1==================== 2How FunctionFS works 3==================== 4 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). 11 12This in particular means that the composite initialisation functions 13may not be in init section (ie. may not use the __init tag). 14 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. 24 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. 32 33When all files are closed the function disables itself. 34 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. 40 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:: 44 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 ) & 50 51On kernel level the gadget checks ffs_data->dev_name to identify 52whether it's FunctionFS designed for MTP ("mtp") or HID ("hid"). 53 54If no "functions" module parameters is supplied, the driver accepts 55just one function with any name. 56 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. 62 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. 66 67Conversely, the gadget is unregistered after the first USB function 68closes its endpoints. 69