其实所谓的开关，就是对底层文件节点的设置。

android\system\netd\server\CommandListener.cpp

int CommandListener::InterfaceCmd::restartModem() {
    const char* WM_AW4G = "/sys/kernel/ril_api/ril";
    const char* WM_DT4G = "/sys/devices/platform/datang_power/datang";
    char MODEM_PATH[512] = {0};

    // for example:
    // echo 0 > /sys/kernel/ril_api/ril
    // sleep little time
    // echo 1 > /sys/kernel/ril_api/ril
    if (!access(WM_AW4G, F_OK)) {
        strcpy(MODEM_PATH, WM_AW4G);
        ALOGI("Use WM_AW4G, kernel node: %s", MODEM_PATH);
    } else if (!access(WM_DT4G, F_OK)) {
        strcpy(MODEM_PATH, WM_DT4G);
        ALOGI("Use WM_DT4G, kernel node: %s", MODEM_PATH);
    } else {
        ALOGE("Can't find modem kernel node,so can't restart modem");
        return -1;
    }

    ALOGI("Now restart modem");
    int fd = -1;

    fd = open(MODEM_PATH, O_WRONLY);
    if (fd < 0) {
        ALOGE("Failed to open %s: %s", MODEM_PATH, strerror(errno));
        return -1;
    }
    if (write(fd, "0", 1) != 1) {
        ALOGE("Failed to write 0 to %s: %s", MODEM_PATH, strerror(errno));
        close(fd);
        return -1;
    }

    // os said need sleep 1s
    usleep(500*1000);
    usleep(500*1000);

    if (write(fd, "1", 1) != 1) {
        ALOGE("Failed to write 1 to %s: %s", MODEM_PATH, strerror(errno));
        close(fd);
        return -1;
    }

    if (fd > 0) close(fd);
    ALOGI("Successful to write restart to %s", MODEM_PATH);
    return 0;
}

CommandListener::InterfaceCmd::runCommand

{

//    0     1     2
+        // interface restart  modem
+        if ((argc == 3) && !strcmp(argv[1], "restart") && !strcmp(argv[2], "modem")) {
+            if (restartModem()) {
+                cli->sendMsg(ResponseCode::CommandOkay,
+                    "restartModem() successful", false);
+            } else {
+                cli->sendMsg(ResponseCode::OperationFailed,
+                    "restartModem() failed", false);
+            }
+            return 0;
+        }

}

有了底层的修改，就可以通过上层的NetworkManagementService.java中通过接收广播，或者通过逻辑继续控制。

核心代码： mConnector.execute("interface", "restart", "modem");