Linux设备模型(五) - uevent kernel实现

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_ENVP            64    /* number of env pointers */
#define UEVENT_BUFFER_SIZE        2048    /* 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buffer
    int 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uvent
    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
};
 
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

4.1 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。

/**
* 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);
    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;
    }
 
    //* 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uevent
    retval = 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 */
        }
    }
#endif
 
exit:
    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

4.2 kobject_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);

4.3 add_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->buf
    va_start(args, format);
    len = vsnprintf(&env->buf[env->buflen],
            sizeof(env->buf) - env->buflen,
            format, args);
    va_end(args);
 
    //检查buffer size
    if (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 pair
    env->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);

4.4 kobject_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