Android的usb gadget配置流程

对于Android系统，和应用，要改变usb gadget是通过配置如下两个prop属性：
persist.sys.usb.config
sys.usb.config

配置“sys.usb.config”之后，系统会执行.rc脚本里面对应的规则，来改变kernel的usb configs对应节点的值，进而控制usb gadget驱动切换到对应的device，例如：

on property:sys.usb.ffs.ready=1 && property:sys.usb.config=adb && property:sys.usb.configfs=1write /config/usb_gadget/g1/configs/b.1/strings/0x409/configuration "adb"symlink /config/usb_gadget/g1/functions/ffs.adb /config/usb_gadget/g1/configs/b.1/f1write /config/usb_gadget/g1/UDC ${sys.usb.controller}setprop sys.usb.state ${sys.usb.config}

开机过程中usb gadget配置

开机过程直到开机完成，这一阶段，有两个地方去配置“sys.usb.config”：一是在.rc脚本里面，在“on boot”或者其他阶段，去执行了“setprop”；另外是在“UsbDeviceManager.java”文件里，“MSG_BOOT_COMPLETED”开机完成消息的时候。

第一种：rc脚本

一般系统都会设置一个“persist.sys.usb.config”的默认配置，比如在文件：
device/mediatek/mt6739/device.mk

ifeq ($(TARGET_BUILD_VARIANT),user)PRODUCT_DEFAULT_PROPERTY_OVERRIDES += persist.sys.usb.config=mtp
elsePRODUCT_DEFAULT_PROPERTY_OVERRIDES += persist.sys.usb.config=adb
endif

区分user和eng模式，设置不同的默认配置，然后用如下python脚本：
build/make/tools/post_process_props.py

# Put the modifications that you need to make into the /system/etc/prop.default into this
# function. The prop object has get(name) and put(name,value) methods.
def mangle_default_prop(prop):# If ro.debuggable is 1, then enable adb on USB by default# (this is for userdebug builds)if prop.get("ro.debuggable") == "1":val = prop.get("persist.sys.usb.config")if "adb" not in val:if val == "":val = "adb"else:val = val + ",adb"prop.put("persist.sys.usb.config", val)# UsbDeviceManager expects a value here.  If it doesn't get it, it will# default to "adb". That might not the right policy there, but it's better# to be explicit.if not prop.get("persist.sys.usb.config"):prop.put("persist.sys.usb.config", "none");

将默认“persist.sys.usb.config”写入out目录输出文件：
/system/etc/prop.default

这个“persist.sys.usb.config”属性起作用是在脚本文件：
system/core/rootdir/init.usb.rc（也有可能在源码device目录下的.rc脚本里）

# Used to set USB configuration at boot and to switch the configuration
# when changing the default configuration
on boot && property:persist.sys.usb.config=*setprop sys.usb.config ${persist.sys.usb.config}

可以看到，最终还是执行了“setprop sys.usb.config”。

这样开机的时候，就会设置usb gadget为“persist.sys.usb.config”默认的配置，我们也可手动添加config，实现想要的device，例如：

on bootsetprop sys.usb.config mtp,adb

第二种：UsbDeviceManager.java

这个文件目录：
frameworks/base/services/usb/java/com/android/server/usb/UsbDeviceManager.java

    /*** The persistent property which stores whether adb is enabled or not.* May also contain vendor-specific default functions for testing purposes.*/private static final String USB_PERSISTENT_CONFIG_PROPERTY = "persist.sys.usb.config";/*** The non-persistent property which stores the current USB settings.*/private static final String USB_CONFIG_PROPERTY = "sys.usb.config";

