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1 UHID - User-space I/O driver support for HID subsystem
2 ========================================================
4 UHID allows user-space to implement HID transport drivers. Please see
5 hid-transport.txt for an introduction into HID transport drivers. This document
6 relies heavily on the definitions declared there.
8 With UHID, a user-space transport driver can create kernel hid-devices for each
9 device connected to the user-space controlled bus. The UHID API defines the I/O
10 events provided from the kernel to user-space and vice versa.
12 There is an example user-space application in ./samples/uhid/uhid-example.c
14 The UHID API
15 ------------
17 UHID is accessed through a character misc-device. The minor-number is allocated
18 dynamically so you need to rely on udev (or similar) to create the device node.
19 This is /dev/uhid by default.
21 If a new device is detected by your HID I/O Driver and you want to register this
22 device with the HID subsystem, then you need to open /dev/uhid once for each
23 device you want to register. All further communication is done by read()'ing or
24 write()'ing "struct uhid_event" objects. Non-blocking operations are supported
25 by setting O_NONBLOCK.
27 struct uhid_event {
28 __u32 type;
29 union {
30 struct uhid_create2_req create2;
31 struct uhid_output_req output;
32 struct uhid_input2_req input2;
33 ...
34 } u;
35 };
37 The "type" field contains the ID of the event. Depending on the ID different
38 payloads are sent. You must not split a single event across multiple read()'s or
39 multiple write()'s. A single event must always be sent as a whole. Furthermore,
40 only a single event can be sent per read() or write(). Pending data is ignored.
41 If you want to handle multiple events in a single syscall, then use vectored
42 I/O with readv()/writev().
43 The "type" field defines the payload. For each type, there is a
44 payload-structure available in the union "u" (except for empty payloads). This
45 payload contains management and/or device data.
47 The first thing you should do is sending an UHID_CREATE2 event. This will
48 register the device. UHID will respond with an UHID_START event. You can now
49 start sending data to and reading data from UHID. However, unless UHID sends the
50 UHID_OPEN event, the internally attached HID Device Driver has no user attached.
51 That is, you might put your device asleep unless you receive the UHID_OPEN
52 event. If you receive the UHID_OPEN event, you should start I/O. If the last
53 user closes the HID device, you will receive an UHID_CLOSE event. This may be
54 followed by an UHID_OPEN event again and so on. There is no need to perform
55 reference-counting in user-space. That is, you will never receive multiple
56 UHID_OPEN events without an UHID_CLOSE event. The HID subsystem performs
57 ref-counting for you.
58 You may decide to ignore UHID_OPEN/UHID_CLOSE, though. I/O is allowed even
59 though the device may have no users.
61 If you want to send data on the interrupt channel to the HID subsystem, you send
62 an HID_INPUT2 event with your raw data payload. If the kernel wants to send data
63 on the interrupt channel to the device, you will read an UHID_OUTPUT event.
64 Data requests on the control channel are currently limited to GET_REPORT and
65 SET_REPORT (no other data reports on the control channel are defined so far).
66 Those requests are always synchronous. That means, the kernel sends
67 UHID_GET_REPORT and UHID_SET_REPORT events and requires you to forward them to
68 the device on the control channel. Once the device responds, you must forward
69 the response via UHID_GET_REPORT_REPLY and UHID_SET_REPORT_REPLY to the kernel.
70 The kernel blocks internal driver-execution during such round-trips (times out
71 after a hard-coded period).
73 If your device disconnects, you should send an UHID_DESTROY event. This will
74 unregister the device. You can now send UHID_CREATE2 again to register a new
75 device.
76 If you close() the fd, the device is automatically unregistered and destroyed
77 internally.
79 write()
80 -------
81 write() allows you to modify the state of the device and feed input data into
82 the kernel. The kernel will parse the event immediately and if the event ID is
83 not supported, it will return -EOPNOTSUPP. If the payload is invalid, then
84 -EINVAL is returned, otherwise, the amount of data that was read is returned and
85 the request was handled successfully. O_NONBLOCK does not affect write() as
86 writes are always handled immediately in a non-blocking fashion. Future requests
87 might make use of O_NONBLOCK, though.
89 UHID_CREATE2:
90 This creates the internal HID device. No I/O is possible until you send this
91 event to the kernel. The payload is of type struct uhid_create2_req and
92 contains information about your device. You can start I/O now.
