.. include:: <isonum.txt>

============
Introduction
============

:Copyright: |copy| 1999-2001 Vojtech Pavlik <vojtech@ucw.cz> - Sponsored by SuSE

Architecture
============

Input subsystem is a collection of drivers that is designed to support
all input devices under Linux. Most of the drivers reside in
drivers/input, although quite a few live in drivers/hid and
drivers/platform.

The core of the input subsystem is the input module, which must be
loaded before any other of the input modules - it serves as a way of
communication between two groups of modules:

Device drivers
--------------

These modules talk to the hardware (for example via USB), and provide
events (keystrokes, mouse movements) to the input module.

Event handlers
--------------

These modules get events from input core and pass them where needed
via various interfaces - keystrokes to the kernel, mouse movements via
a simulated PS/2 interface to GPM and X, and so on.

Simple Usage
============

For the most usual configuration, with one USB mouse and one USB keyboard,
you'll have to load the following modules (or have them built in to the
kernel)::

        input
        mousedev
        usbcore
        uhci_hcd or ohci_hcd or ehci_hcd
        usbhid
        hid_generic

After this, the USB keyboard will work straight away, and the USB mouse
will be available as a character device on major 13, minor 63::

        crw-r--r--   1 root     root      13,  63 Mar 28 22:45 mice

This device is usually created automatically by the system. The commands
to create it by hand are::

        cd /dev
        mkdir input
        mknod input/mice c 13 63

After that you have to point GPM (the textmode mouse cut&paste tool) and
XFree to this device to use it - GPM should be called like::

        gpm -t ps2 -m /dev/input/mice

And in X::

        Section "Pointer"
            Protocol    "ImPS/2"
            Device      "/dev/input/mice"
            ZAxisMapping 4 5
        EndSection

When you do all of the above, you can use your USB mouse and keyboard.

Detailed Description
====================

Event handlers
--------------

Event handlers distribute the events from the devices to userspace and
in-kernel consumers, as needed.

evdev
~~~~~

``evdev`` is the generic input event interface. It passes the events
generated in the kernel straight to the program, with timestamps. The
event codes are the same on all architectures and are hardware
independent.

This is the preferred interface for userspace to consume user
input, and all clients are encouraged to use it.

See :ref:`event-interface` for notes on API.

The devices are in /dev/input::

        crw-r--r--   1 root     root      13,  64 Apr  1 10:49 event0
        crw-r--r--   1 root     root      13,  65 Apr  1 10:50 event1
        crw-r--r--   1 root     root      13,  66 Apr  1 10:50 event2
        crw-r--r--   1 root     root      13,  67 Apr  1 10:50 event3
        ...

There are two ranges of minors: 64 through 95 is the static legacy
range. If there are more than 32 input devices in a system, additional
evdev nodes are created with minors starting with 256.

keyboard
~~~~~~~~

``keyboard`` is in-kernel input handler and is a part of VT code. It
consumes keyboard keystrokes and handles user input for VT consoles.

mousedev
~~~~~~~~

``mousedev`` is a hack to make legacy programs that use mouse input
work. It takes events from either mice or digitizers/tablets and makes
a PS/2-style (a la /dev/psaux) mouse device available to the
userland.

Mousedev devices in /dev/input (as shown above) are::

        crw-r--r--   1 root     root      13,  32 Mar 28 22:45 mouse0
        crw-r--r--   1 root     root      13,  33 Mar 29 00:41 mouse1
        crw-r--r--   1 root     root      13,  34 Mar 29 00:41 mouse2
        crw-r--r--   1 root     root      13,  35 Apr  1 10:50 mouse3
        ...
        ...
        crw-r--r--   1 root     root      13,  62 Apr  1 10:50 mouse30
        crw-r--r--   1 root     root      13,  63 Apr  1 10:50 mice

Each ``mouse`` device is assigned to a single mouse or digitizer, except
the last one - ``mice``. This single character device is shared by all
mice and digitizers, and even if none are connected, the device is
present.  This is useful for hotplugging USB mice, so that older programs
that do not handle hotplug can open the device even when no mice are
present.

CONFIG_INPUT_MOUSEDEV_SCREEN_[XY] in the kernel configuration are
the size of your screen (in pixels) in XFree86. This is needed if you
want to use your digitizer in X, because its movement is sent to X
via a virtual PS/2 mouse and thus needs to be scaled
accordingly. These values won't be used if you use a mouse only.

Mousedev will generate either PS/2, ImPS/2 (Microsoft IntelliMouse) or
ExplorerPS/2 (IntelliMouse Explorer) protocols, depending on what the
program reading the data wishes. You can set GPM and X to any of
these. You'll need ImPS/2 if you want to make use of a wheel on a USB
mouse and ExplorerPS/2 if you want to use extra (up to 5) buttons.

joydev
~~~~~~

``joydev`` implements v0.x and v1.x Linux joystick API. See
:ref:`joystick-api` for details.

