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