设备模型的uevent机制

linux-3.5/include/linux/kobject.h

//    ADD/REMOVE，Kobject（或上层数据结构）的添加/移除事件。
//    ONLINE/OFFLINE，Kobject（或上层数据结构）的上线/下线事件，其实是是否使能。
//    CHANGE，Kobject（或上层数据结构）的状态或者内容发生改变。
//    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）。
//CHANGE，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。
enum kobject_action {
    KOBJ_ADD,
    KOBJ_REMOVE,
    KOBJ_CHANGE,
    KOBJ_MOVE, 
    KOBJ_ONLINE,
    KOBJ_OFFLINE,
    KOBJ_MAX
};

#define UEVENT_HELPER_PATH_LEN      256
#define UEVENT_NUM_ENVP         32  /* number of env pointers */
#define UEVENT_BUFFER_SIZE      2048    /* buffer for the variables */

//在利用kmod模块向用户空间上报event事件时，会直接执行用户空间的可执行文件。而在linux系统中，可执行文件的执行，依赖于环境变量，
//因此kobj_uevent_env用于组织此次事件上报是的环境变量。
struct kobj_uevent_env {
    char *envp[UEVENT_NUM_ENVP];//指针数组，用于保存每个环境变量的地址，最多支持32个环境变量
    int envp_idx;//用户访问环境变量数组的索引
    char buf[UEVENT_BUFFER_SIZE];//保存环境变量的buffer
    int buflen;//???
};

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filter借口过滤，阻止不希望上报的uevent。
    const char *(* const name)(struct kset *kset, struct kobject *kobj);//该接口可以返回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uevent
    int (* 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。
              //因为很多时候上报uevent时的环境变量都是相同的，因此可以由kset统一处理，就不需要让每个Kobject独自添加了。
};

#if defined(CONFIG_HOTPLUG)
int kobject_uevent(struct kobject *kobj, enum kobject_action action);
int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
            char *envp[]);

__printf(2, 3)
int add_uevent_var(struct kobj_uevent_env *env, const char *format, ...);

int kobject_action_type(const char *buf, size_t count,
            enum kobject_action *type);

            
kobject_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。

int kobject_uevent(struct kobject *kobj, enum kobject_action action)
{
    return kobject_uevent_env(kobj, action, NULL);
}

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;
#ifdef CONFIG_NET
    struct uevent_sock *ue_sk;
#endif

    pr_debug("kobject: ‘%s‘ (%p): %s\n",
         kobject_name(kobj), kobj, __func__);

    /* search the kset we belong to */
    //1.查找当前kobject或其parent是否从属于某个kset;如果都不从属于某个kset，则返回错误。(说明一个kobject若没有加入kset，是不会上报uevent的)
    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*/
    //2.查看kobj->uevent_suppress是否被设置；如果设置了，则忽略所有的uevent上报，并返回0.
    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. */
    //3.如果所属的kset有uevent_ops->filter,则调用该函数，若该函数返回0，则过滤此次上报。(kset 可以通过filter接口过滤不希望上报的event)
    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 */
    //4.判断所属的kset是否有合法的名称,若uevent_ops->name存在就用其返回的名称作为subsystem;若uevent_ops->name不存在就用kset本身的kobject的名称作为subsystem;
    //若没有合法的名称，则不上报uevent
    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__);
        return 0;
    }

    /* environment buffer */
    //5.分配一个此次上报的用于保存环境变量的buffer,
    env = kzalloc(sizeof(struct kobj_uevent_env), GFP_KERNEL);
    if (!env)
        return -ENOMEM;

    /* complete object path */
    //6.获得该kobject在sysfs中路径
    devpath = kobject_get_path(kobj, GFP_KERNEL);
    if (!devpath) {
        retval = -ENOENT;
        goto exit;
    }

    /* default keys */
    //7.添加ACTION到env
    retval = add_uevent_var(env, "ACTION=%s", action_string);
    if (retval)
        goto exit;
    //8.添加DEVPATH(kobject路径信息)到env
    retval = add_uevent_var(env, "DEVPATH=%s", devpath);
    if (retval)
        goto exit;
    //9.添加SUBSYSTEM到env
    retval = add_uevent_var(env, "SUBSYSTEM=%s", subsystem);
    if (retval)
        goto exit;

    /* keys passed in from the caller */
    //10.如果传入的envp_ext不空，则解析传入的环境变量中，同样调用add_uevent_var接口，添加到env指针中
    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 */
    //11.如果 uevent_ops->uevent 存在，调用该接口，添加kset统一的环境变量到env指针
    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;
        }
    }

    /*
     * Mark "add" and "remove" events in the object to ensure proper
     * events 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.
     */
    //12.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
    if (action == KOBJ_ADD)
        kobj->state_add_uevent_sent = 1;
    else if (action == KOBJ_REMOVE)
        kobj->state_remove_uevent_sent = 1;

    mutex_lock(&uevent_sock_mutex);
    /* we will send an event, so request a new sequence number */
    //13.调用add_uevent_var接口，添加格式为"SEQNUM=%llu”的序列号
    retval = add_uevent_var(env, "SEQNUM=%llu", (unsigned long long)++uevent_seqnum);
    if (retval) {
        mutex_unlock(&uevent_sock_mutex);
        goto exit;
    }
//14.如果定义了"CONFIG_NET”，则使用netlink发送该uevent
#if defined(CONFIG_NET)
    /* send netlink message */
    list_for_each_entry(ue_sk, &uevent_sock_list, list) {
        struct sock *uevent_sock = ue_sk->sk;
        struct sk_buff *skb;
        size_t len;

        if (!netlink_has_listeners(uevent_sock, 1))
            continue;

        /* allocate message with the maximum possible size */
        len = strlen(action_string) + strlen(devpath) + 2;
        skb = alloc_skb(len + env->buflen, GFP_KERNEL);
        if (skb) {
            char *scratch;

            /* add header */
            scratch = skb_put(skb, len);
            sprintf(scratch, "%s@%s", action_string, devpath);

            /* copy keys to our continuous event payload buffer */
            for (i = 0; i < env->envp_idx; i++) {
                len = strlen(env->envp[i]) + 1;
                scratch = skb_put(skb, len);
                strcpy(scratch, env->envp[i]);
            }

            NETLINK_CB(skb).dst_group = 1;
            retval = netlink_broadcast_filtered(uevent_sock, skb,
                                0, 1, GFP_KERNEL,
                                kobj_bcast_filter,
                                kobj);
            /* ENOBUFS should be handled in userspace */
            if (retval == -ENOBUFS || retval == -ESRCH)
                retval = 0;
        } else
            retval = -ENOMEM;
    }
#endif
    mutex_unlock(&uevent_sock_mutex);

    /* call uevent_helper, usually only enabled during early boot */
    //15.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。
    if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {
        char *argv [3];

        argv [0] = uevent_helper;//在/sys/kernel/uevent_helper文件中可以存入用户空间可执行程序的路径，当内核有事件发生时，将会执行该程序
        argv [1] = (char *)subsystem;
        argv [2] = 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[0], argv,
                         env->envp, UMH_WAIT_EXEC);
    }

exit:
    kfree(devpath);
    kfree(env);
    return retval;
}