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Android 4.4 KitKat 支持 u 盘功能
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.<board>.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.<board>
/block/sda /mnt/media_rw/udisk0 vfat defaults voldmanaged=udisk0:auto
配置 overlay
<storage android:mountPoint="/storage/udisk0" android:storageDescription="@string/storage_usb" android:primary="false" android:removable="true" />
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/system I/Vold ( 820): Vold 2.1 (the revenge) firing up D/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: 0 D/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: 4294967296 D/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: 0 D/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/sdcard1 D/MountService( 1587): volume state changed for /storage/sdcard1 (unmounted -> removed) W/MountService( 1587): Duplicate state transition (unmounted -> unmounted) for /storage/udisk0 I/Vold ( 820): /dev/block/vold/8:1 being considered for volume udisk0 D/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 OK D/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.<hardware> 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. #<src> <mnt_point> <type> <mnt_flags and opt ions> <fs_mgr_flags> # 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,nombl k_io_submit,errors=panic wait,check /dev/block/platform/dw_mmc.0/by-name/userdata /data ext4 noatime,nosuid,nodev,nombl k_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;
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