| Care and feeding of your Human Interface Devices |
| |
| INTRODUCTION |
| |
| In addition to the normal input type HID devices, USB also uses the |
| human interface device protocols for things that are not really human |
| interfaces, but have similar sorts of communication needs. The two big |
| examples for this are power devices (especially uninterruptable power |
| supplies) and monitor control on higher end monitors. |
| |
| To support these disparite requirements, the Linux USB system provides |
| HID events to two separate interfaces: |
| * the input subsystem, which converts HID events into normal input |
| device interfaces (such as keyboard, mouse and joystick) and a |
| normalised event interface - see Documentation/input/input.txt |
| * the hiddev interface, which provides fairly raw HID events |
| |
| The data flow for a HID event produced by a device is something like |
| the following : |
| |
| usb.c ---> hid-core.c ----> hid-input.c ----> [keyboard/mouse/joystick/event] |
| | |
| | |
| --> hiddev.c ----> POWER / MONITOR CONTROL |
| |
| In addition, other subsystems (apart from USB) can potentially feed |
| events into the input subsystem, but these have no effect on the hid |
| device interface. |
| |
| USING THE HID DEVICE INTERFACE |
| |
| The hiddev interface is a char interface using the normal USB major, |
| with the minor numbers starting at 96 and finishing at 111. Therefore, |
| you need the following commands: |
| mknod /dev/usb/hiddev0 c 180 96 |
| mknod /dev/usb/hiddev1 c 180 97 |
| mknod /dev/usb/hiddev2 c 180 98 |
| mknod /dev/usb/hiddev3 c 180 99 |
| mknod /dev/usb/hiddev4 c 180 100 |
| mknod /dev/usb/hiddev5 c 180 101 |
| mknod /dev/usb/hiddev6 c 180 102 |
| mknod /dev/usb/hiddev7 c 180 103 |
| mknod /dev/usb/hiddev8 c 180 104 |
| mknod /dev/usb/hiddev9 c 180 105 |
| mknod /dev/usb/hiddev10 c 180 106 |
| mknod /dev/usb/hiddev11 c 180 107 |
| mknod /dev/usb/hiddev12 c 180 108 |
| mknod /dev/usb/hiddev13 c 180 109 |
| mknod /dev/usb/hiddev14 c 180 110 |
| mknod /dev/usb/hiddev15 c 180 111 |
| |
| So you point your hiddev compliant user-space program at the correct |
| interface for your device, and it all just works. |
| |
| Assuming that you have a hiddev compliant user-space program, of |
| course. If you need to write one, read on. |
| |
| |
| THE HIDDEV API |
| This description should be read in conjunction with the HID |
| specification, freely available from http://www.usb.org, and |
| conveniently linked of http://www.linux-usb.org. |
| |
| The hiddev API uses a read() interface, and a set of ioctl() calls. |
| |
| HID devices exchange data with the host computer using data |
| bundles called "reports". Each report is divided into "fields", |
| each of which can have one or more "usages". In the hid-core, |
| each one of these usages has a single signed 32 bit value. |
| |
| read(): |
| This is the event interface. When the HID device's state changes, |
| it performs an interrupt transfer containing a report which contains |
| the changed value. The hid-core.c module parses the report, and |
| returns to hiddev.c the individual usages that have changed within |
| the report. In its basic mode, the hiddev will make these individual |
| usage changes available to the reader using a struct hiddev_event: |
| |
| struct hiddev_event { |
| unsigned hid; |
| signed int value; |
| }; |
| |
| containing the HID usage identifier for the status that changed, and |
| the value that it was changed to. Note that the structure is defined |
| within <linux/hiddev.h>, along with some other useful #defines and |
| structures. The HID usage identifier is a composite of the HID usage |
| page shifted to the 16 high order bits ORed with the usage code. The |
| behavior of the read() function can be modified using the HIDIOCSFLAG |
| ioctl() described below. |
| |
| |
| ioctl(): |
| This is the control interface. There are a number of controls: |
| |
| HIDIOCGVERSION - int (read) |
| Gets the version code out of the hiddev driver. |
| |
| HIDIOCAPPLICATION - (none) |
| This ioctl call returns the HID application usage associated with the |
| hid device. The third argument to ioctl() specifies which application |
| index to get. This is useful when the device has more than one |
| application collection. If the index is invalid (greater or equal to |
| the number of application collections this device has) the ioctl |
| returns -1. You can find out beforehand how many application |
| collections the device has from the num_applications field from the |
| hiddev_devinfo structure. |
| |
| HIDIOCGCOLLECTIONINFO - struct hiddev_collection_info (read/write) |
| This returns a superset of the information above, providing not only |
| application collections, but all the collections the device has. It |
