Based on kernel version 4.15. Page generated on 2018-01-29 10:01 EST.
1 *How FunctionFS works* 2 3 From kernel point of view it is just a composite function with some 4 unique behaviour. It may be added to an USB configuration only after 5 the user space driver has registered by writing descriptors and 6 strings (the user space program has to provide the same information 7 that kernel level composite functions provide when they are added to 8 the configuration). 9 10 This in particular means that the composite initialisation functions 11 may not be in init section (ie. may not use the __init tag). 12 13 From user space point of view it is a file system which when 14 mounted provides an "ep0" file. User space driver need to 15 write descriptors and strings to that file. It does not need 16 to worry about endpoints, interfaces or strings numbers but 17 simply provide descriptors such as if the function was the 18 only one (endpoints and strings numbers starting from one and 19 interface numbers starting from zero). The FunctionFS changes 20 them as needed also handling situation when numbers differ in 21 different configurations. 22 23 When descriptors and strings are written "ep#" files appear 24 (one for each declared endpoint) which handle communication on 25 a single endpoint. Again, FunctionFS takes care of the real 26 numbers and changing of the configuration (which means that 27 "ep1" file may be really mapped to (say) endpoint 3 (and when 28 configuration changes to (say) endpoint 2)). "ep0" is used 29 for receiving events and handling setup requests. 30 31 When all files are closed the function disables itself. 32 33 What I also want to mention is that the FunctionFS is designed in such 34 a way that it is possible to mount it several times so in the end 35 a gadget could use several FunctionFS functions. The idea is that 36 each FunctionFS instance is identified by the device name used 37 when mounting. 38 39 One can imagine a gadget that has an Ethernet, MTP and HID interfaces 40 where the last two are implemented via FunctionFS. On user space 41 level it would look like this: 42 43 $ insmod g_ffs.ko idVendor=<ID> iSerialNumber=<string> functions=mtp,hid 44 $ mkdir /dev/ffs-mtp && mount -t functionfs mtp /dev/ffs-mtp 45 $ ( cd /dev/ffs-mtp && mtp-daemon ) & 46 $ mkdir /dev/ffs-hid && mount -t functionfs hid /dev/ffs-hid 47 $ ( cd /dev/ffs-hid && hid-daemon ) & 48 49 On kernel level the gadget checks ffs_data->dev_name to identify 50 whether it's FunctionFS designed for MTP ("mtp") or HID ("hid"). 51 52 If no "functions" module parameters is supplied, the driver accepts 53 just one function with any name. 54 55 When "functions" module parameter is supplied, only functions 56 with listed names are accepted. In particular, if the "functions" 57 parameter's value is just a one-element list, then the behaviour 58 is similar to when there is no "functions" at all; however, 59 only a function with the specified name is accepted. 60 61 The gadget is registered only after all the declared function 62 filesystems have been mounted and USB descriptors of all functions 63 have been written to their ep0's. 64 65 Conversely, the gadget is unregistered after the first USB function 66 closes its endpoints.