Android设备节点.doc

第1章 Android设备节点的创建在编写linux设备驱动程序的时候，很多时候都是利用mknod命令来手动创建设备节点的，带上名字和主次设备号就可以在/dev目录下生成设备节点。同样Android沿用了linux内核，很多设备驱动的节点是又是什么时候创建的呢？在kernel自解压模块加载完成之后，会去运行android第一个应用程序init。在init.c的main函数中。System/core/init/init.cint main(int argc, char *argv)action_for_each_trigger(boot, action_add_queue_tail);在init进程解析init.rc脚本完成后，在on boot的最后两句是class start main和core，其中classStart是命令，在keyword.h中定义了class_start对应的function实际就是do_class_start。System/core/init/builtins.cint do_class_start(int nargs, char *args) service_for_each_class(args1, service_start_if_not_disabled); return 0;System/core/init/init_parser.cvoid service_for_each_class(const char *classname, void (*func)(struct service *svc)list_for_each(node, &service_list) svc = node_to_item(node, struct service, slist); if (!strcmp(svc-classname, classname) func(svc); 在之前解析init.rc脚本的时候，service会被放在service_list的链表里。接下来就是要执行service_for_each_class的func(svc)，也就是service_start_if_not_disabled。System/core/init/builtins.cstatic void service_start_if_not_disabled(struct service *svc) if (!(svc-flags & SVC_DISABLED) service_start(svc, NULL); Android的service大都是编译成可执行文件以命令的格式，我们注意到在init.rc中又这么个service值得关注下。service ueventd /sbin/ueventd class core criticalAndroid的服务不是选项不是disabled并且带core和main的选项的服务都是需要开机自动加载的服务。而ueventd是由system/core/init/ueventd.c编译而成的。System/core/init/ueventd.cint ueventd_main(int argc, char *argv)ueventd_parse_config_file(/ueventd.rc);snprintf(tmp, sizeof(tmp), /ueventd.%s.rc, hardware);ueventd_parse_config_file(tmp);device_init();while(1) ufd.revents = 0; nr = poll(&ufd, 1, -1); if (nr = 0) continue; if (ufd.revents = POLLIN) handle_device_fd();Ueventd的main函数做的事情比较多，首先是要解析根文件系统下的ueventd.rc以及ueventd.$hardware.rc。System/core/init/ueventd_parser.cint ueventd_parse_config_file(const char *fn)char *data; data = read_file(fn, 0); if (!data) return -1; parse_config(fn, data); DUMP(); return 0;static void parse_config(const char *fn, char *s)state.parse_line = parse_line_device;for (;) switch (token) case T_EOF: state.parse_line(&state, 0, 0); return; case T_NEWLINE: if (nargs) tate.parse_line(&state, 0, 0);解析过程中，会去循环解析这个文件，对解析的结果调用state.parse_line，对应的回调函数就是parse_line_device。System/core/init/ueventd_parser.cstatic void parse_line_device(struct parse_state* state, int nargs, char *args) set_device_permission(nargs, args);在set_device_permission(nargs, args)中会获取设备的mode，uid，gid。给设备增加操作权限。然后通过device_init给设备添加创建NETLINK套接字。System/core/init/ueventd.cint ueventd_main(int argc, char *argv)ueventd_parse_config_file(/ueventd.rc);snprintf(tmp, sizeof(tmp), /ueventd.%s.rc, hardware);ueventd_parse_config_file(tmp);device_init();while(1) ufd.revents = 0; nr = poll(&ufd, 1, -1); if (nr 0) if(n = UEVENT_MSG_LEN) /* overflow - discard */ continue; msgn = 0; msgn+1 = 0; struct uevent uevent; parse_event(msg, &uevent); handle_device_event(&uevent); handle_firmware_event(&uevent); 接收的uevent的小心不能超过1024个字节，如果超出就算溢出将不会处理。如果接收的uevent有效，解析这个uevent会根据设备的类型来解析。之后handle_device_event会处理设备的event。而handle_firmware_event则是和某些设备需要firmware回去处理firmware的加载。System/core/init/devices.cstatic void handle_device_event(struct uevent *uevent) if (!strcmp(uevent-action,add) fixup_sys_perms(uevent-path); if (!strncmp(uevent-subsystem, block, 5) handle_block_device_event(uevent); else if (!strncmp(uevent-subsystem, platform, 8) handle_platform_device_event(uevent); else handle_generic_device_event(uevent); 如果event的subsystem为block，说明是个块设备，就会交由handle_block_device_event去处理。或者subsystem是platform设备的话，则有handle_platform_device_event来处理。如果两者都不是的话，说明是一个普通设备，handle_generic_device_event会来处理。块设备和平台设备的处理方式比较特殊，先看一般通用设备的处理过程。System/core/init/devices.cstatic void handle_generic_device_event(struct uevent *uevent)name = parse_device_name(uevent, 64);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); else /* ignore other USB events */ return; else if (!strncmp(uevent-subsystem, graphics, 8) base = /dev/graphics/; mkdir(base, 0755); else if (!strncmp(uevent-subsystem, drm, 3) base = /dev/dri/; 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/; handle_device(uevent-action, devpath, uevent-path, 0, uevent-major, uevent-minor, links);首先得到设备的名字，然后是创建一些设备的子目录，如果uevent-subsystem是usb，就需要创建一个/dev/bus/usb的目录。如果是uevent-subsystem是graphic的话，就需要创建一个/dev/graphics的目录，按照这样依次比较下去，创建设备类所需要的子目录。System/core/init/devices.cstatic void handle_device(const char *action, const char *devpath, const char *path, int block, int major, int minor, char *links) if(!strcmp(action, add) make_device(devpath, path, block, major, minor);如果uevent的action是设备添加，就会调用make_device来创建设备节点。System/core/init/devices.cstatic void make_device(const char *path, const char *upath, int block, int major, int minor)mode = get_device_perm(path, &uid, &gi