HID: uhid: add documentation

This describes the protocol used by uhid for user-space applications. It
describes the details like non-blocking I/O and readv/writev for multiple
events per syscall.
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/hid/uhid.txt

+      UHID - User-space I/O driver support for HID subsystem
+     ========================================================
+
+The HID subsystem needs two kinds of drivers. In this document we call them:
+
+ 1. The "HID I/O Driver" is the driver that performs raw data I/O to the
+    low-level device. Internally, they register an hid_ll_driver structure with
+    the HID core. They perform device setup, read raw data from the device and
+    push it into the HID subsystem and they provide a callback so the HID
+    subsystem can send data to the device.
+
+ 2. The "HID Device Driver" is the driver that parses HID reports and reacts on
+    them. There are generic drivers like "generic-usb" and "generic-bluetooth"
+    which adhere to the HID specification and provide the standardizes features.
+    But there may be special drivers and quirks for each non-standard device out
+    there. Internally, they use the hid_driver structure.
+
+Historically, the USB stack was the first subsystem to provide an HID I/O
+Driver. However, other standards like Bluetooth have adopted the HID specs and
+may provide HID I/O Drivers, too. The UHID driver allows to implement HID I/O
+Drivers in user-space and feed the data into the kernel HID-subsystem.
+
+This allows user-space to operate on the same level as USB-HID, Bluetooth-HID
+and similar. It does not provide a way to write HID Device Drivers, though. Use
+hidraw for this purpose.
+
+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_create_req create;
+                struct uhid_data_req data;
+                ...
+        } 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 first thing you should do is sending an UHID_CREATE 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 to the HID subsystem, you send an HID_INPUT event with
+your raw data payload. If the kernel wants to send data to the device, you will
+read an UHID_OUTPUT or UHID_OUTPUT_EV event.
+
+If your device disconnects, you should send an UHID_DESTROY event. This will
+unregister the device. You can now send UHID_CREATE 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 following types are supported: UHID_CREATE, UHID_DESTROY and
+UHID_INPUT. 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.
+
+  UHID_CREATE:
+  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_create_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_CREATE again. There is no need to
+  reopen the character device.
+
+  UHID_INPUT:
+  You must send UHID_CREATE before sending input to the kernel! This event
+  contains a data-payload. This is the raw data that you read from your device.
+  The kernel will parse the HID reports and react on it.
+
+  UHID_FEATURE_ANSWER:
+  If you receive a UHID_FEATURE 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 occured 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 feature request and set "size" correspondingly.
+
+read()
+------
+read() will return a queued ouput report. These output reports can be of type
+UHID_START, UHID_STOP, UHID_OPEN, UHID_CLOSE, UHID_OUTPUT or UHID_OUTPUT_EV. No
+reaction is required to any of them but you should handle them according to your
+needs. Only UHID_OUTPUT and UHID_OUTPUT_EV have payloads.
+
+  UHID_START:
+  This is sent when the HID device is started. Consider this as an answer to
+  UHID_CREATE. This is always the first event that is sent.
+
+  UHID_STOP:
+  This is sent when the HID device is stopped. Consider this as an answer to
+  UHID_DESTROY.
+  If the kernel HID device driver closes the device manually (that is, you
+  didn't send UHID_DESTROY) then you should consider this device closed and send
+  an UHID_DESTROY event. You may want to reregister your device, though. This is
+  always the last message that is sent to you unless you reopen the device with
+  UHID_CREATE.
+
+  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_INPUT 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. 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_OUTPUT_EV:
+  Same as UHID_OUTPUT but this contains a "struct input_event" as payload. This
+  is called for force-feedback, LED or similar events which are received through
+  an input device by the HID subsystem. You should convert this into raw reports
+  and send them to your device similar to events of type UHID_OUTPUT.
+
+  UHID_FEATURE:
+  This event is sent if the kernel driver wants to perform a feature request as
+  described in the HID specs. The report-type and report-number are available in
+  the payload.
+  The kernel serializes feature requests so there will never be two in parallel.
+  However, if you fail to respond with a UHID_FEATURE_ANSWER in a time-span of 5
+  seconds, then the requests will be dropped and a new one might be sent.
+  Therefore, the payload also contains an "id" field that identifies every
+  request.
+
+Document by:
+  David Herrmann <dh.herrmann@googlemail.com>