转自：http://blog.csdn.net/tankai19880619/article/details/11726371
说明：本文基于Android2.3和Linux2.6，其余版本仅供参考。
一、devfs、udev和sysfs是什么关系？
linux2.6之前使用devfs设备文件系统，它存在与内核空间；
linux2.6之后使用udev设备文件系统，它存在与用户空间、但严重依赖与sysfs文件系统。
二、Android(使用linux2.6以后的设备节点创建策略)设备节点的创建
在Android中，没有独立的类似于udev或者mdev的用户程序，这个功能集成到了init中做了。代码见：system/core/init/init.c文件，如下：
- if (ufds[0].revents == POLLIN)
- handle_device_fd(device_fd);
其中handle_device_fd(device_fd)函数在system/core/init/devices.c中实现，参数device_fd 由函数device_init（）->open_uevent_socket()->socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT)函数调用返回。
函数handle_device_fd(device_fd)中，根据传进来的device_fd参数，调用recv(fd, msg, UEVENT_MSG_LEN, 0)函数，将内核探测到的设备并通过NETLINK机制传过来的socket描述符转化成消息。接着调用parse_event(msg, &uevent);函数将消息翻译成uevent事件，并将改事件传递给handle_device_event（&uevent）函数。
handle_device_event（&uevent）函数中，依据参数uevent->subsystem类型创建dev下的相应目录，如：/dev/graphics。紧接着根据uevent->action是"add"还是"remove"来实现设备节点的创建与删除。如果uevent->action是"add"，则调用make_device(devpath, block, uevent->major, uevent->minor)函数生成设备节点。如果uevent->action是"remove"，则调用unlink(devpath)对设备节点进行删除。
三、下边看看Android的ueventd进程
说明：其配置文件uevent.rc不起实际作用，只是对创建的设备节点文件属性做设置。
1.system/core/init/ueventd.c
- int ueventd_main(int argc, char **argv){
- device_init();
- /*
- system/core/init/device.c
- void device_init(void){
- device_fd = uevent_open_socket(64*1024, true);
- coldboot("/sys/class");
- coldboot("/sys/block");
- coldboot("/sys/devices");
- }
- */
- ufd.events = POLLIN;
- ufd.fd = get_device_fd();
- while(1) {
- ufd.revents = 0;
- nr = poll(&ufd, 1, -1);
- if (nr <= 0)
- continue;
- if (ufd.revents == POLLIN)
- handle_device_fd();
- }
- }
2.system/core/init/device.c
注意：只有主次设备号的事件才会创建设备节点
- void handle_device_fd(){
- while ((n = uevent_kernel_multicast_recv(device_fd, msg, UEVENT_MSG_LEN)) > 0) {
- handle_device_event(&uevent);
- }
- }
- static void handle_device_event(struct uevent *uevent){
- if (!strcmp(uevent->action,"add"))
- fixup_sys_perms(uevent->path);
-
- /* if it's not a /dev device, nothing else to do */
- if((uevent->major < 0) || (uevent->minor < 0)){//只有主次设备号的事件才会创建设备节点
- //ERROR("TK------------>>>>device.c>>>>>>handle_device_event>>>>>>uevent->major and minor < 0\n");//add by tankai
- return;
- }
- if(!strncmp(uevent->subsystem, "block", 5)) {
- block = 1;
- base = "/dev/block/";
- mkdir(base, 0755);
- if (!strncmp(uevent->path, "/devices/platform/", 18))
- links = parse_platform_block_device(uevent);
- } else {
- block = 0;
- /* this should probably be configurable somehow */
- if (!strncmp(uevent->subsystem, "usb", 3)) {
- if (!strcmp(uevent->subsystem, "usb")) {
- /* This imitates the file system that would be created
- * if we were using devfs instead.
- * Minors are broken up into groups of 128, starting at "001"
- */
- int bus_id = uevent->minor / 128 + 1;
- int device_id = uevent->minor % 128 + 1;
- /* build directories */
- mkdir("/dev/bus", 0755);
- mkdir("/dev/bus/usb", 0755);
- snprintf(devpath, sizeof(devpath), "/dev/bus/usb/%03d", bus_id);
- mkdir(devpath, 0755);
- snprintf(devpath, sizeof(devpath), "/dev/bus/usb/%03d/%03d", bus_id, device_id);
- devpath_ready = 1;
- } else {
- /* ignore other USB events */
- return;
- }
- } else if (!strncmp(uevent->subsystem, "graphics", 8)) {
- base = "/dev/graphics/";
- mkdir(base, 0755);
- } else if (!strncmp(uevent->subsystem, "oncrpc", 6)) {
- base = "/dev/oncrpc/";
- mkdir(base, 0755);
- } else if (!strncmp(uevent->subsystem, "adsp", 4)) {
- base = "/dev/adsp/";
- mkdir(base, 0755);
- } else if (!strncmp(uevent->subsystem, "msm_camera", 10)) {
- base = "/dev/msm_camera/";
- mkdir(base, 0755);
- } else if(!strncmp(uevent->subsystem, "input", 5)) {
- base = "/dev/input/";
- mkdir(base, 0755);
- } else if(!strncmp(uevent->subsystem, "mtd", 3)) {
- base = "/dev/mtd/";
- mkdir(base, 0755);
- } else if(!strncmp(uevent->subsystem, "sound", 5)) {
- base = "/dev/snd/";
- mkdir(base, 0755);
- } else if(!strncmp(uevent->subsystem, "misc", 4) &&
- !strncmp(name, "log_", 4)) {
- base = "/dev/log/";
- mkdir(base, 0755);
- name += 4;
- } else
- base = "/dev/";
- }
-
- if (!devpath_ready)
- snprintf(devpath, sizeof(devpath), "%s%s", base, name);
-
- if(!strcmp(uevent->action, "add")) {
- //ERROR("TK------_>>>>>>>>>make_devices\n");//add by tankai
- make_device(devpath, uevent->path, block, uevent->major, uevent->minor);
- if (links) {
- for (i = 0; links[i]; i++)
- make_link(devpath, links[i]);
- }
- }
- }
- static void make_device(const char *path,
- const char *upath,
- int block, int major, int minor)
- {
- mode = get_device_perm(path, &uid, &gid) | (block ? S_IFBLK : S_IFCHR);
- setegid(gid);
- mknod(path, mode, dev);
- chown(path, uid, -1);
- setegid(AID_ROOT);
- }
