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