View source for USB Mass Storage Gadget (or MSG)

The USB Gadget File-backed Storage Driver implements the USB Mass Storage class. The System appears to the host as a SCSI disk drive. A file or a block device is used as a backing store for the drive.

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== General Information ==

For further information about details of the USB Gadget File-backed Storage Driver have a look at [http://www.linux-usb.org/gadget/file_storage.html www.linux-usb.org/gadget/file_storage.html]. It is possible to work with kernel-modules. The steps below descripe how to compile the driver statically into the kernel because it is easier to handle.

== Configure the USB-OTG Port ==

To use USB Gadget features the USB-OTG Port has to be initialized in USB slave mode. Add <code>"otg_mode=device"</code> to the kernel commandline on the bootloader console.

On DHCOM boards it is possible to use the u-boot environment variable <code>"linux_add_param"</code>:
# enter bootloader console with DEL/BS Key
# <code>setenv linux_add_param otg_mode=device</code>
# <code>saveenv</code>

== Prepare the Kernel ==

Prepare the Linux Kernel Sources to be able to compile.

Open a console window and go to the root directory of the kernel sources and start the kernel configuration dialog:
* <code> ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- make menuconfig </code>

Enable the following items:
* Device Drivers -->  USB Support -->  USB Gadget Support 
* Device Drivers -->  USB Support -->  USB Gadget Support --> USB Peripheral Controller 
:: choose the matching device controller:
::: * DHCOM AM35x: OMAP USB Device Controller
::: * DHCOM iMX25: Freescale Highspeed USB DR Peripheral Controller
* Device Drivers -->  USB Support -->  USB Gadget Support --> USB Gadget Drivers -->  File-backed Storage Gadget

Apply the following changes to the file <code>linux-2.6.38/drivers/usb/gadget/file_storage.c</code> in the lines 332 and 333:

 <code><font color="#FF1010">- .removable        = 0,
 - .can_stall        = 1, </font>
 <font color="#00FF10">+ .removable        = 1, // old value = 0,
 + .can_stall        = 0, // old value = 1, </font></code>



Start building the kernel:
* <code> ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- make uImage </code>

Install the kernel on your board.


== Using USB Gadget File-backed Storage ==

On the running device you can use the following line to add the FAT32 partition of the microSD-card to the usb gadget driver:
* <code>i.mx25# echo /dev/mmcblk0p1 > /sys/devices/platform/fsl-usb2-udc/gadget/gadget-lun0/file</code>
If you do not work the i.mx25 maybe there are some tiny differences in the path of the sysfs.


Now, if you connect the USB-OTG port to your host pc the specified device appears as a drive.
 

'''Remark:'''
* If you run windows then you can only use block devices with filesystems supported by windows (FAT32).
* Do not use the rootfilesystem partition of your microSD card. This will cause damage to the used microSD-Card partition. 
 
Have a look at [http://www.linux-usb.org/gadget/file_storage.html www.linux-usb.org/gadget/file_storage.html] to get a step by step guide how to work with „Backing Storage Files“.