"/dev/snd/pcmC0D0p"中的snd从何而来

由ueventd创建，代码：

系统：Android M

/*
 * "/init"可执行程序入口
 * system/core/init/init.cpp
 */
int main(int argc, char** argv) {
    if (!strcmp(basename(argv[0]), "ueventd")) {
        return ueventd_main(argc, argv);
    }
    ......
}

/*
 * system/core/init/ueventd.cpp
 */
int ueventd_main(int argc, char **argv)
{
    ......

    device_init();

    ......
}


/* system/core/init/devices.cpp */
void device_init() {
    ......
    coldboot("/sys/class");
    coldboot("/sys/block");
    coldboot("/sys/devices");
    ......
}


static void coldboot(const char *path)
{
    DIR *d = opendir(path);
    if(d) {
        do_coldboot(d);
        closedir(d);
    }
}

/* Coldboot walks parts of the /sys tree and pokes the uevent files
** to cause the kernel to regenerate device add events that happened
** before init's device manager was started
**
** We drain any pending events from the netlink socket every time
** we poke another uevent file to make sure we don't overrun the
** socket's buffer.
*/
static void do_coldboot(DIR *d)
{
    ......

    fd = openat(dfd, "uevent", O_WRONLY);
    if(fd >= 0) {
        write(fd, "add\n", 4);
        close(fd);
        handle_device_fd();
    }

    ......
}

void handle_device_fd()
{
    char msg[UEVENT_MSG_LEN+2];
    int n;
    while ((n = uevent_kernel_multicast_recv(device_fd, msg, UEVENT_MSG_LEN)) > 0) {
        if(n >= UEVENT_MSG_LEN)   /* overflow -- discard */
            continue;

        msg[n] = '\0';
        msg[n+1] = '\0';

        struct uevent uevent;
        parse_event(msg, &uevent);

        if (sehandle && selinux_status_updated() > 0) {
            struct selabel_handle *sehandle2;
            sehandle2 = selinux_android_file_context_handle();
            if (sehandle2) {
                selabel_close(sehandle);
                sehandle = sehandle2;
            }
        }

        handle_device_event(&uevent);
        handle_firmware_event(&uevent);
    }
}

static void handle_device_event(struct uevent *uevent)
{
    if (!strcmp(uevent->action,"add") || !strcmp(uevent->action, "change") || !strcmp(uevent->action, "online"))
        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);
    }
}

static void handle_generic_device_event(struct uevent *uevent)
{
    const char *base;
    const char *name;
    char devpath[DEVPATH_LEN] = {0};
    char **links = NULL;

    name = parse_device_name(uevent, 64);
    if (!name)
        return;

    struct ueventd_subsystem *subsystem =
            ueventd_subsystem_find_by_name(uevent->subsystem);

    if (subsystem) {
        const char *devname;

        switch (subsystem->devname_src) {
        case DEVNAME_UEVENT_DEVNAME:
            devname = uevent->device_name;
            break;

        case DEVNAME_UEVENT_DEVPATH:
            devname = name;
            break;

        default:
            ERROR("%s subsystem's devpath option is not set; ignoring event\n",
                    uevent->subsystem);
            return;
        }

        if (!assemble_devpath(devpath, subsystem->dirname, devname))
            return;
        mkdir_recursive_for_devpath(devpath);
    } else if (!strncmp(uevent->subsystem, "usb", 3)) {
         if (!strcmp(uevent->subsystem, "usb")) {
            if (uevent->device_name) {
                if (!assemble_devpath(devpath, "/dev", uevent->device_name))
                    return;
                mkdir_recursive_for_devpath(devpath);
             }
             else {
                 /* 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 */
                 make_dir("/dev/bus", 0755);
                 make_dir("/dev/bus/usb", 0755);
                 snprintf(devpath, sizeof(devpath), "/dev/bus/usb/%03d", bus_id);
                 make_dir(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/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "drm", 3)) {
         base = "/dev/dri/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "oncrpc", 6)) {
         base = "/dev/oncrpc/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "adsp", 4)) {
         base = "/dev/adsp/";
         make_dir(base, 0755);
     } else if (!strncmp(uevent->subsystem, "msm_camera", 10)) {
         base = "/dev/msm_camera/";
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "input", 5)) {
         base = "/dev/input/";
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "mtd", 3)) {
         base = "/dev/mtd/";
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "sound", 5)) {
         base = "/dev/snd/"; //snd在这里创建
         make_dir(base, 0755);
     } else if(!strncmp(uevent->subsystem, "misc", 4) &&
                 !strncmp(name, "log_", 4)) {
         INFO("kernel logger is deprecated\n");
         base = "/dev/log/";
         make_dir(base, 0755);
         name += 4;
     } else
         base = "/dev/";
     links = get_character_device_symlinks(uevent);

     if (!devpath[0])
         snprintf(devpath, sizeof(devpath), "%s%s", base, name);

     handle_device(uevent->action, devpath, uevent->path, 0,
             uevent->major, uevent->minor, links);
}