Input: convert force feedback documentation into ReST format

Force feedback for Linux.

By Johann Deneux <johann.deneux@gmail.com> on 2001/04/22.

Updated by Anssi Hannula <anssi.hannula@gmail.com> on 2006/04/09.

========================

Force feedback for Linux

========================

:Author: Johann Deneux <johann.deneux@gmail.com> on 2001/04/22.

:Updated: Anssi Hannula <anssi.hannula@gmail.com> on 2006/04/09.

You may redistribute this file. Please remember to include shape.fig and

interactive.fig as well.

----------------------------------------------------------------------------

1. Introduction

~~~~~~~~~~~~~~~

Introduction

~~~~~~~~~~~~

This document describes how to use force feedback devices under Linux. The

goal is not to support these devices as if they were simple input-only devices

(as it is already the case), but to really enable the rendering of force

interface. Please read joystick.txt and input.txt before reading further this

document.

2. Instructions to the user

~~~~~~~~~~~~~~~~~~~~~~~~~~~

Instructions to the user

~~~~~~~~~~~~~~~~~~~~~~~~

To enable force feedback, you have to:

1. have your kernel configured with evdev and a driver that supports your

If you have a serial iforce device, you need to start inputattach. See

joystick.txt for details.

2.1 Does it work ?

~~~~~~~~~~~~~~~~~~

There is an utility called fftest that will allow you to test the driver.

Does it work ?

--------------

There is an utility called fftest that will allow you to test the driver::

    % fftest /dev/input/eventXX

3. Instructions to the developer

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Instructions to the developer

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

All interactions are done using the event API. That is, you can use ioctl()

and write() on /dev/input/eventXX.

This information is subject to change.

3.1 Querying device capabilities

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Querying device capabilities

----------------------------

::

    #include <linux/input.h>

    #include <sys/ioctl.h>

Returns the features supported by the device. features is a bitfield with the

following bits:

- FF_CONSTANT	can render constant force effects

- FF_PERIODIC	can render periodic effects with the following waveforms:

  - FF_SQUARE	  square waveform

  - FF_TRIANGLE	  triangle waveform

  - FF_SINE	  sine waveform

  - FF_SAW_UP	  sawtooth up waveform

  - FF_SAW_DOWN	  sawtooth down waveform

  - FF_CUSTOM	  custom waveform

- FF_RAMP       can render ramp effects

- FF_SPRING	can simulate the presence of a spring

- FF_FRICTION	can simulate friction

- FF_GAIN	gain is adjustable

- FF_AUTOCENTER	autocenter is adjustable

Note: In most cases you should use FF_PERIODIC instead of FF_RUMBLE. All

.. note::

    - In most cases you should use FF_PERIODIC instead of FF_RUMBLE. All

      devices that support FF_RUMBLE support FF_PERIODIC (square, triangle,

      sine) and the other way around.

Note: The exact syntax FF_CUSTOM is undefined for the time being as no driver

    - The exact syntax FF_CUSTOM is undefined for the time being as no driver

      supports it yet.

::

    int ioctl(int fd, EVIOCGEFFECTS, int *n);

Returns the number of effects the device can keep in its memory.

3.2 Uploading effects to the device

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Uploading effects to the device

-------------------------------

::

    #include <linux/input.h>

    #include <sys/ioctl.h>

find help in a few sketches, contained in files shape.fig and interactive.fig.

You need xfig to visualize these files.

3.3 Removing an effect from the device

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Removing an effect from the device

----------------------------------

::

    int ioctl(int fd, EVIOCRMFF, effect.id);

This makes room for new effects in the device's memory. Note that this also

stops the effect if it was playing.

3.4 Controlling the playback of effects

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Controlling the playback of effects

-----------------------------------

Control of playing is done with write(). Below is an example:

::

    #include <linux/input.h>

    #include <unistd.h>

	write(fd, (const void*) &play, sizeof(stop));

3.5 Setting the gain

~~~~~~~~~~~~~~~~~~~~

Setting the gain

----------------

Not all devices have the same strength. Therefore, users should set a gain

factor depending on how strong they want effects to be. This setting is

persistent across access to the driver.

::

    /* Set the gain of the device

    int gain;		/* between 0 and 100 */

    struct input_event ie;	/* structure used to communicate with the driver */

    if (write(fd, &ie, sizeof(ie)) == -1)

	perror("set gain");

3.6 Enabling/Disabling autocenter

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Enabling/Disabling autocenter

-----------------------------

The autocenter feature quite disturbs the rendering of effects in my opinion,

and I think it should be an effect, which computation depends on the game

type. But you can enable it if you want.

::

    int autocenter;		/* between 0 and 100 */

    struct input_event ie;

A value of 0 means "no auto-center".

3.7 Dynamic update of an effect

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Dynamic update of an effect

---------------------------

Proceed as if you wanted to upload a new effect, except that instead of

setting the id field to -1, you set it to the wanted effect id.

Normally, the effect is not stopped and restarted. However, depending on the

Therefore it is recommended to dynamically change direction while the effect

is playing only when it is ok to restart the effect with a replay count of 1.

3.8 Information about the status of effects

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Information about the status of effects

---------------------------------------

Every time the status of an effect is changed, an event is sent. The values

and meanings of the fields of the event are as follows:

and meanings of the fields of the event are as follows::

    struct input_event {

    /* When the status of the effect changed */

    FF_STATUS_STOPPED	The effect stopped playing

    FF_STATUS_PLAYING	The effect started to play

NOTE: Status feedback is only supported by iforce driver. If you have

.. note::

    - Status feedback is only supported by iforce driver. If you have

      a really good reason to use this, please contact

      linux-joystick@atrey.karlin.mff.cuni.cz or anssi.hannula@gmail.com

      so that support for it can be added to the rest of the drivers.