Documentation / hid / uhid.txt
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Based on kernel version 3.9. Page generated on 2013-05-02 23:07 EST.
1 UHID - User-space I/O driver support for HID subsystem 2 ======================================================== 3 4 The HID subsystem needs two kinds of drivers. In this document we call them: 5 6 1. The "HID I/O Driver" is the driver that performs raw data I/O to the 7 low-level device. Internally, they register an hid_ll_driver structure with 8 the HID core. They perform device setup, read raw data from the device and 9 push it into the HID subsystem and they provide a callback so the HID 10 subsystem can send data to the device. 11 12 2. The "HID Device Driver" is the driver that parses HID reports and reacts on 13 them. There are generic drivers like "generic-usb" and "generic-bluetooth" 14 which adhere to the HID specification and provide the standardizes features. 15 But there may be special drivers and quirks for each non-standard device out 16 there. Internally, they use the hid_driver structure. 17 18 Historically, the USB stack was the first subsystem to provide an HID I/O 19 Driver. However, other standards like Bluetooth have adopted the HID specs and 20 may provide HID I/O Drivers, too. The UHID driver allows to implement HID I/O 21 Drivers in user-space and feed the data into the kernel HID-subsystem. 22 23 This allows user-space to operate on the same level as USB-HID, Bluetooth-HID 24 and similar. It does not provide a way to write HID Device Drivers, though. Use 25 hidraw for this purpose. 26 27 There is an example user-space application in ./samples/uhid/uhid-example.c 28 29 The UHID API 30 ------------ 31 32 UHID is accessed through a character misc-device. The minor-number is allocated 33 dynamically so you need to rely on udev (or similar) to create the device node. 34 This is /dev/uhid by default. 35 36 If a new device is detected by your HID I/O Driver and you want to register this 37 device with the HID subsystem, then you need to open /dev/uhid once for each 38 device you want to register. All further communication is done by read()'ing or 39 write()'ing "struct uhid_event" objects. Non-blocking operations are supported 40 by setting O_NONBLOCK. 41 42 struct uhid_event { 43 __u32 type; 44 union { 45 struct uhid_create_req create; 46 struct uhid_data_req data; 47 ... 48 } u; 49 }; 50 51 The "type" field contains the ID of the event. Depending on the ID different 52 payloads are sent. You must not split a single event across multiple read()'s or 53 multiple write()'s. A single event must always be sent as a whole. Furthermore, 54 only a single event can be sent per read() or write(). Pending data is ignored. 55 If you want to handle multiple events in a single syscall, then use vectored 56 I/O with readv()/writev(). 57 58 The first thing you should do is sending an UHID_CREATE event. This will 59 register the device. UHID will respond with an UHID_START event. You can now 60 start sending data to and reading data from UHID. However, unless UHID sends the 61 UHID_OPEN event, the internally attached HID Device Driver has no user attached. 62 That is, you might put your device asleep unless you receive the UHID_OPEN 63 event. If you receive the UHID_OPEN event, you should start I/O. If the last 64 user closes the HID device, you will receive an UHID_CLOSE event. This may be 65 followed by an UHID_OPEN event again and so on. There is no need to perform 66 reference-counting in user-space. That is, you will never receive multiple 67 UHID_OPEN events without an UHID_CLOSE event. The HID subsystem performs 68 ref-counting for you. 69 You may decide to ignore UHID_OPEN/UHID_CLOSE, though. I/O is allowed even 70 though the device may have no users. 71 72 If you want to send data to the HID subsystem, you send an HID_INPUT event with 73 your raw data payload. If the kernel wants to send data to the device, you will 74 read an UHID_OUTPUT or UHID_OUTPUT_EV event. 75 76 If your device disconnects, you should send an UHID_DESTROY event. This will 77 unregister the device. You can now send UHID_CREATE again to register a new 78 device. 79 If you close() the fd, the device is automatically unregistered and destroyed 80 internally. 