1 =========================
2 HID I/O Transport Drivers
3 =========================
4
5 The HID subsystem is independent of the underlying transport driver. Initially,
6 only USB was supported, but other specifications adopted the HID design and
7 provided new transport drivers. The kernel includes at least support for USB,
8 Bluetooth, I2C and user-space I/O drivers.
9
10 1) HID Bus
11 ==========
12
13 The HID subsystem is designed as a bus. Any I/O subsystem may provide HID
14 devices and register them with the HID bus. HID core then loads generic device
15 drivers on top of it. The transport drivers are responsible for raw data
16 transport and device setup/management. HID core is responsible for
17 report-parsing, report interpretation and the user-space API. Device specifics
18 and quirks are handled by all layers depending on the quirk.
19
20 ::
21
22 +-----------+ +-----------+ +-----------+ +-----------+
23 | Device #1 | | Device #i | | Device #j | | Device #k |
24 +-----------+ +-----------+ +-----------+ +-----------+
25 \\ // \\ //
26 +------------+ +------------+
27 | I/O Driver | | I/O Driver |
28 +------------+ +------------+
29 || ||
30 +------------------+ +------------------+
31 | Transport Driver | | Transport Driver |
32 +------------------+ +------------------+
33 \___ ___/
34 \ /
35 +----------------+
36 | HID Core |
37 +----------------+
38 / | | \
39 / | | \
40 ____________/ | | \_________________
41 / | | \
42 / | | \
43 +----------------+ +-----------+ +------------------+ +------------------+
44 | Generic Driver | | MT Driver | | Custom Driver #1 | | Custom Driver #2 |
45 +----------------+ +-----------+ +------------------+ +------------------+
46
47 Example Drivers:
48
49 - I/O: USB, I2C, Bluetooth-l2cap
50 - Transport: USB-HID, I2C-HID, BT-HIDP
51
52 Everything below "HID Core" is simplified in this graph as it is only of
53 interest to HID device drivers. Transport drivers do not need to know the
54 specifics.
55
56 1.1) Device Setup
57 -----------------
58
59 I/O drivers normally provide hotplug detection or device enumeration APIs to the
60 transport drivers. Transport drivers use this to find any suitable HID device.
61 They allocate HID device objects and register them with HID core. Transport
62 drivers are not required to register themselves with HID core. HID core is never
63 aware of which transport drivers are available and is not interested in it. It
64 is only interested in devices.
65
66 Transport drivers attach a constant "struct hid_ll_driver" object with each
67 device. Once a device is registered with HID core, the callbacks provided via
68 this struct are used by HID core to communicate with the device.
69
70 Transport drivers are responsible for detecting device failures and unplugging.
71 HID core will operate a device as long as it is registered regardless of any
72 device failures. Once transport drivers detect unplug or failure events, they
73 must unregister the device from HID core and HID core will stop using the
74 provided callbacks.
75
76 1.2) Transport Driver Requirements
77 ----------------------------------
78
79 The terms "asynchronous" and "synchronous" in this document describe the
80 transmission behavior regarding acknowledgements. An asynchronous channel must
81 not perform any synchronous operations like waiting for acknowledgements or
82 verifications. Generally, HID calls operating on asynchronous channels must be
83 running in atomic-context just fine.
84 On the other hand, synchronous channels can be implemented by the transport
85 driver in whatever way they like. They might just be the same as asynchronous
86 channels, but they can also provide acknowledgement reports, automatic
87 retransmission on failure, etc. in a blocking manner. If such functionality is
88 required on asynchronous channels, a transport-driver must implement that via
89 its own worker threads.
90
91 HID core requires transport drivers to follow a given design. A Transport
92 driver must provide two bi-directional I/O channels to each HID device. These
93 channels must not necessarily be bi-directional in the hardware itself. A
94 transport driver might just provide 4 uni-directional channels. Or it might
95 multiplex all four on a single physical channel. However, in this document we
96 will describe them as two bi-directional channels as they have several
97 properties in common.
98
99 - Interrupt Channel (intr): The intr channel is used for asynchronous data
100 reports. No management commands or data acknowledgements are sent on this
101 channel. Any unrequested incoming or outgoing data report must be sent on
102 this channel and is never acknowledged by the remote side. Devices usually
103 send their input events on this channel. Outgoing events are normally
104 not sent via intr, except if high throughput is required.
105 - Control Channel (ctrl): The ctrl channel is used for synchronous requests and
106 device management. Unrequested data input events must not be sent on this
107 channel and are normally ignored. Instead, devices only send management
108 events or answers to host requests on this channel.
109 The control-channel is used for direct blocking queries to the device
110 independent of any events on the intr-channel.
