HID: uhid: update documentation

      UHID - User-space I/O driver support for HID subsystem

     ========================================================

The HID subsystem needs two kinds of drivers. In this document we call them:

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.

 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.

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

struct uhid_event {

        __u32 type;

        union {

                struct uhid_create_req create;

                struct uhid_data_req data;

                struct uhid_create2_req create2;

                struct uhid_output_req output;

                struct uhid_input2_req input2;

                ...

        } u;

};

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_CREATE event. This will

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.

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 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_CREATE again to register a new

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 following types are supported: UHID_CREATE, UHID_DESTROY and

UHID_INPUT. The kernel will parse the event immediately and if the event ID is

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.

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_CREATE:

  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_create_req and

  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_CREATE2:

  Same as UHID_CREATE, but the HID report descriptor data (rd_data) is an array

  inside struct uhid_create2_req, instead of a pointer to a separate array.

  Enables use from languages that don't support pointers, e.g. Python.

  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

  You can create a new device by sending UHID_CREATE2 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_INPUT2:

  Same as UHID_INPUT, but the data array is the last field of uhid_input2_req.

  Enables userspace to write only the required bytes to kernel (ev.type +

  ev.u.input2.size + the part of the data array that matters), instead of

  the entire struct uhid_input2_req.

  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 occurred or to EIO if an I/O error occurred.

  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 feature request and set "size" correspondingly.

  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. 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.

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_CREATE. This is always the first event that is sent.

  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 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.

  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_INPUT events to the kernel.

  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

  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".

  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_OUTPUT_EV (obsolete):

  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.

  This is no longer sent by newer kernels. Instead, HID core converts it into a

  raw output report and sends it via 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>

  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>