| also returns the level the collection lives in the hierarchy. |
| The user passes in a hiddev_collection_info struct with the index |
| field set to the index that should be returned. The ioctl fills in |
| the other fields. If the index is larger than the last collection |
| index, the ioctl returns -1 and sets errno to -EINVAL. |
| |
| HIDIOCGDEVINFO - struct hiddev_devinfo (read) |
| Gets a hiddev_devinfo structure which describes the device. |
| |
| HIDIOCGSTRING - struct hiddev_string_descriptor (read/write) |
| Gets a string descriptor from the device. The caller must fill in the |
| "index" field to indicate which descriptor should be returned. |
| |
| HIDIOCINITREPORT - (none) |
| Instructs the kernel to retrieve all input and feature report values |
| from the device. At this point, all the usage structures will contain |
| current values for the device, and will maintain it as the device |
| changes. Note that the use of this ioctl is unnecessary in general, |
| since later kernels automatically initialize the reports from the |
| device at attach time. |
| |
| HIDIOCGNAME - string (variable length) |
| Gets the device name |
| |
| HIDIOCGREPORT - struct hiddev_report_info (write) |
| Instructs the kernel to get a feature or input report from the device, |
| in order to selectively update the usage structures (in contrast to |
| INITREPORT). |
| |
| HIDIOCSREPORT - struct hiddev_report_info (write) |
| Instructs the kernel to send a report to the device. This report can |
| be filled in by the user through HIDIOCSUSAGE calls (below) to fill in |
| individual usage values in the report before sending the report in full |
| to the device. |
| |
| HIDIOCGREPORTINFO - struct hiddev_report_info (read/write) |
| Fills in a hiddev_report_info structure for the user. The report is |
| looked up by type (input, output or feature) and id, so these fields |
| must be filled in by the user. The ID can be absolute -- the actual |
| report id as reported by the device -- or relative -- |
| HID_REPORT_ID_FIRST for the first report, and (HID_REPORT_ID_NEXT | |
| report_id) for the next report after report_id. Without a-priori |
| information about report ids, the right way to use this ioctl is to |
| use the relative IDs above to enumerate the valid IDs. The ioctl |
| returns non-zero when there is no more next ID. The real report ID is |
| filled into the returned hiddev_report_info structure. |
| |
| HIDIOCGFIELDINFO - struct hiddev_field_info (read/write) |
| Returns the field information associated with a report in a |
| hiddev_field_info structure. The user must fill in report_id and |
| report_type in this structure, as above. The field_index should also |
| be filled in, which should be a number from 0 and maxfield-1, as |
| returned from a previous HIDIOCGREPORTINFO call. |
| |
| HIDIOCGUCODE - struct hiddev_usage_ref (read/write) |
| Returns the usage_code in a hiddev_usage_ref structure, given that |
| given its report type, report id, field index, and index within the |
| field have already been filled into the structure. |
| |
| HIDIOCGUSAGE - struct hiddev_usage_ref (read/write) |
| Returns the value of a usage in a hiddev_usage_ref structure. The |
| usage to be retrieved can be specified as above, or the user can |
| choose to fill in the report_type field and specify the report_id as |
| HID_REPORT_ID_UNKNOWN. In this case, the hiddev_usage_ref will be |
| filled in with the report and field information associated with this |
| usage if it is found. |
| |
| HIDIOCSUSAGE - struct hiddev_usage_ref (write) |
| Sets the value of a usage in an output report. The user fills in |
| the hiddev_usage_ref structure as above, but additionally fills in |
| the value field. |
| |
| HIDIOGCOLLECTIONINDEX - struct hiddev_usage_ref (write) |
| Returns the collection index associated with this usage. This |
| indicates where in the collection hierarchy this usage sits. |
| |
| HIDIOCGFLAG - int (read) |
| HIDIOCSFLAG - int (write) |
| These operations respectively inspect and replace the mode flags |
| that influence the read() call above. The flags are as follows: |
| |
| HIDDEV_FLAG_UREF - read() calls will now return |
| struct hiddev_usage_ref instead of struct hiddev_event. |
| This is a larger structure, but in situations where the |
| device has more than one usage in its reports with the |
| same usage code, this mode serves to resolve such |
| ambiguity. |
| |
| HIDDEV_FLAG_REPORT - This flag can only be used in conjunction |
| with HIDDEV_FLAG_UREF. With this flag set, when the device |
| sends a report, a struct hiddev_usage_ref will be returned |
| to read() filled in with the report_type and report_id, but |
| with field_index set to FIELD_INDEX_NONE. This serves as |
| additional notification when the device has sent a report. |