1================================================
2Care and feeding of your Human Interface Devices
3================================================
4
5Introduction
6============
7
8In addition to the normal input type HID devices, USB also uses the
9human interface device protocols for things that are not really human
10interfaces, but have similar sorts of communication needs. The two big
11examples for this are power devices (especially uninterruptible power
12supplies) and monitor control on higher end monitors.
13
14To support these disparate requirements, the Linux USB system provides
15HID events to two separate interfaces:
16* the input subsystem, which converts HID events into normal input
17device interfaces (such as keyboard, mouse and joystick) and a
18normalised event interface - see Documentation/input/input.rst
19* the hiddev interface, which provides fairly raw HID events
20
21The data flow for a HID event produced by a device is something like
22the following::
23
24 usb.c ---> hid-core.c  ----> hid-input.c ----> [keyboard/mouse/joystick/event]
25                         |
26                         |
27                          --> hiddev.c ----> POWER / MONITOR CONTROL
28
29In addition, other subsystems (apart from USB) can potentially feed
30events into the input subsystem, but these have no effect on the HID
31device interface.
32
33Using the HID Device Interface
34==============================
35
36The hiddev interface is a char interface using the normal USB major,
37with the minor numbers starting at 96 and finishing at 111. Therefore,
38you need the following commands::
39
40	mknod /dev/usb/hiddev0 c 180 96
41	mknod /dev/usb/hiddev1 c 180 97
42	mknod /dev/usb/hiddev2 c 180 98
43	mknod /dev/usb/hiddev3 c 180 99
44	mknod /dev/usb/hiddev4 c 180 100
45	mknod /dev/usb/hiddev5 c 180 101
46	mknod /dev/usb/hiddev6 c 180 102
47	mknod /dev/usb/hiddev7 c 180 103
48	mknod /dev/usb/hiddev8 c 180 104
49	mknod /dev/usb/hiddev9 c 180 105
50	mknod /dev/usb/hiddev10 c 180 106
51	mknod /dev/usb/hiddev11 c 180 107
52	mknod /dev/usb/hiddev12 c 180 108
53	mknod /dev/usb/hiddev13 c 180 109
54	mknod /dev/usb/hiddev14 c 180 110
55	mknod /dev/usb/hiddev15 c 180 111
56
57So you point your hiddev compliant user-space program at the correct
58interface for your device, and it all just works.
59
60Assuming that you have a hiddev compliant user-space program, of
61course. If you need to write one, read on.
62
63
64The HIDDEV API
65==============
66
67This description should be read in conjunction with the HID
68specification, freely available from https://www.usb.org, and
69conveniently linked of http://www.linux-usb.org.
70
71The hiddev API uses a read() interface, and a set of ioctl() calls.
72
73HID devices exchange data with the host computer using data
74bundles called "reports".  Each report is divided into "fields",
75each of which can have one or more "usages".  In the hid-core,
76each one of these usages has a single signed 32-bit value.
77
78read():
79-------
80
81This is the event interface.  When the HID device's state changes,
82it performs an interrupt transfer containing a report which contains
83the changed value.  The hid-core.c module parses the report, and
84returns to hiddev.c the individual usages that have changed within
85the report.  In its basic mode, the hiddev will make these individual
86usage changes available to the reader using a struct hiddev_event::
87
88       struct hiddev_event {
89           unsigned hid;
90           signed int value;
91       };
92
93containing the HID usage identifier for the status that changed, and
94the value that it was changed to. Note that the structure is defined
95within <linux/hiddev.h>, along with some other useful #defines and
96structures.  The HID usage identifier is a composite of the HID usage
97page shifted to the 16 high order bits ORed with the usage code.  The
98behavior of the read() function can be modified using the HIDIOCSFLAG
99ioctl() described below.
100
101
102ioctl():
103--------
104
105This is the control interface. There are a number of controls:
106
107HIDIOCGVERSION
108  - int (read)
109
110 Gets the version code out of the hiddev driver.
111
112HIDIOCAPPLICATION
113  - (none)
114
115This ioctl call returns the HID application usage associated with the
116HID device. The third argument to ioctl() specifies which application
117index to get. This is useful when the device has more than one
118application collection. If the index is invalid (greater or equal to
119the number of application collections this device has) the ioctl
120returns -1. You can find out beforehand how many application
121collections the device has from the num_applications field from the
122hiddev_devinfo structure.
123
124HIDIOCGCOLLECTIONINFO
125  - struct hiddev_collection_info (read/write)
126
127This returns a superset of the information above, providing not only
128application collections, but all the collections the device has.  It
129also returns the level the collection lives in the hierarchy.
