsystem，和前期的内核版本有区别），否则不允许上报uevent/* originating subsystem */if (uevent_ops && uevent_ops->name)subsystem = uevent_ops->name(kset, kobj);elsesubsystem = 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__);return 0;}//* 分配一个用于此次上报的、存储环境变量的buffer（结果保存在env指针中），并获得该Kobject在sysfs中路径信息（用户空间软件需要依据该路径信息在sysfs中访问它）/* environment buffer */env = kzalloc(sizeof(struct kobj_uevent_env), GFP_KERNEL);if (!env)return -ENOMEM;/* complete object path */devpath = kobject_get_path(kobj, GFP_KERNEL);if (!devpath) {retval = -ENOENT;goto exit;}//调用add_uevent_var接口（下面会介绍），将Action、路径信息、subsystem等信息，添加到env指针中/* default keys */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;/* keys passed in from the caller */if (envp_ext) {for (i = 0; envp_ext[i]; i++) {retval = add_uevent_var(env, "%s", envp_ext[i]);if (retval)goto exit;}}//如果所属的kset存在uevent_ops->uevent接口，调用该接口，添加kset统一的环境变量到env指针/* let the kset specific function add its stuff */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;}}//根据ACTION的类型，设置kobj->state_add_uevent_sent和kobj->state_remove_uevent_sent变量，以记录正确的状态switch (action) {case KOBJ_ADD:/* * Mark "add" event so we can make sure we deliver "remove" * event to userspace during automatic cleanup. If * the object did send an "add" event, "remove" will * automatically generated by the core, if not already done * by the caller. */kobj->state_add_uevent_sent = 1;break;case KOBJ_UNBIND:zap_modalias_env(env);break;default:break;}//调用add_uevent_var接口，添加格式为"SEQNUM=%llu”的序列号mutex_lock(&uevent_sock_mutex);/* we will send an event, so request a new sequence number */retval = add_uevent_var(env, "SEQNUM=%llu", ++uevent_seqnum);if (retval) {mutex_unlock(&uevent_sock_mutex);goto exit;}//如果定义了"CONFIG_NET”，则使用netlink发送该ueventretval = kobject_uevent_net_broadcast(kobj, env, action_string,devpath);mutex_unlock(&uevent_sock_mutex);//以uevent_helper、subsystem以及添加了标准环境变量（HOME=/，PATH=/sbin:/bin:/usr/sbin:/usr/bin）的env指针为参数，调用kmod模块提供的call_usermodehelper函数，上报uevent#ifdef CONFIG_UEVENT_HELPER/* call uevent_helper, usually only enabled during early boot */if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {struct subprocess_info *info;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 = init_uevent_argv(env, subsystem);if (retval)goto exit;retval = -ENOMEM;info = call_usermodehelper_setup(env->argv[0], env->argv, env->envp, GFP_KERNEL, NULL, cleanup_uevent_env, env);if (info) {retval = call_usermodehelper_exec(info, UMH_NO_WAIT);env = NULL;/* freed by cleanup_uevent_env */}}#endifexit:kfree(devpath);kfree(env);return retval;}EXPORT_SYMBOL_GPL(kobject_uevent_env);

Android在源码目录:system/extras/tests/uevents/中，可以监听底层UEvent事件上报的程序，该程序没有自动编译到系统中，需要单独编译。

编译完成后，可以编译成system/bin/uevents的可执行程序，可以通过adb shell，输入uenvets可以查看上报事件：

change@/devices/platform/soc/soc:mmi,charger/power_supply/mmi_battery ACTION=change DEVPATH=/devices/platform/soc/soc:mmi,charger/power_supply/mmi_battery SUBSYSTEM=power_supply POWER_SUPPLY_NAME=mmi_battery POWER_SUPPLY_TYPE=Mains POWER_SUPPLY_STATUS=Full POWER_SUPPLY_HEALTH=Good POWER_SUPPLY_TEMP=250 POWER_SUPPLY_CAPACITY=100 POWER_SUPPLY_CYCLE_COUNT=2 POWER_SUPPLY_CHARGE_FULL=4015000 POWER_SUPPLY_CHARGE_FULL_DESIGN=4015000 POWER_SUPPLY_VOLTAGE_NOW=4379000 POWER_SUPPLY_CURRENT_NOW=-704000 POWER_SUPPLY_CHARGE_COUNTER=4015000 SEQNUM=54900

change@/devices/platform/soc/ae00000.qcom,mdss_mdp/backlight/panel0-backlight ACTION=change DEVPATH=/devices/platform/soc/ae00000.qcom,mdss_mdp/backlight/panel0-backlight SUBSYSTEM=backlight SOURCE=sysfs SEQNUM=54903