主要的逻辑都在：private final class UsbHandler extends Handler {}
这个私有类里面。其构造方法如下：

        //这是私有类“UsbHandler”的构造方法public UsbHandler(Looper looper) {super(looper);try {// Restore default functions.//获取到当前“sys.usb.config”的配置if (isNormalBoot()) {mCurrentFunctions = SystemProperties.get(USB_CONFIG_PROPERTY,UsbManager.USB_FUNCTION_NONE);mCurrentFunctionsApplied = mCurrentFunctions.equals(SystemProperties.get(USB_STATE_PROPERTY));} else {mCurrentFunctions = SystemProperties.get(getPersistProp(true),UsbManager.USB_FUNCTION_NONE);mCurrentFunctionsApplied = SystemProperties.get(USB_CONFIG_PROPERTY,UsbManager.USB_FUNCTION_NONE).equals(SystemProperties.get(USB_STATE_PROPERTY));}/** Use the normal bootmode persistent prop to maintain state of adb across* all boot modes.*///根据“persist.sys.usb.config”的配置是否包含ADB，来记录是否使能ADBmAdbEnabled = UsbManager.containsFunction(SystemProperties.get(USB_PERSISTENT_CONFIG_PROPERTY),UsbManager.USB_FUNCTION_ADB);/** Previous versions can set persist config to mtp/ptp but it does not* get reset on OTA. Reset the property here instead.*///如果“persist.sys.usb.config”里面包含“mtp/ptp”，则去除之后，重新设置“persist.sys.usb.config”String persisted = SystemProperties.get(USB_PERSISTENT_CONFIG_PROPERTY);if (UsbManager.containsFunction(persisted, UsbManager.USB_FUNCTION_MTP)|| UsbManager.containsFunction(persisted, UsbManager.USB_FUNCTION_PTP)) {SystemProperties.set(USB_PERSISTENT_CONFIG_PROPERTY,UsbManager.removeFunction(UsbManager.removeFunction(persisted,UsbManager.USB_FUNCTION_MTP), UsbManager.USB_FUNCTION_PTP));}String state = FileUtils.readTextFile(new File(STATE_PATH), 0, null).trim();updateState(state);// register observer to listen for settings changesmContentResolver.registerContentObserver(Settings.Global.getUriFor(Settings.Global.ADB_ENABLED),false, new AdbSettingsObserver());// Watch for USB configuration changesmUEventObserver.startObserving(USB_STATE_MATCH);mUEventObserver.startObserving(ACCESSORY_START_MATCH);} catch (Exception e) {Slog.e(TAG, "Error initializing UsbHandler", e);}}

其他一些主要的方法有：
1.private void setUsbConfig(String config) {}
设置“sys.usb.config”属性用的方法，等同于“setprop”命令

        private void setUsbConfig(String config) {if (DEBUG) Slog.d(TAG, "setUsbConfig(" + config + ")");// set the new configuration// we always set it due to b/23631400, where adbd was getting killed// and not restarted due to property timeouts on some devicesSystemProperties.set(USB_CONFIG_PROPERTY, config);}

2.private String applyAdbFunction(String functions) {}
根据变量“mAdbEnabled”来决定，给传入的参数增加还是去除ADB配置

        private String applyAdbFunction(String functions) {// Do not pass null pointer to the UsbManager.// There isnt a check there.if (functions == null) {functions = "";}if (mAdbEnabled) {functions = UsbManager.addFunction(functions, UsbManager.USB_FUNCTION_ADB);} else {functions = UsbManager.removeFunction(functions, UsbManager.USB_FUNCTION_ADB);}return functions;}

3.private void setAdbEnabled(boolean enable) {}
控制ADB功能的使能和关闭

        private void setAdbEnabled(boolean enable) {if (DEBUG) Slog.d(TAG, "setAdbEnabled: " + enable);if (enable != mAdbEnabled) {mAdbEnabled = enable;String oldFunctions = mCurrentFunctions;//重新设置“persist.sys.usb.config”属性// Persist the adb settingString newFunction = applyAdbFunction(SystemProperties.get(USB_PERSISTENT_CONFIG_PROPERTY, UsbManager.USB_FUNCTION_NONE));SystemProperties.set(USB_PERSISTENT_CONFIG_PROPERTY, newFunction);// Remove mtp from the config if file transfer is not enabledif (oldFunctions.equals(UsbManager.USB_FUNCTION_MTP) &&!mUsbDataUnlocked && enable) {oldFunctions = UsbManager.USB_FUNCTION_NONE;}setEnabledFunctions(oldFunctions, true, mUsbDataUnlocked);updateAdbNotification(false);}if (mDebuggingManager != null) {mDebuggingManager.setAdbEnabled(mAdbEnabled);}}

