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