Android如何生成设备节点

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在Android中,由于没有mdev和udev,所以它没有办法动态的生成设备节点,那么它是如何做的呢?

我们可以在system/core/init/下的init.c和devices.c中找到答案:

init.c中

  1. int main(int argc, char **argv) 
  2.     ... 
  3.  
  4.         /* Get the basic filesystem setup we need put 
  5.          * together in the initramdisk on / and then we'll 
  6.          * let the rc file figure out the rest. 
  7.          */ 
  8.     mkdir("/dev", 0755); 
  9.     mkdir("/proc", 0755); 
  10.     mkdir("/sys", 0755); 
  11.  
  12.     mount("tmpfs""/dev""tmpfs", 0, "mode=0755"); 
  13.     mkdir("/dev/pts", 0755); 
  14.     mkdir("/dev/socket", 0755); 
  15.     mount("devpts""/dev/pts""devpts", 0, NULL); 
  16.     mount("proc""/proc""proc", 0, NULL); 
  17.     mount("sysfs""/sys""sysfs", 0, NULL); 
  18.  
  19.    for(;;) { 
  20.        ... 
  21.         if (ufds[0].revents == POLLIN) 
  22.             handle_device_fd(device_fd); 
  23.  
  24.         if (ufds[1].revents == POLLIN) 
  25.             handle_property_set_fd(property_set_fd); 
  26.         if (ufds[3].revents == POLLIN) 
  27.             handle_keychord(keychord_fd); 
  28.     } 
  29.  
  30.     return 0; 

我们再来看看handle_device_fd(),该函数定义在devices.c中

  1. void handle_device_fd(int fd) 
  2.         ... 
  3.         handle_device_event(&uevent); 
  4.         handle_firmware_event(&uevent); 
  5.     } 

而handle_device_event定义如下:

  1. static void handle_device_event(struct uevent *uevent) 
  2.     ... 
  3.     if(!strcmp(uevent->action, "add")) { 
  4.         make_device(devpath, block, uevent->major, uevent->minor); 
  5.         return
  6.     } 
  7.     ... 

make_device定义如下:

  1. static void make_device(const char *path, int block, int major, int minor) 
  2.     ... 
  3.     mode = get_device_perm(path, &uid, &gid) | (block ? S_IFBLK : S_IFCHR); 
  4.     dev = (major << 8) | minor; 
  5.     ... 
  6.     setegid(gid); 
  7.     mknod(path, mode, dev); 
  8.     chown(path, uid, -1); 
  9.     setegid(AID_ROOT); 

我们看看get_device_perm如下实现:

  1. static mode_t get_device_perm(const char *path, unsigned *uid, unsigned *gid) 
  2.     mode_t perm; 
  3.  
  4.     if (get_device_perm_inner(qemu_perms, path, uid, gid, &perm) == 0) { 
  5.         return perm; 
  6.     } else if (get_device_perm_inner(devperms, path, uid, gid, &perm) == 0) { 
  7.         return perm; 
  8.     } else { 
  9.         struct listnode *node; 
  10.         struct perm_node *perm_node; 
  11.         struct perms_ *dp; 
  12.  
  13.         /* Check partners list. */ 
  14.         list_for_each(node, &devperms_partners) { 
  15.             perm_node = node_to_item(node, struct perm_node, plist); 
  16.             dp = &perm_node->dp; 
  17.  
  18.             if (dp->prefix) { 
  19.                 if (strncmp(path, dp->name, strlen(dp->name))) 
  20.                     continue
  21.             } else { 
  22.                 if (strcmp(path, dp->name)) 
  23.                     continue
  24.             } 
  25.             /* Found perm in partner list. */ 
  26.             *uid = dp->uid; 
  27.             *gid = dp->gid; 
  28.             return dp->perm; 
  29.         } 
  30.         /* Default if nothing found. */ 
  31.         *uid = 0; 
  32.         *gid = 0; 
  33.         return 0600; 
  34.     } 

我们最后可以看到在devperms中定义了要生成的设备节点:

