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+      UHID - User-space I/O driver support for HID subsystem
+     ========================================================
+
+UHID allows user-space to implement HID transport drivers. Please see
+hid-transport.txt for an introduction into HID transport drivers. This document
+relies heavily on the definitions declared there.
+
+With UHID, a user-space transport driver can create kernel hid-devices for each
+device connected to the user-space controlled bus. The UHID API defines the I/O
+events provided from the kernel to user-space and vice versa.
+
+There is an example user-space application in ./samples/uhid/uhid-example.c
+
+The UHID API
+------------
+
+UHID is accessed through a character misc-device. The minor-number is allocated
+dynamically so you need to rely on udev (or similar) to create the device node.
+This is /dev/uhid by default.
+
+If a new device is detected by your HID I/O Driver and you want to register this
+device with the HID subsystem, then you need to open /dev/uhid once for each
+device you want to register. All further communication is done by read()'ing or
+write()'ing "struct uhid_event" objects. Non-blocking operations are supported
+by setting O_NONBLOCK.
+
+struct uhid_event {
+        __u32 type;
+        union {
+                struct uhid_create2_req create2;
+                struct uhid_output_req output;
+                struct uhid_input2_req input2;
+                ...
+        } u;
+};
+
+The "type" field contains the ID of the event. Depending on the ID different
+payloads are sent. You must not split a single event across multiple read()'s or
+multiple write()'s. A single event must always be sent as a whole. Furthermore,
+only a single event can be sent per read() or write(). Pending data is ignored.
+If you want to handle multiple events in a single syscall, then use vectored
+I/O with readv()/writev().
+The "type" field defines the payload. For each type, there is a
+payload-structure available in the union "u" (except for empty payloads). This
+payload contains management and/or device data.
+
+The first thing you should do is sending an UHID_CREATE2 event. This will
+register the device. UHID will respond with an UHID_START event. You can now
+start sending data to and reading data from UHID. However, unless UHID sends the
+UHID_OPEN event, the internally attached HID Device Driver has no user attached.
+That is, you might put your device asleep unless you receive the UHID_OPEN
+event. If you receive the UHID_OPEN event, you should start I/O. If the last
+user closes the HID device, you will receive an UHID_CLOSE event. This may be
+followed by an UHID_OPEN event again and so on. There is no need to perform
+reference-counting in user-space. That is, you will never receive multiple
+UHID_OPEN events without an UHID_CLOSE event. The HID subsystem performs
+ref-counting for you.
+You may decide to ignore UHID_OPEN/UHID_CLOSE, though. I/O is allowed even
+though the device may have no users.
+
+If you want to send data on the interrupt channel to the HID subsystem, you send
+an HID_INPUT2 event with your raw data payload. If the kernel wants to send data
+on the interrupt channel to the device, you will read an UHID_OUTPUT event.
+Data requests on the control channel are currently limited to GET_REPORT and
+SET_REPORT (no other data reports on the control channel are defined so far).
+Those requests are always synchronous. That means, the kernel sends
+UHID_GET_REPORT and UHID_SET_REPORT events and requires you to forward them to
+the device on the control channel. Once the device responds, you must forward
+the response via UHID_GET_REPORT_REPLY and UHID_SET_REPORT_REPLY to the kernel.
+The kernel blocks internal driver-execution during such round-trips (times out
+after a hard-coded period).
+
+If your device disconnects, you should send an UHID_DESTROY event. This will
+unregister the device. You can now send UHID_CREATE2 again to register a new
+device.
+If you close() the fd, the device is automatically unregistered and destroyed
+internally.
+
+write()
+-------
+write() allows you to modify the state of the device and feed input data into
+the kernel. The kernel will parse the event immediately and if the event ID is
+not supported, it will return -EOPNOTSUPP. If the payload is invalid, then
+-EINVAL is returned, otherwise, the amount of data that was read is returned and
+the request was handled successfully. O_NONBLOCK does not affect write() as
+writes are always handled immediately in a non-blocking fashion. Future requests
+might make use of O_NONBLOCK, though.
+
+  UHID_CREATE2:
+  This creates the internal HID device. No I/O is possible until you send this
+  event to the kernel. The payload is of type struct uhid_create2_req and
+  contains information about your device. You can start I/O now.
+
+  UHID_DESTROY:
+  This destroys the internal HID device. No further I/O will be accepted. There
+  may still be pending messages that you can receive with read() but no further
+  UHID_INPUT events can be sent to the kernel.