130The user passes in a hiddev_collection_info struct with the index
131field set to the index that should be returned.  The ioctl fills in
132the other fields.  If the index is larger than the last collection
133index, the ioctl returns -1 and sets errno to -EINVAL.
134
135HIDIOCGDEVINFO
136  - struct hiddev_devinfo (read)
137
138Gets a hiddev_devinfo structure which describes the device.
139
140HIDIOCGSTRING
141  - struct hiddev_string_descriptor (read/write)
142
143Gets a string descriptor from the device. The caller must fill in the
144"index" field to indicate which descriptor should be returned.
145
146HIDIOCINITREPORT
147  - (none)
148
149Instructs the kernel to retrieve all input and feature report values
150from the device. At this point, all the usage structures will contain
151current values for the device, and will maintain it as the device
152changes.  Note that the use of this ioctl is unnecessary in general,
153since later kernels automatically initialize the reports from the
154device at attach time.
155
156HIDIOCGNAME
157  - string (variable length)
158
159Gets the device name
160
161HIDIOCGREPORT
162  - struct hiddev_report_info (write)
163
164Instructs the kernel to get a feature or input report from the device,
165in order to selectively update the usage structures (in contrast to
166INITREPORT).
167
168HIDIOCSREPORT
169  - struct hiddev_report_info (write)
170
171Instructs the kernel to send a report to the device. This report can
172be filled in by the user through HIDIOCSUSAGE calls (below) to fill in
173individual usage values in the report before sending the report in full
174to the device.
175
176HIDIOCGREPORTINFO
177  - struct hiddev_report_info (read/write)
178
179Fills in a hiddev_report_info structure for the user. The report is
180looked up by type (input, output or feature) and id, so these fields
181must be filled in by the user. The ID can be absolute -- the actual
182report id as reported by the device -- or relative --
183HID_REPORT_ID_FIRST for the first report, and (HID_REPORT_ID_NEXT |
184report_id) for the next report after report_id. Without a priori
185information about report ids, the right way to use this ioctl is to
186use the relative IDs above to enumerate the valid IDs. The ioctl
187returns non-zero when there is no more next ID. The real report ID is
188filled into the returned hiddev_report_info structure.
189
190HIDIOCGFIELDINFO
191  - struct hiddev_field_info (read/write)
192
193Returns the field information associated with a report in a
194hiddev_field_info structure. The user must fill in report_id and
195report_type in this structure, as above. The field_index should also
196be filled in, which should be a number from 0 and maxfield-1, as
197returned from a previous HIDIOCGREPORTINFO call.
198
199HIDIOCGUCODE
200  - struct hiddev_usage_ref (read/write)
201
202Returns the usage_code in a hiddev_usage_ref structure, given that
203its report type, report id, field index, and index within the
204field have already been filled into the structure.
205
206HIDIOCGUSAGE
207  - struct hiddev_usage_ref (read/write)
208
209Returns the value of a usage in a hiddev_usage_ref structure. The
210usage to be retrieved can be specified as above, or the user can
211choose to fill in the report_type field and specify the report_id as
212HID_REPORT_ID_UNKNOWN. In this case, the hiddev_usage_ref will be
213filled in with the report and field information associated with this
214usage if it is found.
215
216HIDIOCSUSAGE
217  - struct hiddev_usage_ref (write)
218
219Sets the value of a usage in an output report.  The user fills in
220the hiddev_usage_ref structure as above, but additionally fills in
221the value field.
222
223HIDIOGCOLLECTIONINDEX
224  - struct hiddev_usage_ref (write)
225
226Returns the collection index associated with this usage.  This
227indicates where in the collection hierarchy this usage sits.
228
229HIDIOCGFLAG
230  - int (read)
231HIDIOCSFLAG
232  - int (write)
233
234These operations respectively inspect and replace the mode flags
235that influence the read() call above.  The flags are as follows:
236
237    HIDDEV_FLAG_UREF
238      - read() calls will now return
239        struct hiddev_usage_ref instead of struct hiddev_event.
240        This is a larger structure, but in situations where the
241        device has more than one usage in its reports with the
242        same usage code, this mode serves to resolve such
243        ambiguity.
244
245    HIDDEV_FLAG_REPORT
246      - This flag can only be used in conjunction
247        with HIDDEV_FLAG_UREF.  With this flag set, when the device
248        sends a report, a struct hiddev_usage_ref will be returned
249        to read() filled in with the report_type and report_id, but
250        with field_index set to FIELD_INDEX_NONE.  This serves as
251        additional notification when the device has sent a report.
252