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1 | Care and feeding of your Human Interface Devices |
2 | ||
3 | INTRODUCTION | |
4 | ||
5 | In addition to the normal input type HID devices, USB also uses the | |
6 | human interface device protocols for things that are not really human | |
7 | interfaces, but have similar sorts of communication needs. The two big | |
8 | examples for this are power devices (especially uninterruptable power | |
9 | supplies) and monitor control on higher end monitors. | |
10 | ||
5d3f083d | 11 | To support these disparate requirements, the Linux USB system provides |
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12 | HID events to two separate interfaces: |
13 | * the input subsystem, which converts HID events into normal input | |
14 | device interfaces (such as keyboard, mouse and joystick) and a | |
15 | normalised event interface - see Documentation/input/input.txt | |
16 | * the hiddev interface, which provides fairly raw HID events | |
17 | ||
18 | The data flow for a HID event produced by a device is something like | |
19 | the following : | |
20 | ||
21 | usb.c ---> hid-core.c ----> hid-input.c ----> [keyboard/mouse/joystick/event] | |
22 | | | |
23 | | | |
24 | --> hiddev.c ----> POWER / MONITOR CONTROL | |
25 | ||
26 | In addition, other subsystems (apart from USB) can potentially feed | |
27 | events into the input subsystem, but these have no effect on the hid | |
28 | device interface. | |
29 | ||
30 | USING THE HID DEVICE INTERFACE | |
31 | ||
32 | The hiddev interface is a char interface using the normal USB major, | |
33 | with the minor numbers starting at 96 and finishing at 111. Therefore, | |
34 | you need the following commands: | |
35 | mknod /dev/usb/hiddev0 c 180 96 | |
36 | mknod /dev/usb/hiddev1 c 180 97 | |
37 | mknod /dev/usb/hiddev2 c 180 98 | |
38 | mknod /dev/usb/hiddev3 c 180 99 | |
39 | mknod /dev/usb/hiddev4 c 180 100 | |
40 | mknod /dev/usb/hiddev5 c 180 101 | |
41 | mknod /dev/usb/hiddev6 c 180 102 | |
42 | mknod /dev/usb/hiddev7 c 180 103 | |
43 | mknod /dev/usb/hiddev8 c 180 104 | |
44 | mknod /dev/usb/hiddev9 c 180 105 | |
45 | mknod /dev/usb/hiddev10 c 180 106 | |
46 | mknod /dev/usb/hiddev11 c 180 107 | |
47 | mknod /dev/usb/hiddev12 c 180 108 | |
48 | mknod /dev/usb/hiddev13 c 180 109 | |
49 | mknod /dev/usb/hiddev14 c 180 110 | |
50 | mknod /dev/usb/hiddev15 c 180 111 | |
51 | ||
52 | So you point your hiddev compliant user-space program at the correct | |
53 | interface for your device, and it all just works. | |
54 | ||
55 | Assuming that you have a hiddev compliant user-space program, of | |
56 | course. If you need to write one, read on. | |
57 | ||
58 | ||
59 | THE HIDDEV API | |
60 | This description should be read in conjunction with the HID | |
61 | specification, freely available from http://www.usb.org, and | |
62 | conveniently linked of http://www.linux-usb.org. | |
63 | ||
64 | The hiddev API uses a read() interface, and a set of ioctl() calls. | |
65 | ||
66 | HID devices exchange data with the host computer using data | |
67 | bundles called "reports". Each report is divided into "fields", | |
68 | each of which can have one or more "usages". In the hid-core, | |
69 | each one of these usages has a single signed 32 bit value. | |
70 | ||
71 | read(): | |
72 | This is the event interface. When the HID device's state changes, | |
73 | it performs an interrupt transfer containing a report which contains | |
74 | the changed value. The hid-core.c module parses the report, and | |
75 | returns to hiddev.c the individual usages that have changed within | |
76 | the report. In its basic mode, the hiddev will make these individual | |
77 | usage changes available to the reader using a struct hiddev_event: | |
78 | ||
79 | struct hiddev_event { | |
80 | unsigned hid; | |
81 | signed int value; | |
82 | }; | |
83 | ||
84 | containing the HID usage identifier for the status that changed, and | |
85 | the value that it was changed to. Note that the structure is defined | |
86 | within <linux/hiddev.h>, along with some other useful #defines and | |
87 | structures. The HID usage identifier is a composite of the HID usage | |
88 | page shifted to the 16 high order bits ORed with the usage code. The | |
89 | behavior of the read() function can be modified using the HIDIOCSFLAG | |
90 | ioctl() described below. | |
91 | ||
92 | ||
93 | ioctl(): | |
94 | This is the control interface. There are a number of controls: | |
95 | ||
96 | HIDIOCGVERSION - int (read) | |
97 | Gets the version code out of the hiddev driver. | |
98 | ||
99 | HIDIOCAPPLICATION - (none) | |
100 | This ioctl call returns the HID application usage associated with the | |
101 | hid device. The third argument to ioctl() specifies which application | |
102 | index to get. This is useful when the device has more than one | |
103 | application collection. If the index is invalid (greater or equal to | |
104 | the number of application collections this device has) the ioctl | |
105 | returns -1. You can find out beforehand how many application | |
106 | collections the device has from the num_applications field from the | |
107 | hiddev_devinfo structure. | |
108 | ||
109 | HIDIOCGCOLLECTIONINFO - struct hiddev_collection_info (read/write) | |
110 | This returns a superset of the information above, providing not only | |
111 | application collections, but all the collections the device has. It | |