主动向uevent节点写入add也会导致内核生成并重新发送当前注册设备的uevent消息：

console 1

/sys/bus/platform/devices/goodix_ts.0 # echo add > uevent

console 2

lynkco:/system/bin # uevents

add@/devices/platform/goodix_ts.0 ACTION=add DEVPATH=/devices/platform/goodix_ts.0 SUBSYSTEM=platform SYNTH_UUID=0 DRIVER=goodix_ts MODALIAS=platform:goodix_ts SEQNUM=55327

4.2kobject_uevent

和kobject_uevent_env功能一样，只是没有指定任何的环境变量。

/*** kobject_uevent - notify userspace by sending an uevent** @kobj: struct kobject that the action is happening to* @action: action that is happening** Returns 0 if kobject_uevent() is completed with success or the* corresponding error when it fails.*/int kobject_uevent(struct kobject *kobj, enum kobject_action action){return kobject_uevent_env(kobj, action, NULL);}EXPORT_SYMBOL_GPL(kobject_uevent);

4.3add_uevent_var

/*** add_uevent_var - add key value string to the environment buffer* @env: environment buffer structure* @format: printf format for the key=value pair** Returns 0 if environment variable was added successfully or -ENOMEM* if no space was available.*/int add_uevent_var(struct kobj_uevent_env *env, const char *format, ...){va_list args;int len;//环境变量的个数不能超过最大值if (env->envp_idx >= ARRAY_SIZE(env->envp)) {WARN(1, KERN_ERR "add_uevent_var: too many keys\n");return -ENOMEM;}//把环境变量格式化输出到 env->bufva_start(args, format);len = vsnprintf(&env->buf[env->buflen],sizeof(env->buf) - env->buflen,format, args);va_end(args);//检查buffer sizeif (len >= (sizeof(env->buf) - env->buflen)) {WARN(1, KERN_ERR "add_uevent_var: buffer size too small\n");return -ENOMEM;}//env->envp[env->envp_idx++]指向此次添加的环境变量，通过envp_idx能直接取出key=value pairenv->envp[env->envp_idx++] = &env->buf[env->buflen];//增加buflen计数，两个key=value中间有空格env->buflen += len + 1;return 0;}EXPORT_SYMBOL_GPL(add_uevent_var);

4.4kobject_action_type

将enum kobject_action类型的Action，转换为字符串。

5，API使用示例

向user space发送自定义的uevent事件。

1）env = kzalloc(sizeof(*env), GFP_KERNEL_ACCOUNT);if (!env)return;add_uevent_var(env, "CREATED=%llu", created);add_uevent_var(env, "COUNT=%llu", active);if (type == KVM_EVENT_CREATE_VM) {add_uevent_var(env, "EVENT=create");kvm->userspace_pid = task_pid_nr(current);} else if (type == KVM_EVENT_DESTROY_VM) {add_uevent_var(env, "EVENT=destroy");}add_uevent_var(env, "PID=%d", kvm->userspace_pid);if (kvm->debugfs_dentry) {char *tmp, *p = kmalloc(PATH_MAX, GFP_KERNEL_ACCOUNT);if (p) {tmp = dentry_path_raw(kvm->debugfs_dentry, p, PATH_MAX);if (!IS_ERR(tmp))add_uevent_var(env, "STATS_PATH=%s", tmp);kfree(p);}}/* no need for checks, since we are adding at most only 5 keys */env->envp[env->envp_idx++] = NULL;kobject_uevent_env(&kvm_dev.this_device->kobj, KOBJ_CHANGE, env->envp);kfree(env);2）char *envp[4] = { "FC_EVENT=nvmediscovery", hostaddr, tgtaddr, NULL };kobject_uevent_env(&fc_udev_device->kobj, KOBJ_CHANGE, envp);

参考链接：

Linux设备模型(3)_Uevent