需控制Wifi的App會透過system service來呼叫到wifi service，像是在packages/apps/Settings裡的wifi設定頁面，它們會用類似這樣的語法來取得wifi system service。
mWifiManager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
本文將探討由wifi system service到hardware做動的部分。
WifiService 註冊
ServiceManager.addService() 為所有service註冊都要用到的函式。WifiService也是，程式如下。
private ConnectivityService(Context context) {
for (int netType : mPriorityList) {
switch (mNetAttributes[netType].mRadio) {
case ConnectivityManager.TYPE_WIFI:
if (DBG) log("Starting Wifi Service.");
WifiStateTracker wst = new WifiStateTracker();
WifiService wifiService = new WifiService(context);
ServiceManager.addService(Context.WIFI_SERVICE, wifiService);
wifiService.checkAndStartWifi();
mNetTrackers[ConnectivityManager.TYPE_WIFI] = wst;
wst.startMonitoring(context, mHandler);
//TODO: as part of WWS refactor, create only when needed
mWifiWatchdogService = new WifiWatchdogService(context);
break;
ConnectivityService是在SystemServer建構函式用到，至於SystemServer如何被叫到，自行去找Zygote的介紹
try {
Slog.i(TAG, "Connectivity Service");
connectivity = ConnectivityService.getInstance(context);
ServiceManager.addService(Context.CONNECTIVITY_SERVICE, connectivity);
} catch (Throwable e) {
Slog.e(TAG, "Failure starting Connectivity Service", e);
}
Service
開關Wifi是透過setWifiEnabled()，其Service內的實作程式如下
public synchronized boolean setWifiEnabled(boolean enable) {
enforceChangePermission();
if (DBG) {
Slog.e(TAG, "Invoking mWifiStateMachine.setWifiEnabled\n");
}
if (enable) {
reportStartWorkSource();
}
mWifiStateMachine.setWifiEnabled(enable);
最重要的是它用了WifiStateMachine::setWifiEnabled()，它實作如下
public void setWifiEnabled(boolean enable) {
mLastEnableUid.set(Binder.getCallingUid());
if (enable) {
/* Argument is the state that is entered prior to load */
sendMessage(obtainMessage(CMD_LOAD_DRIVER, WIFI_STATE_ENABLING, 0));
sendMessage(CMD_START_SUPPLICANT);
} else {
sendMessage(CMD_STOP_SUPPLICANT);
/* Argument is the state that is entered upon success */
sendMessage(obtainMessage(CMD_UNLOAD_DRIVER, WIFI_STATE_DISABLED, 0));
}
}
可以看到它用了很多sendMessage()，這部分用了樹狀的有限狀態機，sendMessage是拿來叫目前所處的State物件，根據sendMessage要求的狀態來呼叫processMessage()處理。WifiStateMachine的樹狀結構如下
addState(mDefaultState);
addState(mInitialState, mDefaultState);
addState(mDriverUnloadingState, mDefaultState);
addState(mDriverUnloadedState, mDefaultState);
addState(mDriverFailedState, mDriverUnloadedState);
addState(mDriverLoadingState, mDefaultState);
addState(mDriverLoadedState, mDefaultState);
addState(mSupplicantStartingState, mDefaultState);
addState(mSupplicantStartedState, mDefaultState);
addState(mDriverStartingState, mSupplicantStartedState);
addState(mDriverStartedState, mSupplicantStartedState);
addState(mScanModeState, mDriverStartedState);
addState(mConnectModeState, mDriverStartedState);
addState(mConnectingState, mConnectModeState);
addState(mConnectedState, mConnectModeState);
addState(mDisconnectingState, mConnectModeState);
addState(mDisconnectedState, mConnectModeState);
addState(mWaitForWpsCompletionState, mConnectModeState);
addState(mDriverStoppingState, mSupplicantStartedState);
addState(mDriverStoppedState, mSupplicantStartedState);
addState(mSupplicantStoppingState, mDefaultState);
addState(mSoftApStartedState, mDefaultState);
setInitialState(mInitialState);
要先確定自己目前的狀態才能知道程式會用那個State物件的processMessage()/enter()來處理。以上面的setWifiEnabled(true)為例，它會要求DriverUnloadedState::processMessage()處理CMD_LOAD_DRIVER。
class DriverUnloadedState extends HierarchicalState {
@Override
public void enter() {
if (DBG) Log.d(TAG, getName() + "\n");
EventLog.writeEvent(EVENTLOG_WIFI_STATE_CHANGED, getName());
}
@Override
public boolean processMessage(Message message) {
if (DBG) Log.d(TAG, getName() + message.toString() + "\n");
switch (message.what) {
case CMD_LOAD_DRIVER:
transitionTo(mDriverLoadingState);
break;
default:
return NOT_HANDLED;
}
EventLog.writeEvent(EVENTLOG_WIFI_EVENT_HANDLED, message.what);
return HANDLED;
}
}
DriverUnloadedState又轉換狀態到DriverLoadingState，轉過去時會先做enter() method如下
class DriverLoadingState extends HierarchicalState {
@Override
public void enter() {
if (DBG) Log.d(TAG, getName() + "\n");
EventLog.writeEvent(EVENTLOG_WIFI_STATE_CHANGED, getName());
final Message message = new Message();
message.copyFrom(getCurrentMessage());
/* TODO: add a timeout to fail when driver load is hung.
Similarly for driver unload.
*/
new Thread(new Runnable() {
public void run() {
mWakeLock.acquire();
//enabling state
switch(message.arg1) {
case WIFI_STATE_ENABLING:
setWifiState(WIFI_STATE_ENABLING);
break;
case WIFI_AP_STATE_ENABLING:
setWifiApState(WIFI_AP_STATE_ENABLING);
break;
}
if(WifiNative.loadDriver()) {
Log.d(TAG, "Driver load successful");
sendMessage(CMD_LOAD_DRIVER_SUCCESS);
在此method中呼叫了WifiNative.loadDriver()，它真正會呼叫到底層叫loading kernel module的動作。終於要進入JNI了。
public class WifiNative {
static final int BLUETOOTH_COEXISTENCE_MODE_ENABLED = 0;
static final int BLUETOOTH_COEXISTENCE_MODE_DISABLED = 1;
static final int BLUETOOTH_COEXISTENCE_MODE_SENSE = 2;
public native static String getErrorString(int errorCode);
public native static boolean loadDriver();
JNI
以上面的loadDriver()會呼叫到
static jboolean android_net_wifi_loadDriver(JNIEnv* env, jobject clazz)
{
return (jboolean)(::wifi_load_driver() == 0);
}
wifi_load_driver()則是HAL提供。
HAL
HAL該提供的函式可參考hardware/libhardware_legacy/include/hardware_legacy/wifi.h，wifi_load_driver()的實作如下，可以看到insmod的關鍵system call。
int wifi_load_driver()
{
char driver_status[PROPERTY_VALUE_MAX];
int count = 100; /* wait at most 20 seconds for completion */
if (is_wifi_driver_loaded()) {
return 0;
}
if (insmod(DRIVER_MODULE_PATH, DRIVER_MODULE_ARG) < 0)
return -1;
if (strcmp(FIRMWARE_LOADER,"") == 0) {
其它
除了loading kernel driver外，其它有關wifi操作的部分皆是透過wpa_supplicant這套library，這套library很大，所以我不介紹了，有興趣的人可以參考external/wpa_supplicant。
圖：PM在講相簿新增的功能
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