Android实现OTG功能
一般来说，接上U盘后，Linux内核是可以识别到U盘的，但Android层需要进行挂载该U盘，才能被读取到。在init.rc中看到一个网址，于是便打开来查看，原来Android有相应的方法来挂载U盘的。摘录如下。
本文摘自https://source.android.com/devices/storage/config-example.html
Android 5.x and earlier
Physical primary only
This is a typical configuration for a device with single external storagedevice which is a physical SD card, like Nexus One.
The raw physical device must first be mounted under/mnt/media_rw where only the system and FUSE daemon can accessit.vold will then manage thefuse_sdcard0 servicewhen media is inserted/removed.
fstab.hardware
[physical device node] auto vfat defaults voldmanaged=sdcard0:auto,noemulatedsd
init.hardware.rc
on init
mkdir /mnt/media_rw/sdcard0 0700 media_rw media_rw
mkdir /storage/sdcard0 0700 root root
export EXTERNAL_STORAGE /storage/sdcard0
service fuse_sdcard0 /system/bin/sdcard -u 1023 -g 1023 -d /mnt/media_rw/sdcard0 /storage/sdcard0
class late_start
disabled
storage_list.xml
<storage
android:mountPoint="/storage/sdcard0"
android:storageDescription="@string/storage_sd_card"
android:removable="true"
android:primary="true"
android:maxFileSize="4096" />
Emulated primary only
This is a typical configuration for a device with single external storagedevice which is backed by internal storage on the device, like Nexus 4.
init.hardware.rc
on init
mkdir /mnt/shell/emulated 0700 shell shell
mkdir /storage/emulated 0555 root root
export EXTERNAL_STORAGE /storage/emulated/legacy
export EMULATED_STORAGE_SOURCE /mnt/shell/emulated
export EMULATED_STORAGE_TARGET /storage/emulated
on fs
setprop ro.crypto.fuse_sdcard true
service sdcard /system/bin/sdcard -u 1023 -g 1023 -l /data/media /mnt/shell/emulated
class late_start
storage_list.xml
<storage
android:storageDescription="@string/storage_internal"
android:emulated="true"
android:mtpReserve="100" />
Emulated primary, physical secondary
This is a typical configuration for a device with multiple externalstorage devices, where the primary device is backed by internal storageon the device, and where the secondary device is a physical SD card, like Xoom.
The raw physical device must first be mounted under/mnt/media_rw where only the system and FUSE daemon canaccess it.vold will then manage thefuse_sdcard1service when media is inserted/removed.
fstab.hardware
[physical device node] auto vfat defaults voldmanaged=sdcard1:auto
init.hardware.rc
on init
mkdir /mnt/shell/emulated 0700 shell shell
mkdir /storage/emulated 0555 root root
mkdir /mnt/media_rw/sdcard1 0700 media_rw media_rw
mkdir /storage/sdcard1 0700 root root
export EXTERNAL_STORAGE /storage/emulated/legacy
export EMULATED_STORAGE_SOURCE /mnt/shell/emulated
export EMULATED_STORAGE_TARGET /storage/emulated
export SECONDARY_STORAGE /storage/sdcard1
on fs
setprop ro.crypto.fuse_sdcard true
service sdcard /system/bin/sdcard -u 1023 -g 1023 -l /data/media /mnt/shell/emulated
class late_start
service fuse_sdcard1 /system/bin/sdcard -u 1023 -g 1023 -w 1023 -d /mnt/media_rw/sdcard1 /storage/sdcard1
class late_start
disabled
storage_list.xml
<storage
android:storageDescription="@string/storage_internal"
android:emulated="true"
android:mtpReserve="100" />
<storage
android:mountPoint="/storage/sdcard1"
android:storageDescription="@string/storage_sd_card"
android:removable="true"
android:maxFileSize="4096" />
Android 6.0
Physical primary only
This is a typical configuration for a device with single external storagedevice which is a physical SD card, like the original Android One. There is nosecondary shared storage and the device cannot support multi-user.
init.device.rc
on init
# By default, primary storage is physical
setprop ro.vold.primary_physical 1
fstab.device
/devices/platform/mtk-msdc.1/mmc_host* auto auto defaults voldmanaged=sdcard0:auto,encryptable=userdata,noemulatedsd
Emulated primary only
This is a typical configuration for a device with single external storagedevice which is backed by internal storage on the device, like Nexus 6.
- Primary shared storage (
/sdcard) is emulated on top of internal storage. - No secondary SD card storage.
- USB OTG storage devices supported.
- Supports multi-user.
fstab.device
/devices/*/xhci-hcd.0.auto/usb* auto auto defaults voldmanaged=usb:auto
Emulated primary, physical secondary
This is a typical configuration for a device with multiple external storagedevices, where the primary device is backed by internal storage on the device,and where the secondary device is a physical SD card, like Xoom.
- Primary shared storage (
/sdcard) is emulated on top of internal storage. - Secondary storage is a physical SD card slot that can be adopted.
- Supports multi-user.
/devices/platform/mtk-msdc.1/mmc_host* auto auto defaults voldmanaged=sdcard1:auto,encryptable=userdata