mdev机制（二）————在Linux内核中的实现

class_device_create
class_device_register
class_device_add
kobject_uevent(&class_dev->kobj, KOBJ_ADD);
kobject_uevent_env(kobj, action, NULL);

// action_string = "add";
action_string = action_to_string(action);

/* 分配保存环境变量的内存 */
/* environment values */
buffer = kmalloc(BUFFER_SIZE, GFP_KERNEL);

/* 设置环境变量 */
envp [i++] = scratch;
scratch += sprintf(scratch, "ACTION=%s", action_string) + 1;
envp [i++] = scratch;
scratch += sprintf (scratch, "DEVPATH=%s", devpath) + 1;
envp [i++] = scratch;
scratch += sprintf(scratch, "SUBSYSTEM=%s", subsystem) + 1;

/* 调用应用程序: 比如mdev */
/* 启动脚本 echo /sbin/mdev > /proc/sys/kernel/hotplug
* 设置了uevent_helper为“/sbin/mdev“
*/

argv [0] = uevent_helper;  // = "/sbin/mdev"
argv [1] = (char *)subsystem;
argv [2] = NULL;
call_usermodehelper (argv[0], argv, envp, 0);

分析: busybox mdev.c

100ask: uevent_helper = /sbin/mdev
envp[0] = HOME=/
envp[1] = PATH=/sbin:/bin:/usr/sbin:/usr/bin
envp[2] = ACTION=add
envp[3] = DEVPATH=/class/sixth_drv/buttons
envp[4] = SUBSYSTEM=sixth_drv
envp[5] = SEQNUM=720
envp[6] = MAJOR=252
envp[7] = MINOR=0

mdev_main
temp = /sys/class/sixth_drv/buttons
make_device(temp, 0);
/* 确定设备文件名,类型,主次设备号 */
device_name = bb_basename(path);  = "buttons"

'c' == > 字符设备节点
根据"/sys/class/sixth_drv/buttons/dev"的内容确定主次设备号

mknod(device_name, mode | type, makedev(major, minor)