示例

比如插拔sim卡会打印如下信息

[90133.351745] report_usim_event: usim uevent [USIM_NAME=usim0 USIM_EVENT=plugout] is sent
[90133.380554] USIM_NAME=usim0, USIM_EVENT=plugout
[90133.380645] serial_atcmd at+cfun=5[90176.496063] report_usim_event: usim uevent [USIM_NAME=usim0 USIM_EVENT=plugin] is sent
[90176.524383] USIM_NAME=usim0, USIM_EVENT=plugin
[90176.533599] CUR CFUN = 5
[90176.533691] serial_atcmd at+cfun=0
[90181.583862] serial_atcmd at+cfun=1

这是热插拔触后，触发的事件给应用层，然后执行的脚本，跟上面的信息是对上的

#!/bin/sh#usim0_state=`cat /sys/class/usim_event/usim0/state`
usim_name=`echo $USIM_NAME`
usim_event=`echo $USIM_EVENT`if [ -d /sys/devices/virtual/usim_event/usim0 -o -d /sys/devices/virtual/usim_event/usim1 ]; thenecho "USIM_NAME=$usim_name, USIM_EVENT=$usim_event" > /dev/kmsgif [ "$usim_event" == "plugin" ]; thencur_cfun=`serial_atcmd "AT+CFUN?" | grep CFUN | awk -F " " '{print $2}'`echo "CUR CFUN =" $cur_cfun > /dev/kmsgif [ "$cur_cfun" != "0" ]; thenecho "serial_atcmd at+cfun=0" > /dev/kmsgserial_atcmd at+cfun=0sleep 5fiecho "serial_atcmd at+cfun=1" > /dev/kmsgserial_atcmd at+cfun=1ubus send "usim.event" '{"status":1}'elif [ "$usim_event" == "plugout" ]; thenecho "serial_atcmd at+cfun=5" > /dev/kmsgserial_atcmd at+cfun=5ubus call network.interface.wan0 removeubus call network.interface.wan60 removeubus send "usim.event" '{"status":0}'fi
elif [ -d /sys/devices/virtual/usim_event/usimtray ]; thenecho "USIM_NAME=$usim_name, USIM_EVENT=$usim_event" > /dev/kmsg
fi

上报事件的函数如下，这个可以是中断脚来检测，或者上层通过attr属性主动调用触发

static void report_usim_event(struct usim_event_device *uedev, int state)
{char name_buf[50];char *env[3];snprintf(name_buf, sizeof(name_buf), "USIM_NAME=%s", uedev->name);env[0] = name_buf;if (strcmp("usimtray", uedev->name) == 0)env[1] = state ? "USIM_EVENT=trayPlugin" :"USIM_EVENT=trayPlugout";elseenv[1] = state ? "USIM_EVENT=plugin" : "USIM_EVENT=plugout";env[2] = NULL;kobject_uevent_env(&uedev->dev->kobj, KOBJ_CHANGE, env);usim_event_debug(GE_DEBUG_INFO, "%s: usim uevent [%s %s] is sent\n",__func__, env[0], env[1]);
}

.config

uevent相关配置如下

CONFIG_NET=y
CONFIG_UEVENT_HELPER=y
CONFIG_UEVENT_HELPER_PATH=""

uevent把事件上报给用户空间的两种途径：根据上述配置来选择用那种
1.通过kmod模块，直接调用用户空间的可执行程序或脚本。
2..通过netlink通信机制，将事件从内核空间传递到用户空间。

定义了CONFIG_UEVENT_HELPER和CONFIG_UEVENT_HELPER_PATH不为空，就会掉用户空间的程序

/*** 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;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;/* 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;}/* 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;}/* 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;}/* 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;}/* 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;}}/* 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;}}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;}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;}retval = kobject_uevent_net_broadcast(kobj, env, action_string,devpath);mutex_unlock(&uevent_sock_mutex);#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;
}

定义了CONFIG_NET就会走netlink

static int kobject_uevent_net_broadcast(struct kobject *kobj,struct kobj_uevent_env *env,const char *action_string,const char *devpath)
{int ret = 0;#ifdef CONFIG_NETconst struct kobj_ns_type_operations *ops;const struct net *net = NULL;ops = kobj_ns_ops(kobj);if (!ops && kobj->kset) {struct kobject *ksobj = &kobj->kset->kobj;if (ksobj->parent != NULL)ops = kobj_ns_ops(ksobj->parent);}/* kobjects currently only carry network namespace tags and they* are the only tag relevant here since we want to decide which* network namespaces to broadcast the uevent into.*/if (ops && ops->netlink_ns && kobj->ktype->namespace)if (ops->type == KOBJ_NS_TYPE_NET)net = kobj->ktype->namespace(kobj);if (!net)ret = uevent_net_broadcast_untagged(env, action_string,devpath);elseret = uevent_net_broadcast_tagged(net->uevent_sock->sk, env,action_string, devpath);
#endifreturn ret;
}

netlink

netlink就是socket的一种表现方式，在内核中通过NETLINK_KOBJECT_UEVENT参数，用netlink_kernel_create来创建一个netlink的socket

