Android U 盘功能实现和分析

u 盘功能实现结果：

u 盘会当成 usb storage 在 Settings Storage 里面显示。

准备工作

内核需支持 usb host，需支持 FUSE 文件系统，在 FS 选项里面配置即可（CONFIG_FUSE_FS=y）。

Android 的配置

配置 init..rc

    mkdir /storage/udisk0 0000 system system    mkdir /mnt/media_rw/udisk0 0700 media_rw media_rw

service fuse_udisk0 /system/bin/sdcard -u 1023 -g 1023 -w 1023 -d /mnt/media_rw/udisk0 /storage/udisk0    class late_start    disabled

配置 fstab.

/block/sda      /mnt/media_rw/udisk0  vfat  defaults  voldmanaged=udisk0:auto

配置 overlay

挂载过程：

/dev/block/vold/8:1 /mnt/media_rw/udisk0 vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0/dev/fuse /storage/udisk0 fuse rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0

root@android:/ # logcat -s Vold MountService--------- beginning of /dev/log/main--------- beginning of /dev/log/systemI/Vold    (  820): Vold 2.1 (the revenge) firing upD/Vold    (  820): Volume sdcard1 state changing -1 (Initializing) -> 0 (No-Media)D/Vold    (  820): Volume sdcard1 state changing -1 (Initializing) -> 0 (No-Media)D/Vold    (  820): Volume udisk0 state changing -1 (Initializing) -> 0 (No-Media)D/Vold    (  820): Volume udisk0 state changing 0 (No-Media) -> 2 (Pending)D/Vold    (  820): Volume udisk0 state changing 2 (Pending) -> 1 (Idle-Unmounted)D/Vold    (  820): Volume sdcard1 state changing 0 (No-Media) -> 2 (Pending)D/Vold    (  820): Volume sdcard1 state changing 2 (Pending) -> 1 (Idle-Unmounted)D/MountService( 1587): got storage path: /storage/sdcard0 description: Internal storage primary: true removable: false emulated: true mtpReserve: 50 allowMassStorage: false maxFileSize: 0D/MountService( 1587): addVolumeLocked() StorageVolume:D/MountService( 1587):     mStorageId=65537 mPath=/storage/emulated/0 mDescriptionId=17040662 D/MountService( 1587):     mPrimary=true mRemovable=false mEmulated=true mMtpReserveSpace=50 D/MountService( 1587):     mAllowMassStorage=false mMaxFileSize=0 mOwner=UserHandle{0} mUuid=null D/MountService( 1587):     mUserLabel=null mState=null D/MountService( 1587): got storage path: /storage/sdcard1 description: SD card primary: false removable: true emulated: false mtpReserve: 0 allowMassStorage: false maxFileSize: 4294967296D/MountService( 1587): addVolumeLocked() StorageVolume:D/MountService( 1587):     mStorageId=0 mPath=/storage/sdcard1 mDescriptionId=17040663 mPrimary=false D/MountService( 1587):     mRemovable=true mEmulated=false mMtpReserveSpace=0 mAllowMassStorage=false D/MountService( 1587):     mMaxFileSize=4294967296 mOwner=null mUuid=null mUserLabel=null mState=null D/MountService( 1587): got storage path: /storage/udisk0 description: USB storage primary: false removable: true emulated: false mtpReserve: 0 allowMassStorage: false maxFileSize: 0D/MountService( 1587): addVolumeLocked() StorageVolume:D/MountService( 1587):     mStorageId=0 mPath=/storage/udisk0 mDescriptionId=17040664 mPrimary=false D/MountService( 1587):     mRemovable=true mEmulated=false mMtpReserveSpace=0 mAllowMassStorage=false D/MountService( 1587):     mMaxFileSize=0 mOwner=null mUuid=null mUserLabel=null mState=null W/MountService( 1587): Duplicate state transition (unmounted -> unmounted) for /storage/sdcard1D/MountService( 1587): volume state changed for /storage/sdcard1 (unmounted -> removed)W/MountService( 1587): Duplicate state transition (unmounted -> unmounted) for /storage/udisk0I/Vold    (  820): /dev/block/vold/8:1 being considered for volume udisk0D/Vold    (  820): Volume udisk0 state changing 1 (Idle-Unmounted) -> 3 (Checking)D/MountService( 1587): volume state changed for /storage/udisk0 (unmounted -> checking)D/MountService( 1587): sendStorageIntent Intent { act=android.intent.action.MEDIA_CHECKING dat=file:///storage/udisk0 (has extras) } to UserHandle{-1}I/Vold    (  820): Filesystem check completed OKD/Vold    (  820): blkid identified as /dev/block/vold/8:1: UUID="402E-0EE3" TYPE="vfat" D/Vold    (  820): Volume udisk0 state changing 3 (Checking) -> 4 (Mounted)D/MountService( 1587): volume state changed for /storage/udisk0 (checking -> mounted)D/MountService( 1587): sendStorageIntent Intent { act=android.intent.action.MEDIA_MOUNTED dat=file:///storage/udisk0 (has extras) } to UserHandle{-1}W/MountService( 1587): Duplicate state transition (mounted -> mounted) for /storage/emulated/0