111 Outgoing reports are usually sent on the ctrl channel via synchronous
112 SET_REPORT requests.
113
114 Communication between devices and HID core is mostly done via HID reports. A
115 report can be of one of three types:
116
117 - INPUT Report: Input reports provide data from device to host. This
118 data may include button events, axis events, battery status or more. This
119 data is generated by the device and sent to the host with or without
120 requiring explicit requests. Devices can choose to send data continuously or
121 only on change.
122 - OUTPUT Report: Output reports change device states. They are sent from host
123 to device and may include LED requests, rumble requests or more. Output
124 reports are never sent from device to host, but a host can retrieve their
125 current state.
126 Hosts may choose to send output reports either continuously or only on
127 change.
128 - FEATURE Report: Feature reports are used for specific static device features
129 and never reported spontaneously. A host can read and/or write them to access
130 data like battery-state or device-settings.
131 Feature reports are never sent without requests. A host must explicitly set
132 or retrieve a feature report. This also means, feature reports are never sent
133 on the intr channel as this channel is asynchronous.
134
135 INPUT and OUTPUT reports can be sent as pure data reports on the intr channel.
136 For INPUT reports this is the usual operational mode. But for OUTPUT reports,
137 this is rarely done as OUTPUT reports are normally quite scarce. But devices are
138 free to make excessive use of asynchronous OUTPUT reports (for instance, custom
139 HID audio speakers make great use of it).
140
141 Plain reports must not be sent on the ctrl channel, though. Instead, the ctrl
142 channel provides synchronous GET/SET_REPORT requests. Plain reports are only
143 allowed on the intr channel and are the only means of data there.
144
145 - GET_REPORT: A GET_REPORT request has a report ID as payload and is sent
146 from host to device. The device must answer with a data report for the
147 requested report ID on the ctrl channel as a synchronous acknowledgement.
148 Only one GET_REPORT request can be pending for each device. This restriction
149 is enforced by HID core as several transport drivers don't allow multiple
150 simultaneous GET_REPORT requests.
151 Note that data reports which are sent as answer to a GET_REPORT request are
152 not handled as generic device events. That is, if a device does not operate
153 in continuous data reporting mode, an answer to GET_REPORT does not replace
154 the raw data report on the intr channel on state change.
155 GET_REPORT is only used by custom HID device drivers to query device state.
156 Normally, HID core caches any device state so this request is not necessary
157 on devices that follow the HID specs except during device initialization to
158 retrieve the current state.
159 GET_REPORT requests can be sent for any of the 3 report types and shall
160 return the current report state of the device. However, OUTPUT reports as
161 payload may be blocked by the underlying transport driver if the
162 specification does not allow them.
163 - SET_REPORT: A SET_REPORT request has a report ID plus data as payload. It is
164 sent from host to device and a device must update its current report state
165 according to the given data. Any of the 3 report types can be used. However,
166 INPUT reports as payload might be blocked by the underlying transport driver
167 if the specification does not allow them.
168 A device must answer with a synchronous acknowledgement. However, HID core
169 does not require transport drivers to forward this acknowledgement to HID
170 core.
171 Same as for GET_REPORT, only one SET_REPORT can be pending at a time. This
172 restriction is enforced by HID core as some transport drivers do not support
173 multiple synchronous SET_REPORT requests.
174
175 Other ctrl-channel requests are supported by USB-HID but are not available
176 (or deprecated) in most other transport level specifications:
177
178 - GET/SET_IDLE: Only used by USB-HID and I2C-HID.
179 - GET/SET_PROTOCOL: Not used by HID core.
180 - RESET: Used by I2C-HID, not hooked up in HID core.
181 - SET_POWER: Used by I2C-HID, not hooked up in HID core.
182
183 2) HID API
184 ==========
185
186 2.1) Initialization
187 -------------------
188
189 Transport drivers normally use the following procedure to register a new device
190 with HID core::
191
192 struct hid_device *hid;
193 int ret;
194
195 hid = hid_allocate_device();
196 if (IS_ERR(hid)) {
197 ret = PTR_ERR(hid);
198 goto err_<...>;
199 }
200
201 strscpy(hid->name, <device-name-src>, sizeof(hid->name));
202 strscpy(hid->phys, <device-phys-src>, sizeof(hid->phys));
203 strscpy(hid->uniq, <device-uniq-src>, sizeof(hid->uniq));
204
205 hid->ll_driver = &custom_ll_driver;
206 hid->bus = <device-bus>;
207 hid->vendor = <device-vendor>;
208 hid->product = <device-product>;
209 hid->version = <device-version>;
210 hid->country = <device-country>;
211 hid->dev.parent = <pointer-to-parent-device>;
212 hid->driver_data = <transport-driver-data-field>;
213
214 ret = hid_add_device(hid);
215 if (ret)
216 goto err_<...>;
217
218 Once hid_add_device() is entered, HID core might use the callbacks provided in
219 "custom_ll_driver". Note that fields like "country" can be ignored by underlying
220 transport-drivers if not supported.