81 82 write() 83 ------- 84 write() allows you to modify the state of the device and feed input data into 85 the kernel. The following types are supported: UHID_CREATE, UHID_DESTROY and 86 UHID_INPUT. The kernel will parse the event immediately and if the event ID is 87 not supported, it will return -EOPNOTSUPP. If the payload is invalid, then 88 -EINVAL is returned, otherwise, the amount of data that was read is returned and 89 the request was handled successfully. 90 91 UHID_CREATE: 92 This creates the internal HID device. No I/O is possible until you send this 93 event to the kernel. The payload is of type struct uhid_create_req and 94 contains information about your device. You can start I/O now. 95 96 UHID_DESTROY: 97 This destroys the internal HID device. No further I/O will be accepted. There 98 may still be pending messages that you can receive with read() but no further 99 UHID_INPUT events can be sent to the kernel. 100 You can create a new device by sending UHID_CREATE again. There is no need to 101 reopen the character device. 102 103 UHID_INPUT: 104 You must send UHID_CREATE before sending input to the kernel! This event 105 contains a data-payload. This is the raw data that you read from your device. 106 The kernel will parse the HID reports and react on it. 107 108 UHID_FEATURE_ANSWER: 109 If you receive a UHID_FEATURE request you must answer with this request. You 110 must copy the "id" field from the request into the answer. Set the "err" field 111 to 0 if no error occurred or to EIO if an I/O error occurred. 112 If "err" is 0 then you should fill the buffer of the answer with the results 113 of the feature request and set "size" correspondingly. 114 115 read() 116 ------ 117 read() will return a queued ouput report. These output reports can be of type 118 UHID_START, UHID_STOP, UHID_OPEN, UHID_CLOSE, UHID_OUTPUT or UHID_OUTPUT_EV. No 119 reaction is required to any of them but you should handle them according to your 120 needs. Only UHID_OUTPUT and UHID_OUTPUT_EV have payloads. 121 122 UHID_START: 123 This is sent when the HID device is started. Consider this as an answer to 124 UHID_CREATE. This is always the first event that is sent. 125 126 UHID_STOP: 127 This is sent when the HID device is stopped. Consider this as an answer to 128 UHID_DESTROY. 129 If the kernel HID device driver closes the device manually (that is, you 130 didn't send UHID_DESTROY) then you should consider this device closed and send 131 an UHID_DESTROY event. You may want to reregister your device, though. This is 132 always the last message that is sent to you unless you reopen the device with 133 UHID_CREATE. 134 135 UHID_OPEN: 136 This is sent when the HID device is opened. That is, the data that the HID 137 device provides is read by some other process. You may ignore this event but 138 it is useful for power-management. As long as you haven't received this event 139 there is actually no other process that reads your data so there is no need to 140 send UHID_INPUT events to the kernel. 141 142 UHID_CLOSE: 143 This is sent when there are no more processes which read the HID data. It is 144 the counterpart of UHID_OPEN and you may as well ignore this event. 145 146 UHID_OUTPUT: 147 This is sent if the HID device driver wants to send raw data to the I/O 148 device. You should read the payload and forward it to the device. The payload 149 is of type "struct uhid_data_req". 150 This may be received even though you haven't received UHID_OPEN, yet. 151 152 UHID_OUTPUT_EV: 153 Same as UHID_OUTPUT but this contains a "struct input_event" as payload. This 154 is called for force-feedback, LED or similar events which are received through 155 an input device by the HID subsystem. You should convert this into raw reports 156 and send them to your device similar to events of type UHID_OUTPUT. 157 158 UHID_FEATURE: 159 This event is sent if the kernel driver wants to perform a feature request as 160 described in the HID specs. The report-type and report-number are available in 161 the payload. 162 The kernel serializes feature requests so there will never be two in parallel. 163 However, if you fail to respond with a UHID_FEATURE_ANSWER in a time-span of 5 164 seconds, then the requests will be dropped and a new one might be sent. 165 Therefore, the payload also contains an "id" field that identifies every 166 request. 167 168 Document by: 169 David Herrmann <dh.herrmann@googlemail.com>
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