4.private String getDefaultFunctions() {}
获取系统默认的配置

        private String getDefaultFunctions() {String func = SystemProperties.get(getPersistProp(true),UsbManager.USB_FUNCTION_NONE);// if ADB is enabled, reset functions to ADB// else enable MTP as usual.if (UsbManager.containsFunction(func, UsbManager.USB_FUNCTION_ADB)) {return UsbManager.USB_FUNCTION_ADB;} else {return UsbManager.USB_FUNCTION_MTP;}}

5.private void setEnabledFunctions(String functions, boolean forceRestart,boolean usbDataUnlocked) {}
判断并设置系统需要的配置，设置的动作调用的方法“trySetEnabledFunctions”

        private void setEnabledFunctions(String functions, boolean forceRestart,boolean usbDataUnlocked) {if (DEBUG) {Slog.d(TAG, "setEnabledFunctions functions=" + functions + ", "+ "forceRestart=" + forceRestart + ", usbDataUnlocked=" + usbDataUnlocked);}if (usbDataUnlocked != mUsbDataUnlocked) {mUsbDataUnlocked = usbDataUnlocked;updateUsbNotification(false);forceRestart = true;}// Try to set the enabled functions.final String oldFunctions = mCurrentFunctions;final boolean oldFunctionsApplied = mCurrentFunctionsApplied;if (trySetEnabledFunctions(functions, forceRestart)) {return;}......}

6.private boolean trySetEnabledFunctions(String functions, boolean forceRestart) {}
有上面“setEnabledFunctions”方法调用，执行最终的设置动作

        private boolean trySetEnabledFunctions(String functions, boolean forceRestart) {//对参数“functions”做判断，不符合条件的话，使用默认配置if (functions == null || applyAdbFunction(functions).equals(UsbManager.USB_FUNCTION_NONE)) {functions = getDefaultFunctions();}//判断是否需要ADBfunctions = applyAdbFunction(functions);String oemFunctions = applyOemOverrideFunction(functions);if (!isNormalBoot() && !mCurrentFunctions.equals(functions)) {SystemProperties.set(getPersistProp(true), functions);}if ((!functions.equals(oemFunctions) &&(mCurrentOemFunctions == null ||!mCurrentOemFunctions.equals(oemFunctions)))|| !mCurrentFunctions.equals(functions)|| !mCurrentFunctionsApplied|| forceRestart) {Slog.i(TAG, "Setting USB config to " + functions);mCurrentFunctions = functions;mCurrentOemFunctions = oemFunctions;mCurrentFunctionsApplied = false;// Kick the USB stack to close existing connections.//先关闭usb gadgetsetUsbConfig(UsbManager.USB_FUNCTION_NONE);if (!waitForState(UsbManager.USB_FUNCTION_NONE)) {Slog.e(TAG, "Failed to kick USB config");return false;}// Set the new USB configuration.//再设置需要的配置setUsbConfig(oemFunctions);if (mBootCompleted&& (UsbManager.containsFunction(functions, UsbManager.USB_FUNCTION_MTP)|| UsbManager.containsFunction(functions, UsbManager.USB_FUNCTION_PTP))) {// Start up dependent services.updateUsbStateBroadcastIfNeeded(true);}if (!waitForState(oemFunctions)) {Slog.e(TAG, "Failed to switch USB config to " + functions);return false;}mCurrentFunctionsApplied = true;}return true;}

7.public void handleMessage(Message msg) {}
消息处理方法，开机完成的“MSG_BOOT_COMPLETED”消息就是在这里处理

        @Overridepublic void handleMessage(Message msg) {switch (msg.what) {......case MSG_BOOT_COMPLETED:mBootCompleted = true;if (mPendingBootBroadcast) {updateUsbStateBroadcastIfNeeded(false);mPendingBootBroadcast = false;}//可以在这里重新设置usb gadget//例如：null --> SystemProperties.get(USB_PERSISTENT_CONFIG_PROPERTY)setEnabledFunctions(null, false, false);if (mCurrentAccessory != null) {getCurrentSettings().accessoryAttached(mCurrentAccessory);}if (mDebuggingManager != null) {mDebuggingManager.setAdbEnabled(mAdbEnabled);}break;......}}

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