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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	      UHID - User-space I/O driver support for HID subsystem
2	     ========================================================
3	
4	The HID subsystem needs two kinds of drivers. In this document we call them:
5	
6	 1. The "HID I/O Driver" is the driver that performs raw data I/O to the
7	    low-level device. Internally, they register an hid_ll_driver structure with
8	    the HID core. They perform device setup, read raw data from the device and
9	    push it into the HID subsystem and they provide a callback so the HID
10	    subsystem can send data to the device.
11	
12	 2. The "HID Device Driver" is the driver that parses HID reports and reacts on
13	    them. There are generic drivers like "generic-usb" and "generic-bluetooth"
14	    which adhere to the HID specification and provide the standardizes features.
15	    But there may be special drivers and quirks for each non-standard device out
16	    there. Internally, they use the hid_driver structure.
17	
18	Historically, the USB stack was the first subsystem to provide an HID I/O
19	Driver. However, other standards like Bluetooth have adopted the HID specs and
20	may provide HID I/O Drivers, too. The UHID driver allows to implement HID I/O
21	Drivers in user-space and feed the data into the kernel HID-subsystem.
22	
23	This allows user-space to operate on the same level as USB-HID, Bluetooth-HID
24	and similar. It does not provide a way to write HID Device Drivers, though. Use
25	hidraw for this purpose.
26	
27	There is an example user-space application in ./samples/uhid/uhid-example.c
28	
29	The UHID API
30	------------
31	
32	UHID is accessed through a character misc-device. The minor-number is allocated
33	dynamically so you need to rely on udev (or similar) to create the device node.
34	This is /dev/uhid by default.
35	
36	If a new device is detected by your HID I/O Driver and you want to register this
37	device with the HID subsystem, then you need to open /dev/uhid once for each
38	device you want to register. All further communication is done by read()'ing or
39	write()'ing "struct uhid_event" objects. Non-blocking operations are supported
40	by setting O_NONBLOCK.
41	
42	struct uhid_event {
43	        __u32 type;
44	        union {
45	                struct uhid_create_req create;
46	                struct uhid_data_req data;
47	                ...
48	        } u;
49	};
50	
51	The "type" field contains the ID of the event. Depending on the ID different
52	payloads are sent. You must not split a single event across multiple read()'s or
53	multiple write()'s. A single event must always be sent as a whole. Furthermore,
54	only a single event can be sent per read() or write(). Pending data is ignored.
55	If you want to handle multiple events in a single syscall, then use vectored
56	I/O with readv()/writev().
57	
58	The first thing you should do is sending an UHID_CREATE event. This will
59	register the device. UHID will respond with an UHID_START event. You can now
60	start sending data to and reading data from UHID. However, unless UHID sends the
61	UHID_OPEN event, the internally attached HID Device Driver has no user attached.
62	That is, you might put your device asleep unless you receive the UHID_OPEN
63	event. If you receive the UHID_OPEN event, you should start I/O. If the last
64	user closes the HID device, you will receive an UHID_CLOSE event. This may be
65	followed by an UHID_OPEN event again and so on. There is no need to perform
66	reference-counting in user-space. That is, you will never receive multiple
67	UHID_OPEN events without an UHID_CLOSE event. The HID subsystem performs
68	ref-counting for you.
69	You may decide to ignore UHID_OPEN/UHID_CLOSE, though. I/O is allowed even
70	though the device may have no users.
71	
72	If you want to send data to the HID subsystem, you send an HID_INPUT event with
73	your raw data payload. If the kernel wants to send data to the device, you will
74	read an UHID_OUTPUT or UHID_OUTPUT_EV event.
75	
76	If your device disconnects, you should send an UHID_DESTROY event. This will
77	unregister the device. You can now send UHID_CREATE again to register a new
78	device.
79	If you close() the fd, the device is automatically unregistered and destroyed
80	internally.
81	
82	write()
83	-------
84	write() allows you to modify the state of the device and feed input data into
85	the kernel. The following types are supported: UHID_CREATE, UHID_DESTROY and
86	UHID_INPUT. The kernel will parse the event immediately and if the event ID is
87	not supported, it will return -EOPNOTSUPP. If the payload is invalid, then
88	-EINVAL is returned, otherwise, the amount of data that was read is returned and
89	the request was handled successfully.
