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diff --git a/Documentation/hid/uhid.txt b/Documentation/hid/uhid.txt new file mode 100644 index 0000000..4627c42 --- /dev/null +++ b/Documentation/hid/uhid.txt @@ -0,0 +1,169 @@ + 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> |
