QCRIL send RIL_REQUEST_ALLOW_DATA to QMI
QMI Request and Response and Indication
Call Manager(Phone Object) Indicate PM（Policy Manager）about new DDS and active Subs,After evaluate configuration item in PM, then processes system selection preferences in CMPH(call manager phone object)
During step 3, we got CM phone event--CM_PH_EVENT_SYS_SEL_PREF CM
After DDS switch success, Low layer indicate UP layer, and then UP layer(UI telephony)tell qcril to stop the old network interface and start a new network interface to trigger Rmnet call on the target DDS sub, refer to those below QMI message
How does Data Service module Action?
How does NAS action? See LTE OTA LOG give you an overview
结论：
UI触发DDS切换，qcril处理Allow data请求，把DDS设置到主卡或者副卡上去，这是AP侧的底层处理。BP侧，Qcril通过QMI message 请求Call Manager完成Dual standby preference的设置，此时system selection preference发生变化，这促使Multi-mode stack controller维护的协议栈状态以及与协议栈与sub的映射关系发生变化。
切换DDS后，需要重新建立数据通路。即有后面的stop/start network interface的过程以及LTE detach/attach 信令交互过程。
Data serevice过滤关键字：DSSNET6_LINK_DOWN_EV|CM_CALL_END_CMD|rmnet call|Net interface is
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