112 | also returns the level the collection lives in the hierarchy. | |
113 | The user passes in a hiddev_collection_info struct with the index | |
114 | field set to the index that should be returned. The ioctl fills in | |
115 | the other fields. If the index is larger than the last collection | |
116 | index, the ioctl returns -1 and sets errno to -EINVAL. | |
117 | ||
118 | HIDIOCGDEVINFO - struct hiddev_devinfo (read) | |
119 | Gets a hiddev_devinfo structure which describes the device. | |
120 | ||
670e9f34 | 121 | HIDIOCGSTRING - struct hiddev_string_descriptor (read/write) |
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122 | Gets a string descriptor from the device. The caller must fill in the |
123 | "index" field to indicate which descriptor should be returned. | |
124 | ||
125 | HIDIOCINITREPORT - (none) | |
126 | Instructs the kernel to retrieve all input and feature report values | |
127 | from the device. At this point, all the usage structures will contain | |
128 | current values for the device, and will maintain it as the device | |
129 | changes. Note that the use of this ioctl is unnecessary in general, | |
130 | since later kernels automatically initialize the reports from the | |
131 | device at attach time. | |
132 | ||
133 | HIDIOCGNAME - string (variable length) | |
134 | Gets the device name | |
135 | ||
136 | HIDIOCGREPORT - struct hiddev_report_info (write) | |
137 | Instructs the kernel to get a feature or input report from the device, | |
138 | in order to selectively update the usage structures (in contrast to | |
139 | INITREPORT). | |
140 | ||
141 | HIDIOCSREPORT - struct hiddev_report_info (write) | |
142 | Instructs the kernel to send a report to the device. This report can | |
143 | be filled in by the user through HIDIOCSUSAGE calls (below) to fill in | |
144 | individual usage values in the report before sending the report in full | |
145 | to the device. | |
146 | ||
147 | HIDIOCGREPORTINFO - struct hiddev_report_info (read/write) | |
148 | Fills in a hiddev_report_info structure for the user. The report is | |
149 | looked up by type (input, output or feature) and id, so these fields | |
150 | must be filled in by the user. The ID can be absolute -- the actual | |
151 | report id as reported by the device -- or relative -- | |
152 | HID_REPORT_ID_FIRST for the first report, and (HID_REPORT_ID_NEXT | | |
153 | report_id) for the next report after report_id. Without a-priori | |
154 | information about report ids, the right way to use this ioctl is to | |
155 | use the relative IDs above to enumerate the valid IDs. The ioctl | |
156 | returns non-zero when there is no more next ID. The real report ID is | |
157 | filled into the returned hiddev_report_info structure. | |
158 | ||
159 | HIDIOCGFIELDINFO - struct hiddev_field_info (read/write) | |
160 | Returns the field information associated with a report in a | |
161 | hiddev_field_info structure. The user must fill in report_id and | |
162 | report_type in this structure, as above. The field_index should also | |
163 | be filled in, which should be a number from 0 and maxfield-1, as | |
164 | returned from a previous HIDIOCGREPORTINFO call. | |
165 | ||
166 | HIDIOCGUCODE - struct hiddev_usage_ref (read/write) | |
167 | Returns the usage_code in a hiddev_usage_ref structure, given that | |
168 | given its report type, report id, field index, and index within the | |
169 | field have already been filled into the structure. | |
170 | ||
171 | HIDIOCGUSAGE - struct hiddev_usage_ref (read/write) | |
172 | Returns the value of a usage in a hiddev_usage_ref structure. The | |
173 | usage to be retrieved can be specified as above, or the user can | |
174 | choose to fill in the report_type field and specify the report_id as | |
175 | HID_REPORT_ID_UNKNOWN. In this case, the hiddev_usage_ref will be | |
176 | filled in with the report and field information associated with this | |
177 | usage if it is found. | |
178 | ||
179 | HIDIOCSUSAGE - struct hiddev_usage_ref (write) | |
180 | Sets the value of a usage in an output report. The user fills in | |
181 | the hiddev_usage_ref structure as above, but additionally fills in | |
182 | the value field. | |
183 | ||
184 | HIDIOGCOLLECTIONINDEX - struct hiddev_usage_ref (write) | |
185 | Returns the collection index associated with this usage. This | |
186 | indicates where in the collection hierarchy this usage sits. | |
187 | ||
188 | HIDIOCGFLAG - int (read) | |
189 | HIDIOCSFLAG - int (write) | |
190 | These operations respectively inspect and replace the mode flags | |
191 | that influence the read() call above. The flags are as follows: | |
192 | ||
193 | HIDDEV_FLAG_UREF - read() calls will now return | |
194 | struct hiddev_usage_ref instead of struct hiddev_event. | |
195 | This is a larger structure, but in situations where the | |
196 | device has more than one usage in its reports with the | |
197 | same usage code, this mode serves to resolve such | |
198 | ambiguity. | |
199 | ||
200 | HIDDEV_FLAG_REPORT - This flag can only be used in conjunction | |
201 | with HIDDEV_FLAG_UREF. With this flag set, when the device | |
202 | sends a report, a struct hiddev_usage_ref will be returned | |
203 | to read() filled in with the report_type and report_id, but | |
204 | with field_index set to FIELD_INDEX_NONE. This serves as | |
205 | additional notification when the device has sent a report. |