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diff --git a/Documentation/hid/hiddev.rst b/Documentation/hid/hiddev.rst new file mode 100644 index 000000000..9b28a97c0 --- /dev/null +++ b/Documentation/hid/hiddev.rst @@ -0,0 +1,251 @@ +================================================ +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 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.rst +* 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 https://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. |