staging: usb: gadget: Add FunctionFS support to Configurable Composite Gadget driver

Add FunctionFS support.
It allows certain USB functions to be provided from userspace, e.g. MTP,
PTP, adb.

The functions provided by the gadget itself are enumerated in
/sys/class/ccg0/functions. The functions which can be supplied
from userspace must be enumerated in /sys/class/ccg0/f_fs/user_functions.
No other userspace functions can be used than specified in the above mentioned
file, but just specifying them there is not enough to activate them.
The userspace functions in order to be activated need also be enumerated
in /sys/class/ccg0/functions.

An example sequence of operations can be as follows:

$ echo 0 > /sys/class/ccg_usb/ccg0/enable

$ echo -n 0x2d01 > /sys/module/g_ccg/parameters/idProduct
$ echo -n MyDevice > /sys/module/g_ccg/parameters/iSerialNumber
$ echo -n 0x1d6b > /sys/module/g_ccg/parameters/idVendor
$ echo -n Manufacturer > /sys/module/g_ccg/parameters/iManufacturer
$ echo -n Product > /sys/module/g_ccg/parameters/iProduct
$ echo -n bcdDevice > /sys/module/g_ccg/parameters/bcdDevice

$ echo adb,mtp,ptp > /sys/class/ccg_usb/ccg0/f_fs/user_functions
$ echo mass_storage,ptp > /sys/class/ccg_usb/ccg0/functions
$ echo /file.img > /sys/class/ccg_usb/ccg0/f_mass_storage/lun/file

$ mkdir -p /dev/usbgadget/ptp
$ mount -t functionfs ptp /dev/usbgadget/ptp
$ ./ptp &

$ echo 1 > /sys/class/ccg_usb/ccg0/enable

The above example declares that adb, mtp and ptp functions can be supplied
from userspace through FunctionFS. But of them all only ptp is actually
activated, together with mass_storage, the latter being implemented in
the gadget itself (in kernel, not in userspace). The list of functions
can be modified at runtime while the gadget is not enabled, that is,
after

$ echo 0 > /sys/class/ccg_usb/ccg0/enable

The FunctionFS function is implicitly handled by the gadget, that is,
if a userspace function name is provided in
/sys/class/ccg_usb/ccg0/functions, then the FunctionFS function
is activated.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>