/lib/kobject_uevent.c
static int uevent_net_init(struct net *net)
{struct uevent_sock *ue_sk;struct netlink_kernel_cfg cfg = {.groups	= 1,.input = uevent_net_rcv,.flags	= NL_CFG_F_NONROOT_RECV};ue_sk = kzalloc(sizeof(*ue_sk), GFP_KERNEL);if (!ue_sk)return -ENOMEM;ue_sk->sk = netlink_kernel_create(net, NETLINK_KOBJECT_UEVENT, &cfg);if (!ue_sk->sk) {pr_err("kobject_uevent: unable to create netlink socket!\n");kfree(ue_sk);return -ENODEV;}net->uevent_sock = ue_sk;/* Restrict uevents to initial user namespace. */if (sock_net(ue_sk->sk)->user_ns == &init_user_ns) {mutex_lock(&uevent_sock_mutex);list_add_tail(&ue_sk->list, &uevent_sock_list);mutex_unlock(&uevent_sock_mutex);}return 0;
}static void uevent_net_exit(struct net *net)
{struct uevent_sock *ue_sk = net->uevent_sock;if (sock_net(ue_sk->sk)->user_ns == &init_user_ns) {mutex_lock(&uevent_sock_mutex);list_del(&ue_sk->list);mutex_unlock(&uevent_sock_mutex);}netlink_kernel_release(ue_sk->sk);kfree(ue_sk);
}static struct pernet_operations uevent_net_ops = {.init	= uevent_net_init,.exit	= uevent_net_exit,
};static int __init kobject_uevent_init(void)
{return register_pernet_subsys(&uevent_net_ops);
}postcore_initcall(kobject_uevent_init);

在用户层，通过一个进程来创建一个netlink，这个应用程序可以是mdev,udev或者openwrt的procd,这三种方式都是基于netlink来管理热插拔的，比如procd通过NETLINK_KOBJECT_UEVENT参数来通过socket系统调用创建一个fd来与内核的netlink交互数据，内核上报uevent被这个进程接受后，然后调用/etc/hotplug.d/的各脚本来执行热插拔操作

void hotplug(char *rules)
{struct sockaddr_nl nls = {};int nlbufsize = 512 * 1024;rule_file = strdup(rules);nls.nl_family = AF_NETLINK;nls.nl_pid = 0;nls.nl_groups = -1;if ((hotplug_fd.fd = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC, NETLINK_KOBJECT_UEVENT)) == -1) {ERROR("Failed to open hotplug socket: %m\n");exit(1);}if (bind(hotplug_fd.fd, (void *)&nls, sizeof(struct sockaddr_nl))) {ERROR("Failed to bind hotplug socket: %m\n");exit(1);}if (setsockopt(hotplug_fd.fd, SOL_SOCKET, SO_RCVBUFFORCE, &nlbufsize, sizeof(nlbufsize)))ERROR("Failed to resize receive buffer: %m\n");json_script_init(&jctx);queue_proc.cb = queue_proc_cb;uloop_fd_add(&hotplug_fd, ULOOP_READ);
}

 当device_add添加设备时会调用kobject_uevent(&dev->kobj, KOBJ_ADD);来上报事件给procd的netlink,然后根据是否有主从设备号等信息，来创建设备节点文件

static void handle_makedev(struct blob_attr *msg, struct blob_attr *data)
{unsigned int oldumask = umask(0);static struct blobmsg_policy mkdev_policy[3] = {{ .type = BLOBMSG_TYPE_STRING },{ .type = BLOBMSG_TYPE_STRING },{ .type = BLOBMSG_TYPE_STRING },};struct blob_attr *tb[3];char *minor = hotplug_msg_find_var(msg, "MINOR");char *major = hotplug_msg_find_var(msg, "MAJOR");char *subsystem = hotplug_msg_find_var(msg, "SUBSYSTEM");blobmsg_parse_array(mkdev_policy, 3, tb, blobmsg_data(data), blobmsg_data_len(data));if (tb[0] && tb[1] && minor && major && subsystem) {mode_t m = S_IFCHR;char *d = strdup(blobmsg_get_string(tb[0]));d = dirname(d);mkdir_p(d, 0755);free(d);if (!strcmp(subsystem, "block"))m = S_IFBLK;mknod(blobmsg_get_string(tb[0]),m | strtoul(blobmsg_data(tb[1]), NULL, 8),makedev(atoi(major), atoi(minor)));if (tb[2])chgrp_target(tb[2], tb[0]);}umask(oldumask);
}

或者收到sim的切换事件后，调用最上面的位于/etc/hotplug.d里的sim热插拔脚本，来执行对应的行为