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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
+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
+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
+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.