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Documentation / usb / functionfs.txt

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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	*How FunctionFS works*
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).
10	This in particular means that the composite initialisation functions
11	may not be in init section (ie. may not use the __init tag).
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.
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.
31	When all files are closed the function disables itself.
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.
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:
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 ) &
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").
52	If no "functions" module parameters is supplied, the driver accepts
53	just one function with any name.
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.
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.
65	Conversely, the gadget is unregistered after the first USB function
66	closes its endpoints.
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