94 UHID_DESTROY:
95 This destroys the internal HID device. No further I/O will be accepted. There
96 may still be pending messages that you can receive with read() but no further
97 UHID_INPUT events can be sent to the kernel.
98 You can create a new device by sending UHID_CREATE2 again. There is no need to
99 reopen the character device.
101 UHID_INPUT2:
102 You must send UHID_CREATE2 before sending input to the kernel! This event
103 contains a data-payload. This is the raw data that you read from your device
104 on the interrupt channel. The kernel will parse the HID reports.
106 UHID_GET_REPORT_REPLY:
107 If you receive a UHID_GET_REPORT request you must answer with this request.
108 You must copy the "id" field from the request into the answer. Set the "err"
109 field to 0 if no error occurred or to EIO if an I/O error occurred.
110 If "err" is 0 then you should fill the buffer of the answer with the results
111 of the GET_REPORT request and set "size" correspondingly.
113 UHID_SET_REPORT_REPLY:
114 This is the SET_REPORT equivalent of UHID_GET_REPORT_REPLY. Unlike GET_REPORT,
115 SET_REPORT never returns a data buffer, therefore, it's sufficient to set the
116 "id" and "err" fields correctly.
118 read()
119 ------
120 read() will return a queued output report. No reaction is required to any of
121 them but you should handle them according to your needs.
123 UHID_START:
124 This is sent when the HID device is started. Consider this as an answer to
125 UHID_CREATE2. This is always the first event that is sent. Note that this
126 event might not be available immediately after write(UHID_CREATE2) returns.
127 Device drivers might required delayed setups.
128 This event contains a payload of type uhid_start_req. The "dev_flags" field
129 describes special behaviors of a device. The following flags are defined:
130 UHID_DEV_NUMBERED_FEATURE_REPORTS:
131 UHID_DEV_NUMBERED_OUTPUT_REPORTS:
132 UHID_DEV_NUMBERED_INPUT_REPORTS:
133 Each of these flags defines whether a given report-type uses numbered
134 reports. If numbered reports are used for a type, all messages from
135 the kernel already have the report-number as prefix. Otherwise, no
136 prefix is added by the kernel.
137 For messages sent by user-space to the kernel, you must adjust the
138 prefixes according to these flags.
140 UHID_STOP:
141 This is sent when the HID device is stopped. Consider this as an answer to
142 UHID_DESTROY.
143 If you didn't destroy your device via UHID_DESTROY, but the kernel sends an
144 UHID_STOP event, this should usually be ignored. It means that the kernel
145 reloaded/changed the device driver loaded on your HID device (or some other
146 maintenance actions happened).
147 You can usually ignored any UHID_STOP events safely.
149 UHID_OPEN:
150 This is sent when the HID device is opened. That is, the data that the HID
151 device provides is read by some other process. You may ignore this event but
152 it is useful for power-management. As long as you haven't received this event
153 there is actually no other process that reads your data so there is no need to
154 send UHID_INPUT2 events to the kernel.
156 UHID_CLOSE:
157 This is sent when there are no more processes which read the HID data. It is
158 the counterpart of UHID_OPEN and you may as well ignore this event.
160 UHID_OUTPUT:
161 This is sent if the HID device driver wants to send raw data to the I/O
162 device on the interrupt channel. You should read the payload and forward it to
163 the device. The payload is of type "struct uhid_data_req".
164 This may be received even though you haven't received UHID_OPEN, yet.
166 UHID_GET_REPORT:
167 This event is sent if the kernel driver wants to perform a GET_REPORT request
168 on the control channeld as described in the HID specs. The report-type and
169 report-number are available in the payload.
170 The kernel serializes GET_REPORT requests so there will never be two in
171 parallel. However, if you fail to respond with a UHID_GET_REPORT_REPLY, the
172 request might silently time out.
173 Once you read a GET_REPORT request, you shall forward it to the hid device and
174 remember the "id" field in the payload. Once your hid device responds to the
175 GET_REPORT (or if it fails), you must send a UHID_GET_REPORT_REPLY to the
176 kernel with the exact same "id" as in the request. If the request already
177 timed out, the kernel will ignore the response silently. The "id" field is
178 never re-used, so conflicts cannot happen.
180 UHID_SET_REPORT:
181 This is the SET_REPORT equivalent of UHID_GET_REPORT. On receipt, you shall
182 send a SET_REPORT request to your hid device. Once it replies, you must tell
183 the kernel about it via UHID_SET_REPORT_REPLY.
184 The same restrictions as for UHID_GET_REPORT apply.
186 ----------------------------------------------------
187 Written 2012, David Herrmann <dh.herrmann@gmail.com>