linux的udev机制

Lnux2.6内核udev机制（基于2.6.26）

作者：guolele1990                                               2011年1月12日

         最近在研究输入子系统时关于应用层与handler层时有点迷惑，查找了很多资料，发现是设备节点问题，于是看起udev（嵌入式中的是mdev）机制，下面是这个机制的分析。

首先我们知道udev或者mdev都是应用程序，他们的作用就是起自动创建节点作用，这点是必须清楚，也就是它是工作在用户空间的，但是我们设备驱动，无论是event handler层还是driver 层都是工作在内核空间的，那么内核是怎样处理的？其实内核就是利用kobject_uevent来将这件事报告给用户空间，用户空间再调用相应程序（udev、mdev）来创建设备文件（或叫设备节点）。

下面分析kobject_uevent，老风格，先上内核描述

/**

 * kobject_uevent - notify userspace by ending an uevent

 *

 * @action: action that is happening

 * @kobj: struct kobject that the action is happening to

 *

 * Returns 0 if kobject_uevent() is completed with success or the

 * corresponding error when it fails.

 */

这里就是说这函数是通过结束一个uevent来给用户空间报告，不过感觉翻译不对头，因为后面分析这不是结束这uevent，而是最终调用指定的uevent_helper或者是通过netlink发出（这是一种特殊类型的socket，专门用于内核空间与用户空间的异步通信，比较有效率，就是通过网络发送通知）去。这函数就是kobject_uevent_env的包装。

上kobject_uevent_env描述，更清楚

/**

 * kobject_uevent_env - send an uevent with environmental data

 *

 * @action: action that is happening

 * @kobj: struct kobject that the action is happening to

 * @envp_ext: pointer to environmental data

 *

 * Returns 0 if kobject_uevent() is completed with success or the

 * corresponding error when it fails.

 */

它的作用就是发送一个有参数的uevent到用户空间。

int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,

                          char *envp_ext[])

{

         ……



         /* 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;

         }

/*这里要说明一下，这个uevent_ops其实就是对应内核里的（系统一共有 block_uevent_ops  memory_uevent_ops   device_uevent_ops   class_uevent_ops   module_uevent_ops ）这几大类 kset_uevent_ops*/

//这里是为device_uevent_ops，因为现在调用的是创建设备的文件，更深一层的就是一开始device_init时定义的kset，现在我们用的都是这kset所以也就指定了这一个。

         kset = top_kobj->kset;

         uevent_ops = kset->uevent_ops;



         /* 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;

                   }

//如果定义了uevent_ops里的name就调用，因为这里有，调用的翻回名字

         /* originating subsystem */

         if (uevent_ops && uevent_ops->name)

                   subsystem = uevent_ops->name(kset, kobj);

         else

                   subsystem = kobject_name(&kset->kobj);

…….

         /* environment buffer */

         ……

//获得路径

         /* complete object path */

         devpath = kobject_get_path(kobj, GFP_KERNEL);

         if (!devpath) {

                   retval = -ENOENT;

                   goto exit;

         }

//填充参数

         /* 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, 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);

                   ……

         }



         /*

          * 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.

          */

         if (action == KOBJ_ADD)

                   kobj->state_add_uevent_sent = 1;

         else if (action == KOBJ_REMOVE)

                   kobj->state_remove_uevent_sent = 1;

//这个uevent_seqnum是会在/sys/kernel/ /uevent_sequence里显示，是表示产生·过几个uevent

         /* we will send an event, so request a new sequence number */

         spin_lock(&sequence_lock);

         seq = ++uevent_seqnum;

         spin_unlock(&sequence_lock);

         retval = add_uevent_var(env, "SEQNUM=%llu", (unsigned long long)seq);

         if (retval)

                   goto exit;



//这个在新版里如果是采用网络通报的话，应用层里就采用socket来接受这event，这样比较有效率，不用整天去那读

#if defined(CONFIG_NET)

         /* send netlink message */

         if (uevent_sock) {

                   struct sk_buff *skb;

                   size_t len;



                   /* 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;

                            netlink_broadcast(uevent_sock, skb, 0, 1, GFP_KERNEL);

                   }

         }

#endif

//这里是先判断uevent_helper是否为空，它是什么?他默认是/proc/sys/kernel/hotplug，但是我们可以通过echo /sbin/mdev > /proc/sys/kernel/hotplug来修改，这也是很多嵌入式初始化文件里要做的事

         /* call uevent_helper, usually only enabled during early boot */

         if (uevent_helper[0]) {

                   char *argv [3];



                   argv [0] = uevent_helper;

                   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;



                   call_usermodehelper(argv[0], argv, env->envp, UMH_WAIT_EXEC);

         }



exit:

         kfree(devpath);

         kfree(env);

         return retval;

}

而call_usermodehelper，里setup就是将一些信息放在struct subprocess_info结构里，方便调用call_usermodehelper_exec，关于这函数，上一描述

/**

 * call_usermodehelper_exec - start a usermode application

 * @sub_info: information about the subprocessa

 * @wait: wait for the application to finish and return status.

 *        when -1 don't wait at all, but you get no useful error back when

 *        the program couldn't be exec'ed. This makes it safe to call

 *        from interrupt context.

 *

 * Runs a user-space application.  The application is started

 * asynchronously if wait is not set, and runs as a child of keventd.

 * (ie. it runs with full root capabilities).

 */

这函数的就是调用用户模式的应用程序，也就是调用mdev或者udev.其中udev、mdev是内核通过vfork创建一个进程执行的。具体是汇编，比较难懂。

/*

          * We were successful.  We won't be returning to our caller, but

          * instead to user space by manipulating the kernel stack.

          */

asm(         "add          r0, %0, %1/n/t"

                   "mov         r1, %2/n/t"

                   "mov         r2, %3/n/t"

                   "bl    memmove/n/t"        /* copy regs to top of stack */

                   "mov         r8, #0/n/t"        /* not a syscall */

                   "mov         r9, %0/n/t"       /* thread structure */

                   "mov         sp, r0/n/t"        /* reposition stack pointer */

                   "b     ret_to_user"

                   :

                   : "r" (current_thread_info()),

                     "Ir" (THREAD_START_SP - sizeof(regs)),

                     "r" (®s),

                     "Ir" (sizeof(regs))

                   : "r0", "r1", "r2", "r3", "ip", "lr", "memory");

  

还有几个具体也不分析了，大概了解一下它的主要流程就好了。