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-*How FunctionFS works*
-From kernel point of view it is just a composite function with some
-unique behaviour. It may be added to an USB configuration only after
-the user space driver has registered by writing descriptors and
-strings (the user space program has to provide the same information
-that kernel level composite functions provide when they are added to
-the configuration).
-This in particular means that the composite initialisation functions
-may not be in init section (ie. may not use the __init tag).
-From user space point of view it is a file system which when
-mounted provides an "ep0" file. User space driver need to
-write descriptors and strings to that file. It does not need
-to worry about endpoints, interfaces or strings numbers but
-simply provide descriptors such as if the function was the
-only one (endpoints and strings numbers starting from one and
-interface numbers starting from zero). The FunctionFS changes
-them as needed also handling situation when numbers differ in
-different configurations.
-When descriptors and strings are written "ep#" files appear
-(one for each declared endpoint) which handle communication on
-a single endpoint. Again, FunctionFS takes care of the real
-numbers and changing of the configuration (which means that
-"ep1" file may be really mapped to (say) endpoint 3 (and when
-configuration changes to (say) endpoint 2)). "ep0" is used
-for receiving events and handling setup requests.
-When all files are closed the function disables itself.
-What I also want to mention is that the FunctionFS is designed in such
-a way that it is possible to mount it several times so in the end
-a gadget could use several FunctionFS functions. The idea is that
-each FunctionFS instance is identified by the device name used
-when mounting.
-One can imagine a gadget that has an Ethernet, MTP and HID interfaces
-where the last two are implemented via FunctionFS. On user space
-level it would look like this:
-$ insmod g_ffs.ko idVendor=<ID> iSerialNumber=<string> functions=mtp,hid
-$ mkdir /dev/ffs-mtp && mount -t functionfs mtp /dev/ffs-mtp
-$ ( cd /dev/ffs-mtp && mtp-daemon ) &
-$ mkdir /dev/ffs-hid && mount -t functionfs hid /dev/ffs-hid
-$ ( cd /dev/ffs-hid && hid-daemon ) &
-On kernel level the gadget checks ffs_data->dev_name to identify
-whether it's FunctionFS designed for MTP ("mtp") or HID ("hid").
-If no "functions" module parameters is supplied, the driver accepts
-just one function with any name.
-When "functions" module parameter is supplied, only functions
-with listed names are accepted. In particular, if the "functions"
-parameter's value is just a one-element list, then the behaviour
-is similar to when there is no "functions" at all; however,
-only a function with the specified name is accepted.
-The gadget is registered only after all the declared function
-filesystems have been mounted and USB descriptors of all functions
-have been written to their ep0's.
-Conversely, the gadget is unregistered after the first USB function
-closes its endpoints.