2How FunctionFS works
   5From kernel point of view it is just a composite function with some
   6unique behaviour.  It may be added to an USB configuration only after
   7the user space driver has registered by writing descriptors and
   8strings (the user space program has to provide the same information
   9that kernel level composite functions provide when they are added to
  10the configuration).
  12This in particular means that the composite initialisation functions
  13may not be in init section (ie. may not use the __init tag).
  15From user space point of view it is a file system which when
  16mounted provides an "ep0" file.  User space driver need to
  17write descriptors and strings to that file.  It does not need
  18to worry about endpoints, interfaces or strings numbers but
  19simply provide descriptors such as if the function was the
  20only one (endpoints and strings numbers starting from one and
  21interface numbers starting from zero).  The FunctionFS changes
  22them as needed also handling situation when numbers differ in
  23different configurations.
  25When descriptors and strings are written "ep#" files appear
  26(one for each declared endpoint) which handle communication on
  27a single endpoint.  Again, FunctionFS takes care of the real
  28numbers and changing of the configuration (which means that
  29"ep1" file may be really mapped to (say) endpoint 3 (and when
  30configuration changes to (say) endpoint 2)).  "ep0" is used
  31for receiving events and handling setup requests.
  33When all files are closed the function disables itself.
  35What I also want to mention is that the FunctionFS is designed in such
  36a way that it is possible to mount it several times so in the end
  37a gadget could use several FunctionFS functions. The idea is that
  38each FunctionFS instance is identified by the device name used
  39when mounting.
  41One can imagine a gadget that has an Ethernet, MTP and HID interfaces
  42where the last two are implemented via FunctionFS.  On user space
  43level it would look like this::
  45  $ insmod g_ffs.ko idVendor=<ID> iSerialNumber=<string> functions=mtp,hid
  46  $ mkdir /dev/ffs-mtp && mount -t functionfs mtp /dev/ffs-mtp
  47  $ ( cd /dev/ffs-mtp && mtp-daemon ) &
  48  $ mkdir /dev/ffs-hid && mount -t functionfs hid /dev/ffs-hid
  49  $ ( cd /dev/ffs-hid && hid-daemon ) &
  51On kernel level the gadget checks ffs_data->dev_name to identify
  52whether it's FunctionFS designed for MTP ("mtp") or HID ("hid").
  54If no "functions" module parameters is supplied, the driver accepts
  55just one function with any name.
  57When "functions" module parameter is supplied, only functions
  58with listed names are accepted. In particular, if the "functions"
  59parameter's value is just a one-element list, then the behaviour
  60is similar to when there is no "functions" at all; however,
  61only a function with the specified name is accepted.
  63The gadget is registered only after all the declared function
  64filesystems have been mounted and USB descriptors of all functions
  65have been written to their ep0's.
  67Conversely, the gadget is unregistered after the first USB function
  68closes its endpoints.