Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

Force feedback for Linux.

By Johann Deneux  on 2001/04/22.

Updated by Anssi Hannula  on 2006/04/09.

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

interactive.fig as well.

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

1. 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

effects.

This document only describes the force feedback part of the Linux input

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

document.

2. Instructions to the user

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

To enable force feedback, you have to:

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

   device.

2. make sure evdev module is loaded and /dev/input/event* device files are

   created.

Before you start, let me WARN you that some devices shake violently during the

initialisation phase. This happens for example with my "AVB Top Shot Pegasus".

To stop this annoying behaviour, move you joystick to its limits. Anyway, you

should keep a hand on your device, in order to avoid it to break down if

something goes wrong.

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.

% fftest /dev/input/eventXX

3. 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

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

#include 

#include 

unsigned long features[1 + FF_MAX/sizeof(unsigned long)];

int ioctl(int file_descriptor, int request, unsigned long *features);

"request" must be EVIOCGBIT(EV_FF, size of features array in bytes )

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_DAMPER     can simulate damper effects

- FF_RUMBLE     rumble effects

- FF_INERTIA    can simulate inertia

- FF_GAIN       gain is adjustable

- FF_AUTOCENTER autocenter is adjustable

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

      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

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

#include 

#include 

int ioctl(int file_descriptor, int request, struct ff_effect *effect);

"request" must be EVIOCSFF.

"effect" points to a structure describing the effect to upload. The effect is

uploaded, but not played.

The content of effect may be modified. In particular, its field "id" is set

to the unique id assigned by the driver. This data is required for performing

some operations (removing an effect, controlling the playback).

This if field must be set to -1 by the user in order to tell the driver to

allocate a new effect.

Effects are file descriptor specific.

See  for a description of the ff_effect struct. You should also

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

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

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

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

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

#include 

#include 

        struct input_event play;

        struct input_event stop;

        struct ff_effect effect;

        int fd;

...

        fd = open("/dev/input/eventXX", O_RDWR);

...

        /* Play three times */

        play.type = EV_FF;

        play.code = effect.id;

        play.value = 3;

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

...

        /* Stop an effect */

        stop.type = EV_FF;

        stop.code = effect.id;

        stop.value = 0;

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

3.5 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 */

ie.type = EV_FF;

ie.code = FF_GAIN;

ie.value = 0xFFFFUL * gain / 100;

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

        perror("set gain");

3.6 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;

ie.type = EV_FF;

ie.code = FF_AUTOCENTER;

ie.value = 0xFFFFUL * autocenter / 100;

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

        perror("set auto-center");

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

3.7 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

type of device, not all parameters can be dynamically updated. For example,

the direction of an effect cannot be updated with iforce devices. In this

case, the driver stops the effect, up-load it, and restart it.

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

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

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:

struct input_event {

/* When the status of the effect changed */

        struct timeval time;

/* Set to EV_FF_STATUS */

        unsigned short type;

/* Contains the id of the effect */

        unsigned short code;

/* Indicates the status */

        unsigned int value;

};

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

      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.