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+Care and feeding of your Human Interface Devices
+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 disparate 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.
+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.
+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.
+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.
+This is the control interface. There are a number of controls:
+HIDIOCGVERSION - int (read)
+Gets the version code out of the hiddev driver.
+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.
+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
+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.