1==================== 2How FunctionFS works 3==================== 4 5Overview 6======== 7 8From kernel point of view it is just a composite function with some 9unique behaviour. It may be added to an USB configuration only after 10the user space driver has registered by writing descriptors and 11strings (the user space program has to provide the same information 12that kernel level composite functions provide when they are added to 13the configuration). 14 15This in particular means that the composite initialisation functions 16may not be in init section (ie. may not use the __init tag). 17 18From user space point of view it is a file system which when 19mounted provides an "ep0" file. User space driver need to 20write descriptors and strings to that file. It does not need 21to worry about endpoints, interfaces or strings numbers but 22simply provide descriptors such as if the function was the 23only one (endpoints and strings numbers starting from one and 24interface numbers starting from zero). The FunctionFS changes 25them as needed also handling situation when numbers differ in 26different configurations. 27 28When descriptors and strings are written "ep#" files appear 29(one for each declared endpoint) which handle communication on 30a single endpoint. Again, FunctionFS takes care of the real 31numbers and changing of the configuration (which means that 32"ep1" file may be really mapped to (say) endpoint 3 (and when 33configuration changes to (say) endpoint 2)). "ep0" is used 34for receiving events and handling setup requests. 35 36When all files are closed the function disables itself. 37 38What I also want to mention is that the FunctionFS is designed in such 39a way that it is possible to mount it several times so in the end 40a gadget could use several FunctionFS functions. The idea is that 41each FunctionFS instance is identified by the device name used 42when mounting. 43 44One can imagine a gadget that has an Ethernet, MTP and HID interfaces 45where the last two are implemented via FunctionFS. On user space 46level it would look like this:: 47 48 $ insmod g_ffs.ko idVendor=<ID> iSerialNumber=<string> functions=mtp,hid 49 $ mkdir /dev/ffs-mtp && mount -t functionfs mtp /dev/ffs-mtp 50 $ ( cd /dev/ffs-mtp && mtp-daemon ) & 51 $ mkdir /dev/ffs-hid && mount -t functionfs hid /dev/ffs-hid 52 $ ( cd /dev/ffs-hid && hid-daemon ) & 53 54On kernel level the gadget checks ffs_data->dev_name to identify 55whether its FunctionFS is designed for MTP ("mtp") or HID ("hid"). 56 57If no "functions" module parameters is supplied, the driver accepts 58just one function with any name. 59 60When "functions" module parameter is supplied, only functions 61with listed names are accepted. In particular, if the "functions" 62parameter's value is just a one-element list, then the behaviour 63is similar to when there is no "functions" at all; however, 64only a function with the specified name is accepted. 65 66The gadget is registered only after all the declared function 67filesystems have been mounted and USB descriptors of all functions 68have been written to their ep0's. 69 70Conversely, the gadget is unregistered after the first USB function 71closes its endpoints. 72 73DMABUF interface 74================ 75 76FunctionFS additionally supports a DMABUF based interface, where the 77userspace can attach DMABUF objects (externally created) to an endpoint, 78and subsequently use them for data transfers. 79 80A userspace application can then use this interface to share DMABUF 81objects between several interfaces, allowing it to transfer data in a 82zero-copy fashion, for instance between IIO and the USB stack. 83 84As part of this interface, three new IOCTLs have been added. These three 85IOCTLs have to be performed on a data endpoint (ie. not ep0). They are: 86 87 ``FUNCTIONFS_DMABUF_ATTACH(int)`` 88 Attach the DMABUF object, identified by its file descriptor, to the 89 data endpoint. Returns zero on success, and a negative errno value 90 on error. 91 92 ``FUNCTIONFS_DMABUF_DETACH(int)`` 93 Detach the given DMABUF object, identified by its file descriptor, 94 from the data endpoint. Returns zero on success, and a negative 95 errno value on error. Note that closing the endpoint's file 96 descriptor will automatically detach all attached DMABUFs. 97 98 ``FUNCTIONFS_DMABUF_TRANSFER(struct usb_ffs_dmabuf_transfer_req *)`` 99 Enqueue the previously attached DMABUF to the transfer queue. 100 The argument is a structure that packs the DMABUF's file descriptor, 101 the size in bytes to transfer (which should generally correspond to 102 the size of the DMABUF), and a 'flags' field which is unused 103 for now. Returns zero on success, and a negative errno value on 104 error. 105