Documentation/hid/uhid.txt

   1       UHID - User-space I/O driver support for HID subsystem
   2      ========================================================
   3
   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.
   7
   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.
  11
  12 There is an example user-space application in ./samples/uhid/uhid-example.c
  13
  14 The UHID API
  15 ------------
  16
  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.
  20
  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.
  26
  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 };
  36
  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.
  46
  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.
  60
  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).
  72
  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.
  78
  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.
  88
  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.
  93
  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.
 100
 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.
 105
 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.
 112
 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.
 117
 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.
 122
 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.
 139
 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.
 148
 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.
 155
 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.
 159
 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.
 165
 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.
 179
 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.
 185
 186 ----------------------------------------------------
 187 Written 2012, David Herrmann <dh.herrmann@gmail.com>