+  You can create a new device by sending UHID_CREATE2 again. There is no need to
+  reopen the character device.
+
+  UHID_INPUT2:
+  You must send UHID_CREATE2 before sending input to the kernel! This event
+  contains a data-payload. This is the raw data that you read from your device
+  on the interrupt channel. The kernel will parse the HID reports.
+
+  UHID_GET_REPORT_REPLY:
+  If you receive a UHID_GET_REPORT request you must answer with this request.
+  You  must copy the "id" field from the request into the answer. Set the "err"
+  field to 0 if no error occurred or to EIO if an I/O error occurred.
+  If "err" is 0 then you should fill the buffer of the answer with the results
+  of the GET_REPORT request and set "size" correspondingly.
+
+  UHID_SET_REPORT_REPLY:
+  This is the SET_REPORT equivalent of UHID_GET_REPORT_REPLY. Unlike GET_REPORT,
+  SET_REPORT never returns a data buffer, therefore, it's sufficient to set the
+  "id" and "err" fields correctly.
+
+read()
+------
+read() will return a queued output report. No reaction is required to any of
+them but you should handle them according to your needs.
+
+  UHID_START:
+  This is sent when the HID device is started. Consider this as an answer to
+  UHID_CREATE2. This is always the first event that is sent. Note that this
+  event might not be available immediately after write(UHID_CREATE2) returns.
+  Device drivers might required delayed setups.
+  This event contains a payload of type uhid_start_req. The "dev_flags" field
+  describes special behaviors of a device. The following flags are defined:
+      UHID_DEV_NUMBERED_FEATURE_REPORTS:
+      UHID_DEV_NUMBERED_OUTPUT_REPORTS:
+      UHID_DEV_NUMBERED_INPUT_REPORTS:
+          Each of these flags defines whether a given report-type uses numbered
+          reports. If numbered reports are used for a type, all messages from
+          the kernel already have the report-number as prefix. Otherwise, no
+          prefix is added by the kernel.
+          For messages sent by user-space to the kernel, you must adjust the
+          prefixes according to these flags.
+
+  UHID_STOP:
+  This is sent when the HID device is stopped. Consider this as an answer to
+  UHID_DESTROY.
+  If you didn't destroy your device via UHID_DESTROY, but the kernel sends an
+  UHID_STOP event, this should usually be ignored. It means that the kernel
+  reloaded/changed the device driver loaded on your HID device (or some other
+  maintenance actions happened).
+  You can usually ignored any UHID_STOP events safely.
+
+  UHID_OPEN:
+  This is sent when the HID device is opened. That is, the data that the HID
+  device provides is read by some other process. You may ignore this event but
+  it is useful for power-management. As long as you haven't received this event
+  there is actually no other process that reads your data so there is no need to
+  send UHID_INPUT2 events to the kernel.
+
+  UHID_CLOSE:
+  This is sent when there are no more processes which read the HID data. It is
+  the counterpart of UHID_OPEN and you may as well ignore this event.
+
+  UHID_OUTPUT:
+  This is sent if the HID device driver wants to send raw data to the I/O
+  device on the interrupt channel. You should read the payload and forward it to
+  the device. The payload is of type "struct uhid_data_req".
+  This may be received even though you haven't received UHID_OPEN, yet.
+
+  UHID_GET_REPORT:
+  This event is sent if the kernel driver wants to perform a GET_REPORT request
+  on the control channeld as described in the HID specs. The report-type and
+  report-number are available in the payload.
+  The kernel serializes GET_REPORT requests so there will never be two in
+  parallel. However, if you fail to respond with a UHID_GET_REPORT_REPLY, the
+  request might silently time out.
+  Once you read a GET_REPORT request, you shall forward it to the hid device and
+  remember the "id" field in the payload. Once your hid device responds to the
+  GET_REPORT (or if it fails), you must send a UHID_GET_REPORT_REPLY to the
+  kernel with the exact same "id" as in the request. If the request already
+  timed out, the kernel will ignore the response silently. The "id" field is
+  never re-used, so conflicts cannot happen.
+
+  UHID_SET_REPORT:
+  This is the SET_REPORT equivalent of UHID_GET_REPORT. On receipt, you shall
+  send a SET_REPORT request to your hid device. Once it replies, you must tell
+  the kernel about it via UHID_SET_REPORT_REPLY.
+  The same restrictions as for UHID_GET_REPORT apply.
+
+----------------------------------------------------
+Written 2012, David Herrmann <dh.herrmann@gmail.com>