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