1 HID I/O Transport Drivers 2 =========================== 3 4The HID subsystem is independent of the underlying transport driver. Initially, 5only USB was supported, but other specifications adopted the HID design and 6provided new transport drivers. The kernel includes at least support for USB, 7Bluetooth, I2C and user-space I/O drivers. 8 91) HID Bus 10========== 11 12The HID subsystem is designed as a bus. Any I/O subsystem may provide HID 13devices and register them with the HID bus. HID core then loads generic device 14drivers on top of it. The transport drivers are responsible of raw data 15transport and device setup/management. HID core is responsible of 16report-parsing, report interpretation and the user-space API. Device specifics 17and quirks are handled by all layers depending on the quirk. 18 19 +-----------+ +-----------+ +-----------+ +-----------+ 20 | Device #1 | | Device #i | | Device #j | | Device #k | 21 +-----------+ +-----------+ +-----------+ +-----------+ 22 \\ // \\ // 23 +------------+ +------------+ 24 | I/O Driver | | I/O Driver | 25 +------------+ +------------+ 26 || || 27 +------------------+ +------------------+ 28 | Transport Driver | | Transport Driver | 29 +------------------+ +------------------+ 30 \___ ___/ 31 \ / 32 +----------------+ 33 | HID Core | 34 +----------------+ 35 / | | \ 36 / | | \ 37 ____________/ | | \_________________ 38 / | | \ 39 / | | \ 40 +----------------+ +-----------+ +------------------+ +------------------+ 41 | Generic Driver | | MT Driver | | Custom Driver #1 | | Custom Driver #2 | 42 +----------------+ +-----------+ +------------------+ +------------------+ 43 44Example Drivers: 45 I/O: USB, I2C, Bluetooth-l2cap 46 Transport: USB-HID, I2C-HID, BT-HIDP 47 48Everything below "HID Core" is simplified in this graph as it is only of 49interest to HID device drivers. Transport drivers do not need to know the 50specifics. 51 521.1) Device Setup 53----------------- 54 55I/O drivers normally provide hotplug detection or device enumeration APIs to the 56transport drivers. Transport drivers use this to find any suitable HID device. 57They allocate HID device objects and register them with HID core. Transport 58drivers are not required to register themselves with HID core. HID core is never 59aware of which transport drivers are available and is not interested in it. It 60is only interested in devices. 61 62Transport drivers attach a constant "struct hid_ll_driver" object with each 63device. Once a device is registered with HID core, the callbacks provided via 64this struct are used by HID core to communicate with the device. 65 66Transport drivers are responsible of detecting device failures and unplugging. 67HID core will operate a device as long as it is registered regardless of any 68device failures. Once transport drivers detect unplug or failure events, they 69must unregister the device from HID core and HID core will stop using the 70provided callbacks. 71 721.2) Transport Driver Requirements 73---------------------------------- 74 75The terms "asynchronous" and "synchronous" in this document describe the 76transmission behavior regarding acknowledgements. An asynchronous channel must 77not perform any synchronous operations like waiting for acknowledgements or 78verifications. Generally, HID calls operating on asynchronous channels must be 79running in atomic-context just fine. 80On the other hand, synchronous channels can be implemented by the transport 81driver in whatever way they like. They might just be the same as asynchronous 82channels, but they can also provide acknowledgement reports, automatic 83retransmission on failure, etc. in a blocking manner. If such functionality is 84required on asynchronous channels, a transport-driver must implement that via 85its own worker threads. 86 87HID core requires transport drivers to follow a given design. A Transport 88driver must provide two bi-directional I/O channels to each HID device. These 89channels must not necessarily be bi-directional in the hardware itself. A 90transport driver might just provide 4 uni-directional channels. Or it might 91multiplex all four on a single physical channel. However, in this document we 92will describe them as two bi-directional channels as they have several 93properties in common. 94 95 - Interrupt Channel (intr): The intr channel is used for asynchronous data 96 reports. No management commands or data acknowledgements are sent on this 97 channel. Any unrequested incoming or outgoing data report must be sent on 98 this channel and is never acknowledged by the remote side. Devices usually 99 send their input events on this channel. Outgoing events are normally 100 not send via intr, except if high throughput is required. 101 - Control Channel (ctrl): The ctrl channel is used for synchronous requests and 102 device management. Unrequested data input events must not be sent on this 103 channel and are normally ignored. Instead, devices only send management 104 events or answers to host requests on this channel. 105 The control-channel is used for direct blocking queries to the device 106 independent of any events on the intr-channel. 