Vold 解析 fstab 文件

在 Android 4.4 中， Vold 会解析 vold. 文件

   property_get("ro.hardware", propbuf, "");    snprintf(fstab_filename, sizeof(fstab_filename), FSTAB_PREFIX"%s", propbuf);    fstab = fs_mgr_read_fstab(fstab_filename);    if (!fstab) {        SLOGE("failed to open %s\n", fstab_filename);        return -1;    }

其中 #define FSTAB_PREFIX "/fstab." 所以，配置的 fatab 文件必须放在根目录， Vold 是 android 的后台进程，将会一直监听 fatab 里面标记为 voldmanaged 的项目如果没有标记，则略过。没有标记的项目。可以交给 init 的 mount 命令处理，但是不会监听，所以你可以将recovery 和 cache 目录这样只需启动是挂载的目录项放在 fatab 里面交给 init 的 mount 命令处理，例如三星 device/samsung/manta/fstab.manta

# Android fstab file.#                                                                                      # The filesystem that contains the filesystem checker binary (typically /system) cannot# specify MF_CHECK, and must come before any filesystems that do specify MF_CHECK/dev/block/platform/dw_mmc.0/by-name/system     /system             ext4      ro                            wait/dev/block/platform/dw_mmc.0/by-name/cache      /cache              ext4      noatime,nosuid,nodev,nomblk_io_submit,errors=panic    wait,check/dev/block/platform/dw_mmc.0/by-name/userdata   /data               ext4      noatime,nosuid,nodev,nomblk_io_submit,errors=panic    wait,check,encryptable=/dev/block/platform/dw_mmc.0/by-name/metadata/dev/block/platform/dw_mmc.0/by-name/efs        /factory            ext4      noatime,nosuid,nodev,ro                            wait/dev/block/platform/dw_mmc.0/by-name/boot       /boot               emmc      defaults                            defaults/dev/block/platform/dw_mmc.0/by-name/recovery   /recovery           emmc      defaults                            defaults/dev/block/platform/dw_mmc.0/by-name/misc       /misc               emmc      defaults                            defaults/dev/block/mmcblk0boot0                         /bootloader         emmc      defaults                            defaults

在系统启动调用 mount ：

on fs    mkdir /factory 0775 radio radio    mount_all /fstab.manta

同理， init 的 mount 也会忽略标记为 voldmanaged 的项目 init 的 mount 命令与 vold 的 mount 管理大有不同，两者不能混用。

MountService 与 Vold 联合挂载 u 盘

MountService 与 Vold 会建立 socket 通信，具体的挂载操作都是下发命令，交给 Vold 去完成。 MountService 会解析我们配置的 storage_list.xml

   private void readStorageListLocked() {        mVolumes.clear();        mVolumeStates.clear();        Resources resources = mContext.getResources();        int id = com.android.internal.R.xml.storage_list;        XmlResourceParser parser = resources.getXml(id);        AttributeSet attrs = Xml.asAttributeSet(parser);        try {            XmlUtils.beginDocument(parser, TAG_STORAGE_LIST);            while (true) {                XmlUtils.nextElement(parser);

Vold 获取到 list 里面的设备变化时将会发送 state change event， MountService 在 onEvent 处理各种不同 state ，其中获取到设备插入，将会执行挂载操作：

            if (code == VoldResponseCode.VolumeDiskInserted) {                new Thread("MountService#VolumeDiskInserted") {                    @Override                    public void run() {                        try {                            int rc;                            if ((rc = doMountVolume(path)) != StorageResultCode.OperationSucceeded) {                                Slog.w(TAG, String.format("Insertion mount failed (%d)", rc));                            }                        } catch (Exception ex) {                            Slog.w(TAG, "Failed to mount media on insertion", ex);                        }                    }                }.start();

doMountVolume 只是发送 mount 命令给 Vold

   private int doMountVolume(String path) {        int rc = StorageResultCode.OperationSucceeded;        final StorageVolume volume;        synchronized (mVolumesLock) {            volume = mVolumesByPath.get(path);        }        if (DEBUG_EVENTS) Slog.i(TAG, "doMountVolume: Mouting " + path);        try {            mConnector.execute("volume", "mount", path);

最终 Vold 里面执行相应的 mount 操作，我们的 u 盘是 vfat 格式，我只测试了这个格式， Linux 下的格式没有测试，因为 Vold 只支持 fat 和 ext4

int VolumeManager::mountVolume(const char *label) {    Volume *v = lookupVolume(label);    if (!v) {        errno = ENOENT;        return -1;    }    return v->mountVol();}

       if (Fat::doMount(devicePath, getMountpoint(), false, false, false,                AID_MEDIA_RW, AID_MEDIA_RW, 0007, true)) {            SLOGE("%s failed to mount via VFAT (%s)\n", devicePath, strerror(errno));            continue;

Android 4.4 KitKat 支持 u 盘功能