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 the

fuse_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 the

fuse_sdcard1

service 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