107 Outgoing reports are usually sent on the ctrl channel via synchronous 108 SET_REPORT requests. 109 110Communication between devices and HID core is mostly done via HID reports. A 111report can be of one of three types: 112 113 - INPUT Report: Input reports provide data from device to host. This 114 data may include button events, axis events, battery status or more. This 115 data is generated by the device and sent to the host with or without 116 requiring explicit requests. Devices can choose to send data continuously or 117 only on change. 118 - OUTPUT Report: Output reports change device states. They are sent from host 119 to device and may include LED requests, rumble requests or more. Output 120 reports are never sent from device to host, but a host can retrieve their 121 current state. 122 Hosts may choose to send output reports either continuously or only on 123 change. 124 - FEATURE Report: Feature reports are used for specific static device features 125 and never reported spontaneously. A host can read and/or write them to access 126 data like battery-state or device-settings. 127 Feature reports are never sent without requests. A host must explicitly set 128 or retrieve a feature report. This also means, feature reports are never sent 129 on the intr channel as this channel is asynchronous. 130 131INPUT and OUTPUT reports can be sent as pure data reports on the intr channel. 132For INPUT reports this is the usual operational mode. But for OUTPUT reports, 133this is rarely done as OUTPUT reports are normally quite scarce. But devices are 134free to make excessive use of asynchronous OUTPUT reports (for instance, custom 135HID audio speakers make great use of it). 136 137Plain reports must not be sent on the ctrl channel, though. Instead, the ctrl 138channel provides synchronous GET/SET_REPORT requests. Plain reports are only 139allowed on the intr channel and are the only means of data there. 140 141 - GET_REPORT: A GET_REPORT request has a report ID as payload and is sent 142 from host to device. The device must answer with a data report for the 143 requested report ID on the ctrl channel as a synchronous acknowledgement. 144 Only one GET_REPORT request can be pending for each device. This restriction 145 is enforced by HID core as several transport drivers don't allow multiple 146 simultaneous GET_REPORT requests. 147 Note that data reports which are sent as answer to a GET_REPORT request are 148 not handled as generic device events. That is, if a device does not operate 149 in continuous data reporting mode, an answer to GET_REPORT does not replace 150 the raw data report on the intr channel on state change. 151 GET_REPORT is only used by custom HID device drivers to query device state. 152 Normally, HID core caches any device state so this request is not necessary 153 on devices that follow the HID specs except during device initialization to 154 retrieve the current state. 155 GET_REPORT requests can be sent for any of the 3 report types and shall 156 return the current report state of the device. However, OUTPUT reports as 157 payload may be blocked by the underlying transport driver if the 158 specification does not allow them. 159 - SET_REPORT: A SET_REPORT request has a report ID plus data as payload. It is 160 sent from host to device and a device must update it's current report state 161 according to the given data. Any of the 3 report types can be used. However, 162 INPUT reports as payload might be blocked by the underlying transport driver 163 if the specification does not allow them. 164 A device must answer with a synchronous acknowledgement. However, HID core 165 does not require transport drivers to forward this acknowledgement to HID 166 core. 167 Same as for GET_REPORT, only one SET_REPORT can be pending at a time. This 168 restriction is enforced by HID core as some transport drivers do not support 169 multiple synchronous SET_REPORT requests. 170 171Other ctrl-channel requests are supported by USB-HID but are not available 172(or deprecated) in most other transport level specifications: 173 174 - GET/SET_IDLE: Only used by USB-HID and I2C-HID. 175 - GET/SET_PROTOCOL: Not used by HID core. 176 - RESET: Used by I2C-HID, not hooked up in HID core. 177 - SET_POWER: Used by I2C-HID, not hooked up in HID core. 178 1792) HID API 180========== 181 1822.1) Initialization 183------------------- 184 185Transport drivers normally use the following procedure to register a new device 186with HID core: 187 188 struct hid_device *hid; 189 int ret; 190 191 hid = hid_allocate_device(); 192 if (IS_ERR(hid)) { 193 ret = PTR_ERR(hid); 194 goto err_<...>; 195 } 196 197 strlcpy(hid->name, <device-name-src>, 127); 198 strlcpy(hid->phys, <device-phys-src>, 63); 199 strlcpy(hid->uniq, <device-uniq-src>, 63); 200 201 hid->ll_driver = &custom_ll_driver; 202 hid->bus = <device-bus>; 203 hid->vendor = <device-vendor>; 204 hid->product = <device-product>; 205 hid->version = <device-version>; 206 hid->country = <device-country>; 207 hid->dev.parent = <pointer-to-parent-device>; 208 hid->driver_data = <transport-driver-data-field>; 209 210 ret = hid_add_device(hid); 211 if (ret) 212 goto err_<...