USB Gadget/Mass storage

= USB storage support =

This allows your devices act as a USB mass storage like external hard drive or thumb drive.

Kernel support
Currently, the g_mass_storage module is not compiled as part of default kernel configuration.

To enable this, follow same kernel building information as previous section USB Ethernet support but instead of compiling "Ethernet Gadget", select the following to "m": Device Drivers ---> USB support ---> USB Gadget Support --->     Mass Storage Gadget

You can now continue following manual build howto to continue kernel compilation and installation.

Preparing shared storage device
See "Backing Storage for the Mass Storage Gadget" for full instructions and recommendations.

Be aware that shared storage cannot be used in "read write" mode if both systems (device and host) are using it at same time except if you accept to corrupt your data...

An existing physical partition or a logical volume can be used as shared storage device; another option is to use a flat file as shared storage.

Creating a sparse file
Use "dd" command's "count" and "seek" options to create a 1GB file that will not use storage until it is used:

-rw-r--r-- 1 root root 1073741824 Feb 15 16:40 /mass_storage 0      /mass_storage
 * 1) dd if=/dev/zero of=/mass_storage bs=1M seek=1024 count=0
 * 2) ls -l /mass_storage
 * 1) du -hs /mass_storage

Partitioning and formatting sparse file
To be recognized by most Operating Systems, create a single FAT type partition and format it as DOS filesystem using Linux loop device driver.

Create a single partition
,,c EOT
 * 1) cat <<EOT | sfdisk --in-order -L -uM /mass_storage

Find partition offset

 * 1) fdisk -lu /mass_storage

Disk /mass_storage: 1073 MB, 1073741824 bytes 139 heads, 8 sectors/track, 1885 cylinders, total 2097152 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000

Device Boot     Start         End      Blocks   Id  System /mass_storage1              1     2097151     1048575+   c  W95 FAT32 (LBA)

First partition starts at first sector: offset = 1 * 512 = 512 bytes

Set up loop device
/dev/loop0: [b302]:14867 (/mass_storage), offset 512
 * 1) losetup -o512 /dev/loop0 /mass_storage
 * 2) losetup -a

Format device

 * 1) apt-get install dosfstools
 * 2) mkdosfs /dev/loop0

Test device
Filesystem     Size  Used Avail Use% Mounted on /dev/loop0     1022M  4.0K 1022M   1% /mnt /dev/loop0 on /mnt type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=cp437,iocharset=ascii,shortname=mixed,errors=remount-ro)
 * 1) mount -t vfat /dev/loop0 /mnt/
 * 2) df -h /mnt
 * 1) mount | grep mnt

Release device

 * 1) umount  /mnt/
 * 2) losetup -d /dev/loop0
 * 3) losetup -a

As we can see below, 1GB sparse file is only using 2.1MB storage (size of filesystem metadata):

2.1M   /mass_storage -rw-r--r-- 1 root root 1.0G Feb 15 16:54 /mass_storage
 * 1) du -sh /mass_storage
 * 1) ls -lh /mass_storage

Exporting mass storage / Loading the driver (on the device)
It looks like g_mass_storage conflicts with g_ether, remove g_ether first (if previously loaded):


 * 1) modprobe -r g_ether

Load g_mass_storage specifying storage to share (can also be a physical partition or a logical volume):


 * 1) modprobe g_mass_storage file=/mass_storage

We can now plug device to another host and use it as USB connected storage.

Notes: Surprisingly it works well my Mac OS hosts (device automatically appears in Finder, I can copy and read files) but not with my Linux hosts (device appears and disappears constantly and cannot be mounted). Not tested with Windows hosts.

Emulating a USB CD-ROM
The g_mass_storage module can also present an optical drive to the USB host. For this, your backing storage should be a suitable filesystem image - usually an .ISO file. Pass a "cdrom" module option like this:


 * 1) modprobe g_mass_storage file=/path/to/your-image-file.iso cdrom=y

(This can be useful with old PCs where the BIOS may have trouble starting from USB sticks, but supports booting via USB-CDROM.)


 * That means some restrictions, especially a hardcoded limit on the size of a backing (.iso) file. In case the file size exceeds 1152000 2KiB blocks (approx. 2.36GB / 2.2GiB), the driver will complain "file too big" and limit the usable range (sector count). If the filesystem driver on the USB host requests any sector beyond that, it will encounter I/O errors ("attempt to access beyond end of device").


 * In order to use larger images and properly emulate DVD media, the g_mass_storage would have to be extended. Some ideas and patches for that have been around for a while, e.g. https://lkml.org/lkml/2015/3/7/388 and http://linuxehacking.blogspot.de/2013/07/how-to-emulatore-dvd-rom-hardware-usb.html.