  1. static struct perms_ devperms[] = { 
  2.     { "/dev/null",          0666,   AID_ROOT,       AID_ROOT,       0 }, 
  3.     { "/dev/zero",          0666,   AID_ROOT,       AID_ROOT,       0 }, 
  4.     { "/dev/full",          0666,   AID_ROOT,       AID_ROOT,       0 }, 
  5.     { "/dev/ptmx",          0666,   AID_ROOT,       AID_ROOT,       0 }, 
  6.     { "/dev/tty",           0666,   AID_ROOT,       AID_ROOT,       0 }, 
  7.     { "/dev/random",        0666,   AID_ROOT,       AID_ROOT,       0 }, 
  8.     { "/dev/urandom",       0666,   AID_ROOT,       AID_ROOT,       0 }, 
  9.     { "/dev/ashmem",        0666,   AID_ROOT,       AID_ROOT,       0 }, 
  10.     { "/dev/binder",        0666,   AID_ROOT,       AID_ROOT,       0 }, 
  11.  
  12.         /* logger should be world writable (for logging) but not readable */ 
  13.     { "/dev/log/",          0662,   AID_ROOT,       AID_LOG,        1 }, 
  14.  
  15.     /* the msm hw3d client device node is world writable/readable. */ 
  16.     { "/dev/msm_hw3dc",     0666,   AID_ROOT,       AID_ROOT,       0 }, 
  17.  
  18.     /* gpu driver for adreno200 is globally accessible */ 
  19.     { "/dev/kgsl",          0666,   AID_ROOT,       AID_ROOT,       0 }, 
  20.  
  21.         /* these should not be world writable */ 
  22.     { "/dev/diag",          0660,   AID_RADIO,      AID_RADIO,        0 }, 
  23.     { "/dev/diag_arm9",     0660,   AID_RADIO,      AID_RADIO,        0 }, 
  24.     { "/dev/android_adb",   0660,   AID_ADB,        AID_ADB,        0 }, 
  25.     { "/dev/android_adb_enable",   0660,   AID_ADB,        AID_ADB,        0 }, 
  26.     { "/dev/ttyMSM0",       0600,   AID_BLUETOOTH,  AID_BLUETOOTH,  0 }, 
  27.     { "/dev/ttyHS0",        0600,   AID_BLUETOOTH,  AID_BLUETOOTH,  0 }, 
  28.     { "/dev/uinput",        0660,   AID_SYSTEM,     AID_BLUETOOTH,  0 }, 
  29.     { "/dev/alarm",         0664,   AID_SYSTEM,     AID_RADIO,      0 }, 
  30.     { "/dev/tty0",          0660,   AID_ROOT,       AID_SYSTEM,     0 }, 
  31.     { "/dev/graphics/",     0660,   AID_ROOT,       AID_GRAPHICS,   1 }, 
  32.     { "/dev/msm_hw3dm",     0660,   AID_SYSTEM,     AID_GRAPHICS,   0 }, 
  33.     { "/dev/input/",        0660,   AID_ROOT,       AID_INPUT,      1 }, 
  34.     { "/dev/eac",           0660,   AID_ROOT,       AID_AUDIO,      0 }, 
  35.     { "/dev/cam",           0660,   AID_ROOT,       AID_CAMERA,     0 }, 
  36.     { "/dev/pmem",          0660,   AID_SYSTEM,     AID_GRAPHICS,   0 }, 
  37.     { "/dev/pmem_adsp",     0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  38.     { "/dev/pmem_camera",   0660,   AID_SYSTEM,     AID_CAMERA,     1 }, 
  39.     { "/dev/oncrpc/",       0660,   AID_ROOT,       AID_SYSTEM,     1 }, 
  40.     { "/dev/adsp/",         0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  41.     { "/dev/snd/",          0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  42.     { "/dev/mt9t013",       0660,   AID_SYSTEM,     AID_SYSTEM,     0 }, 
  43.     { "/dev/msm_camera/",   0660,   AID_SYSTEM,     AID_SYSTEM,     1 }, 
  44.     { "/dev/akm8976_daemon",0640,   AID_COMPASS,    AID_SYSTEM,     0 }, 
  45.     { "/dev/akm8976_aot",   0640,   AID_COMPASS,    AID_SYSTEM,     0 }, 
  46.     { "/dev/akm8973_daemon",0640,   AID_COMPASS,    AID_SYSTEM,     0 }, 
  47.     { "/dev/akm8973_aot",   0640,   AID_COMPASS,    AID_SYSTEM,     0 }, 
  48.     { "/dev/bma150",        0640,   AID_COMPASS,    AID_SYSTEM,     0 }, 
  49.     { "/dev/cm3602",        0640,   AID_COMPASS,    AID_SYSTEM,     0 }, 
  50.     { "/dev/akm8976_pffd",  0640,   AID_COMPASS,    AID_SYSTEM,     0 }, 
  51.     { "/dev/lightsensor",   0640,   AID_SYSTEM,     AID_SYSTEM,     0 }, 
  52.     { "/dev/msm_pcm_out",   0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  53.     { "/dev/msm_pcm_in",    0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  54.     { "/dev/msm_pcm_ctl",   0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  55.     { "/dev/msm_snd",       0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  56.     { "/dev/msm_mp3",       0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  57.     { "/dev/audience_a1026", 0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  58.     { "/dev/tpa2018d1",     0660,   AID_SYSTEM,     AID_AUDIO,      1 }, 
  59.     { "/dev/msm_audpre",    0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  60.     { "/dev/msm_audio_ctl", 0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  61.     { "/dev/htc-acoustic",  0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  62.     { "/dev/vdec",          0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  63.     { "/dev/q6venc",        0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  64.     { "/dev/snd/dsp",       0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  65.     { "/dev/snd/dsp1",      0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  66.     { "/dev/snd/mixer",     0660,   AID_SYSTEM,     AID_AUDIO,      0 }, 
  67.     { "/dev/smd0",          0640,   AID_RADIO,      AID_RADIO,      0 }, 
  68.     { "/dev/qemu_trace",    0666,   AID_SYSTEM,     AID_SYSTEM,     0 }, 
  69.     { "/dev/qmi",           0640,   AID_RADIO,      AID_RADIO,      0 }, 
  70.     { "/dev/qmi0",          0640,   AID_RADIO,      AID_RADIO,      0 }, 
  71.     { "/dev/qmi1",          0640,   AID_RADIO,      AID_RADIO,      0 }, 
  72.     { "/dev/qmi2",          0640,   AID_RADIO,      AID_RADIO,      0 }, 
  73.         /* CDMA radio interface MUX */ 
  74.     { "/dev/ts0710mux",     0640,   AID_RADIO,      AID_RADIO,      1 }, 
  75.     { "/dev/ppp",           0660,   AID_RADIO,      AID_VPN,        0 }, 
  76.     { "/dev/tun",           0640,   AID_VPN,        AID_VPN,        0 }, 
  77.     { NULL, 0, 0, 0, 0 }, 
  78. }; 

 

 

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。

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