221
222 To unregister a device, use::
223
224 hid_destroy_device(hid);
225
226 Once hid_destroy_device() returns, HID core will no longer make use of any
227 driver callbacks.
228
229 2.2) hid_ll_driver operations
230 -----------------------------
231
232 The available HID callbacks are:
233
234 ::
235
236 int (*start) (struct hid_device *hdev)
237
238 Called from HID device drivers once they want to use the device. Transport
239 drivers can choose to setup their device in this callback. However, normally
240 devices are already set up before transport drivers register them to HID core
241 so this is mostly only used by USB-HID.
242
243 ::
244
245 void (*stop) (struct hid_device *hdev)
246
247 Called from HID device drivers once they are done with a device. Transport
248 drivers can free any buffers and deinitialize the device. But note that
249 ->start() might be called again if another HID device driver is loaded on the
250 device.
251
252 Transport drivers are free to ignore it and deinitialize devices after they
253 destroyed them via hid_destroy_device().
254
255 ::
256
257 int (*open) (struct hid_device *hdev)
258
259 Called from HID device drivers once they are interested in data reports.
260 Usually, while user-space didn't open any input API/etc., device drivers are
261 not interested in device data and transport drivers can put devices asleep.
262 However, once ->open() is called, transport drivers must be ready for I/O.
263 ->open() calls are nested for each client that opens the HID device.
264
265 ::
266
267 void (*close) (struct hid_device *hdev)
268
269 Called from HID device drivers after ->open() was called but they are no
270 longer interested in device reports. (Usually if user-space closed any input
271 devices of the driver).
272
273 Transport drivers can put devices asleep and terminate any I/O of all
274 ->open() calls have been followed by a ->close() call. However, ->start() may
275 be called again if the device driver is interested in input reports again.
276
277 ::
278
279 int (*parse) (struct hid_device *hdev)
280
281 Called once during device setup after ->start() has been called. Transport
282 drivers must read the HID report-descriptor from the device and tell HID core
283 about it via hid_parse_report().
284
285 ::
286
287 int (*power) (struct hid_device *hdev, int level)
288
289 Called by HID core to give PM hints to transport drivers. Usually this is
290 analogical to the ->open() and ->close() hints and redundant.
291
292 ::
293
294 void (*request) (struct hid_device *hdev, struct hid_report *report,
295 int reqtype)
296
297 Send a HID request on the ctrl channel. "report" contains the report that
298 should be sent and "reqtype" the request type. Request-type can be
299 HID_REQ_SET_REPORT or HID_REQ_GET_REPORT.
300
301 This callback is optional. If not provided, HID core will assemble a raw
302 report following the HID specs and send it via the ->raw_request() callback.
303 The transport driver is free to implement this asynchronously.
304
305 ::
306
307 int (*wait) (struct hid_device *hdev)
308
309 Used by HID core before calling ->request() again. A transport driver can use
310 it to wait for any pending requests to complete if only one request is
311 allowed at a time.
312
313 ::
314
315 int (*raw_request) (struct hid_device *hdev, unsigned char reportnum,
316 __u8 *buf, size_t count, unsigned char rtype,
317 int reqtype)
318
319 Same as ->request() but provides the report as raw buffer. This request shall
320 be synchronous. A transport driver must not use ->wait() to complete such
321 requests. This request is mandatory and hid core will reject the device if
322 it is missing.
323
324 ::
325
326 int (*output_report) (struct hid_device *hdev, __u8 *buf, size_t len)
327
328 Send raw output report via intr channel. Used by some HID device drivers
329 which require high throughput for outgoing requests on the intr channel. This
330 must not cause SET_REPORT calls! This must be implemented as asynchronous
331 output report on the intr channel!
332
333 ::
334
335 int (*idle) (struct hid_device *hdev, int report, int idle, int reqtype)
336
337 Perform SET/GET_IDLE request. Only used by USB-HID, do not implement!
338
339 2.3) Data Path
340 --------------
341
342 Transport drivers are responsible of reading data from I/O devices. They must
343 handle any I/O-related state-tracking themselves. HID core does not implement
344 protocol handshakes or other management commands which can be required by the
345 given HID transport specification.
346
347 Every raw data packet read from a device must be fed into HID core via
348 hid_input_report(). You must specify the channel-type (intr or ctrl) and report
349 type (input/output/feature). Under normal conditions, only input reports are
350 provided via this API.
351
352 Responses to GET_REPORT requests via ->request() must also be provided via this
353 API. Responses to ->raw_request() are synchronous and must be intercepted by the
354 transport driver and not passed to hid_input_report().
355 Acknowledgements to SET_REPORT requests are not of interest to HID core.
356
357 ----------------------------------------------------
358
359 Written 2013, David Herrmann <dh.herrmann@gmail.com>
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