>; 213 214Once hid_add_device() is entered, HID core might use the callbacks provided in 215"custom_ll_driver". Note that fields like "country" can be ignored by underlying 216transport-drivers if not supported. 217 218To unregister a device, use: 219 220 hid_destroy_device(hid); 221 222Once hid_destroy_device() returns, HID core will no longer make use of any 223driver callbacks. 224 2252.2) hid_ll_driver operations 226----------------------------- 227 228The available HID callbacks are: 229 - int (*start) (struct hid_device *hdev) 230 Called from HID device drivers once they want to use the device. Transport 231 drivers can choose to setup their device in this callback. However, normally 232 devices are already set up before transport drivers register them to HID core 233 so this is mostly only used by USB-HID. 234 235 - void (*stop) (struct hid_device *hdev) 236 Called from HID device drivers once they are done with a device. Transport 237 drivers can free any buffers and deinitialize the device. But note that 238 ->start() might be called again if another HID device driver is loaded on the 239 device. 240 Transport drivers are free to ignore it and deinitialize devices after they 241 destroyed them via hid_destroy_device(). 242 243 - int (*open) (struct hid_device *hdev) 244 Called from HID device drivers once they are interested in data reports. 245 Usually, while user-space didn't open any input API/etc., device drivers are 246 not interested in device data and transport drivers can put devices asleep. 247 However, once ->open() is called, transport drivers must be ready for I/O. 248 ->open() calls are nested for each client that opens the HID device. 249 250 - void (*close) (struct hid_device *hdev) 251 Called from HID device drivers after ->open() was called but they are no 252 longer interested in device reports. (Usually if user-space closed any input 253 devices of the driver). 254 Transport drivers can put devices asleep and terminate any I/O of all 255 ->open() calls have been followed by a ->close() call. However, ->start() may 256 be called again if the device driver is interested in input reports again. 257 258 - int (*parse) (struct hid_device *hdev) 259 Called once during device setup after ->start() has been called. Transport 260 drivers must read the HID report-descriptor from the device and tell HID core 261 about it via hid_parse_report(). 262 263 - int (*power) (struct hid_device *hdev, int level) 264 Called by HID core to give PM hints to transport drivers. Usually this is 265 analogical to the ->open() and ->close() hints and redundant. 266 267 - void (*request) (struct hid_device *hdev, struct hid_report *report, 268 int reqtype) 269 Send an HID request on the ctrl channel. "report" contains the report that 270 should be sent and "reqtype" the request type. Request-type can be 271 HID_REQ_SET_REPORT or HID_REQ_GET_REPORT. 272 This callback is optional. If not provided, HID core will assemble a raw 273 report following the HID specs and send it via the ->raw_request() callback. 274 The transport driver is free to implement this asynchronously. 275 276 - int (*wait) (struct hid_device *hdev) 277 Used by HID core before calling ->request() again. A transport driver can use 278 it to wait for any pending requests to complete if only one request is 279 allowed at a time. 280 281 - int (*raw_request) (struct hid_device *hdev, unsigned char reportnum, 282 __u8 *buf, size_t count, unsigned char rtype, 283 int reqtype) 284 Same as ->request() but provides the report as raw buffer. This request shall 285 be synchronous. A transport driver must not use ->wait() to complete such 286 requests. This request is mandatory and hid core will reject the device if 287 it is missing. 288 289 - int (*output_report) (struct hid_device *hdev, __u8 *buf, size_t len) 290 Send raw output report via intr channel. Used by some HID device drivers 291 which require high throughput for outgoing requests on the intr channel. This 292 must not cause SET_REPORT calls! This must be implemented as asynchronous 293 output report on the intr channel! 294 295 - int (*idle) (struct hid_device *hdev, int report, int idle, int reqtype) 296 Perform SET/GET_IDLE request. Only used by USB-HID, do not implement! 297 2982.3) Data Path 299-------------- 300 301Transport drivers are responsible of reading data from I/O devices. They must 302handle any I/O-related state-tracking themselves. HID core does not implement 303protocol handshakes or other management commands which can be required by the 304given HID transport specification. 305 306Every raw data packet read from a device must be fed into HID core via 307hid_input_report(). You must specify the channel-type (intr or ctrl) and report 308type (input/output/feature). Under normal conditions, only input reports are 309provided via this API. 310 311Responses to GET_REPORT requests via ->request() must also be provided via this 312API. Responses to ->raw_request() are synchronous and must be intercepted by the 313transport driver and not passed to hid_input_report(). 314Acknowledgements to SET_REPORT requests are not of interest to HID core. 315 316---------------------------------------------------- 317Written 2013, David Herrmann <dh.herrmann@gmail.com> 318
