Q：热插拔
A：一个完整的热插拔系统包括热插拔系统的硬件，支持热插拔的软件和操作系统，支持热
插拔的设备驱动程序和支持热插拔的用户接口。
- 硬件支持：插入，先数据，后电源；拔出，先电源，后数据，最重要的是检测电平变化的引脚
- uevent, user space event. 内核与用户空间的一种通信机制
http://www.wowotech.net/linux_kenrel/uevent.html 主要讲述的是头文件热插拔事件从内核空间到用户空间
http://blog.csdn.net/bingqingsuimeng/article/details/7922217 一般，并且知识体系太老了
http://blog.csdn.net/bingqingsuimeng/article/details/7924300 有源代码的讲解，比较清晰
http://www.cnblogs.com/image-eye/archive/2011/08/19/2145858.html 内核启动过程的驱动加载
http://blog.chinaunix.net/uid-25721104-id-3023525.html 讨论的关于热插拔
http://blog.csdn.net/zirconsdu/article/details/8792184 图解很清晰
讲清楚了koject_uevent_env在做了什么，如何进入用户空间：https://elixir.bootlin.com/linux/latest/source/lib/kobject_uevent.c#L456
/**
* 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);
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 */
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 */
}
}
#endif
exit:
kfree(devpath);
kfree(env);
return retval;
}
EXPORT_SYMBOL_GPL(kobject_uevent_env);
如果定义了"CONFIG_NET”，则使用netlink发送该uevent；其中的call_usermodehelper是个最终切换接口；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。
而在Linux系统，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时的环境变量。
软件支持：插入设备-> device_add-> kobject_uevent-> kobject_uevent_env（之前都是内核空间动作）
->call_usermodehelper_setup->call_usermodehelper_exec->调用用户空间应用(call_usermodehelper=setup+exec)，udevd (udev 后台程序)
1 外设插入
2 总线发现（usb中断hub_irq）新设备，调用device_add;添加新设备到设备管理器
3 device_add调用kobject_uevent(,KOBJ_ADD),向用户空间传递环境变量以及action；
netlink方式（广播方式）【主要就是Uevent】
4 用户空间应用程序收到消息【udev ->> udevd】
5 udevd根据消息和环境变量，查询/sys的变化，按照规则(/etc/udev/rules.d/*)，在/dev
目录下自动创建设备节点；
同时可以总结，设备插入系统时，相应驱动的关联情况：
① 若直接编译进内核或在启动时加载，则无需在udev中加载驱动模块，在bus_probe_device()中会为其找到相应的驱动；
② 若驱动需要动态加载，则需要在udev(目前的情况是这样，以前也有其他方式如/sbin/hotplug，cardmgr等)中，动态加载其驱动，在驱动的register函数中，找到该device进行关联；udev 是Linux kernel 2.6系列的设备管理器，它主要的功能是管理/dev目录底下的设备节点的程序。udevd是根据一定的规则在/dev/目录下生成设备文件的后台服务进程。
比如你插入一个u盘， udevd服务就会自动生成/dev/sda 这样的文件。
http://eeepage.info/sysfs/ 对新的sysfs的讲解非常到位
http://www.linuxidc.com/Linux/2012-05/60757.htm 同样讲解sysfs文件系统
http://blog.chinaunix.net/uid-24227137-id-3266449.html 讲解kobject,kset,sysfs
在此，感觉各位将资源无私放在网上的各位大神
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