碰上一個寫在init.rc里面的service不能自動啟動的問題，加上class好了。
service yourservice /sbin/yourservice
class core
user adb
group adb
class <name>
Specify a class name for the service. All services in a
named class may be started or stopped together. A service
is in the class "default" if one is not specified via the
class option.
看起來是因為
All services in a named class may be started or stopped together.
而如果default的class都沒有起來的話，這個service也就不會自動啟動了
所以 class_start default. 還是有用的....
Ref:
[Android init language]
http://www.netmite.com/android/mydroid/1.6/system/core/init/readme.txt