As soon as any joystick is connected, it can be accessed in /dev/input on::

        crw-r--r--   1 root     root      13,   0 Apr  1 10:50 js0
        crw-r--r--   1 root     root      13,   1 Apr  1 10:50 js1
        crw-r--r--   1 root     root      13,   2 Apr  1 10:50 js2
        crw-r--r--   1 root     root      13,   3 Apr  1 10:50 js3
        ...

And so on up to js31 in legacy range, and additional nodes with minors
above 256 if there are more joystick devices.

Device drivers
--------------

Device drivers are the modules that generate events.

hid-generic
~~~~~~~~~~~

``hid-generic`` is one of the largest and most complex driver of the
whole suite. It handles all HID devices, and because there is a very
wide variety of them, and because the USB HID specification isn't
simple, it needs to be this big.

Currently, it handles USB mice, joysticks, gamepads, steering wheels,
keyboards, trackballs and digitizers.

However, USB uses HID also for monitor controls, speaker controls, UPSs,
LCDs and many other purposes.

The monitor and speaker controls should be easy to add to the hid/input
interface, but for the UPSs and LCDs it doesn't make much sense. For this,
the hiddev interface was designed. See Documentation/hid/hiddev.rst
for more information about it.

The usage of the usbhid module is very simple, it takes no parameters,
detects everything automatically and when a HID device is inserted, it
detects it appropriately.

However, because the devices vary wildly, you might happen to have a
device that doesn't work well. In that case #define DEBUG at the beginning
of hid-core.c and send me the syslog traces.

usbmouse
~~~~~~~~

For embedded systems, for mice with broken HID descriptors and just any
other use when the big usbhid wouldn't be a good choice, there is the
usbmouse driver. It handles USB mice only. It uses a simpler HIDBP
protocol. This also means the mice must support this simpler protocol. Not
all do. If you don't have any strong reason to use this module, use usbhid
instead.

usbkbd
~~~~~~

Much like usbmouse, this module talks to keyboards with a simplified
HIDBP protocol. It's smaller, but doesn't support any extra special keys.
Use usbhid instead if there isn't any special reason to use this.

psmouse
~~~~~~~

This is driver for all flavors of pointing devices using PS/2
protocol, including Synaptics and ALPS touchpads, Intellimouse
Explorer devices, Logitech PS/2 mice and so on.

atkbd
~~~~~

This is driver for PS/2 (AT) keyboards.

iforce
~~~~~~

A driver for I-Force joysticks and wheels, both over USB and RS232.
It includes Force Feedback support now, even though Immersion
Corp. considers the protocol a trade secret and won't disclose a word
about it.

Verifying if it works
=====================

Typing a couple keys on the keyboard should be enough to check that
a keyboard works and is correctly connected to the kernel keyboard
driver.

Doing a ``cat /dev/input/mouse0`` (c, 13, 32) will verify that a mouse
is also emulated; characters should appear if you move it.

You can test the joystick emulation with the ``jstest`` utility,
available in the joystick package (see :ref:`joystick-doc`).

You can test the event devices with the ``evtest`` utility.

.. _event-interface:

Event interface
===============

You can use blocking and nonblocking reads, and also select() on the
/dev/input/eventX devices, and you'll always get a whole number of input
events on a read. Their layout is::

    struct input_event {
            struct timeval time;
            unsigned short type;
            unsigned short code;
            unsigned int value;
    };

``time`` is the timestamp, it returns the time at which the event happened.
Type is for example EV_REL for relative movement, EV_KEY for a keypress or
release. More types are defined in include/uapi/linux/input-event-codes.h.

``code`` is event code, for example REL_X or KEY_BACKSPACE, again a complete
list is in include/uapi/linux/input-event-codes.h.

``value`` is the value the event carries. Either a relative change for
EV_REL, absolute new value for EV_ABS (joysticks ...), or 0 for EV_KEY for
release, 1 for keypress and 2 for autorepeat.

See :ref:`input-event-codes` for more information about various even codes.