What do you mean by mDataConnectionReason=dataAttached? What do you mean by the output that we obtain on running telephony registry using adb?

mDataConnectionReason=dataAttached
mDataConnectionReason stores the last connection/disconnection reason of the mobile data network. The values can be found in com.android.internal.telephony.Phone.java.

0159     /**
0160      * Optional reasons for disconnect and connect
0161      */
0162     static final String REASON_ROAMING_ON = "roamingOn";
0163     static final String REASON_ROAMING_OFF = "roamingOff";
0164     static final String REASON_DATA_DISABLED = "dataDisabled";
0165     static final String REASON_DATA_ENABLED = "dataEnabled";
0166     static final String REASON_DATA_ATTACHED = "dataAttached";
0167     static final String REASON_DATA_DETACHED = "dataDetached";
0168     static final String REASON_CDMA_DATA_ATTACHED = "cdmaDataAttached";
0169     static final String REASON_CDMA_DATA_DETACHED = "cdmaDataDetached";
0170     static final String REASON_APN_CHANGED = "apnChanged";
0171     static final String REASON_APN_SWITCHED = "apnSwitched";
0172     static final String REASON_APN_FAILED = "apnFailed";
0173     static final String REASON_RESTORE_DEFAULT_APN = "restoreDefaultApn";
0174     static final String REASON_RADIO_TURNED_OFF = "radioTurnedOff";
0175     static final String REASON_PDP_RESET = "pdpReset";
0176     static final String REASON_VOICE_CALL_ENDED = "2GVoiceCallEnded";
0177     static final String REASON_VOICE_CALL_STARTED = "2GVoiceCallStarted";
0178     static final String REASON_PS_RESTRICT_ENABLED = "psRestrictEnabled";
0179     static final String REASON_PS_RESTRICT_DISABLED = "psRestrictDisabled";
0180     static final String REASON_SIM_LOADED = "simLoaded";
0181     static final String REASON_NW_TYPE_CHANGED = "nwTypeChanged";
0182     static final String REASON_DATA_DEPENDENCY_MET = "dependencyMet";
0183     static final String REASON_DATA_DEPENDENCY_UNMET = "dependencyUnmet";
0184     static final String REASON_LINK_PROPERTIES_CHANGED = "linkPropertiesChanged";


As far as I can tell, dataAttached just means that a data connection has been enabled and the connection has been established correctly.