90	
91	  UHID_CREATE:
92	  This creates the internal HID device. No I/O is possible until you send this
93	  event to the kernel. The payload is of type struct uhid_create_req and
94	  contains information about your device. You can start I/O now.
95	
96	  UHID_DESTROY:
97	  This destroys the internal HID device. No further I/O will be accepted. There
98	  may still be pending messages that you can receive with read() but no further
99	  UHID_INPUT events can be sent to the kernel.
100	  You can create a new device by sending UHID_CREATE again. There is no need to
101	  reopen the character device.
102	
103	  UHID_INPUT:
104	  You must send UHID_CREATE before sending input to the kernel! This event
105	  contains a data-payload. This is the raw data that you read from your device.
106	  The kernel will parse the HID reports and react on it.
107	
108	  UHID_FEATURE_ANSWER:
109	  If you receive a UHID_FEATURE request you must answer with this request. You
110	  must copy the "id" field from the request into the answer. Set the "err" field
111	  to 0 if no error occurred or to EIO if an I/O error occurred.
112	  If "err" is 0 then you should fill the buffer of the answer with the results
113	  of the feature request and set "size" correspondingly.
114	
115	read()
116	------
117	read() will return a queued ouput report. These output reports can be of type
118	UHID_START, UHID_STOP, UHID_OPEN, UHID_CLOSE, UHID_OUTPUT or UHID_OUTPUT_EV. No
119	reaction is required to any of them but you should handle them according to your
120	needs. Only UHID_OUTPUT and UHID_OUTPUT_EV have payloads.
121	
122	  UHID_START:
123	  This is sent when the HID device is started. Consider this as an answer to
124	  UHID_CREATE. This is always the first event that is sent.
125	
126	  UHID_STOP:
127	  This is sent when the HID device is stopped. Consider this as an answer to
128	  UHID_DESTROY.
129	  If the kernel HID device driver closes the device manually (that is, you
130	  didn't send UHID_DESTROY) then you should consider this device closed and send
131	  an UHID_DESTROY event. You may want to reregister your device, though. This is
132	  always the last message that is sent to you unless you reopen the device with
133	  UHID_CREATE.
134	
135	  UHID_OPEN:
136	  This is sent when the HID device is opened. That is, the data that the HID
137	  device provides is read by some other process. You may ignore this event but
138	  it is useful for power-management. As long as you haven't received this event
139	  there is actually no other process that reads your data so there is no need to
140	  send UHID_INPUT events to the kernel.
141	
142	  UHID_CLOSE:
143	  This is sent when there are no more processes which read the HID data. It is
144	  the counterpart of UHID_OPEN and you may as well ignore this event.
145	
146	  UHID_OUTPUT:
147	  This is sent if the HID device driver wants to send raw data to the I/O
148	  device. You should read the payload and forward it to the device. The payload
149	  is of type "struct uhid_data_req".
150	  This may be received even though you haven't received UHID_OPEN, yet.
151	
152	  UHID_OUTPUT_EV (obsolete):
153	  Same as UHID_OUTPUT but this contains a "struct input_event" as payload. This
154	  is called for force-feedback, LED or similar events which are received through
155	  an input device by the HID subsystem. You should convert this into raw reports
156	  and send them to your device similar to events of type UHID_OUTPUT.
157	  This is no longer sent by newer kernels. Instead, HID core converts it into a
158	  raw output report and sends it via UHID_OUTPUT.
159	
160	  UHID_FEATURE:
161	  This event is sent if the kernel driver wants to perform a feature request as
162	  described in the HID specs. The report-type and report-number are available in
163	  the payload.
164	  The kernel serializes feature requests so there will never be two in parallel.
165	  However, if you fail to respond with a UHID_FEATURE_ANSWER in a time-span of 5
166	  seconds, then the requests will be dropped and a new one might be sent.
167	  Therefore, the payload also contains an "id" field that identifies every
168	  request.
169	
170	Document by:
171	  David Herrmann <dh.herrmann@googlemail.com>
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