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Force feedback for Linux.

By Johann Deneux  on 2001/04/22.

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0. 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.

At the moment, only I-Force devices are supported, and not officially. That

means I had to find out how the protocol works on my own. Of course, the

information I managed to grasp is far from being complete, and I can not

guarranty that this driver will work for you.

This document only describes the force feedback part of the driver for I-Force

devices. Please read joystick.txt before reading further this document.

2. Instructions to the user

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

Here are instructions on how to compile and use the driver. In fact, this

driver is the normal iforce.o, input.o and evdev.o drivers written by Vojtech

Pavlik, plus additions to support force feedback.

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 brake down if

something goes wrong.

At the kernel's compilation:

        - Enable IForce/Serial

        - Enable Event interface

Compile the modules, install them.

You also need inputattach.

You then need to insert the modules into the following order:

% modprobe joydev

% modprobe serport

% modprobe iforce

% modprobe evdev

% ./inputattach -ifor $2 &      # Only for serial

For convenience, you may use the shell script named "ff" available from

the cvs tree of the Linux Console Project at sourceforge. You can also

retrieve it from http://www.esil.univ-mrs.fr/~jdeneux/projects/ff/.

If you are using USB, you don't need the inputattach step.

Please check that you have all the /dev/input entries needed:

cd /dev

rm js*

mkdir input

mknod input/js0 c 13 0

mknod input/js1 c 13 1

mknod input/js2 c 13 2

mknod input/js3 c 13 3

ln -s input/js0 js0

ln -s input/js1 js1

ln -s input/js2 js2

ln -s input/js3 js3

mknod input/event0 c 13 64

mknod input/event1 c 13 65

mknod input/event2 c 13 66

mknod input/event3 c 13 67

2.1 Does it work ?

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

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

% fftest /dev/eventXX

3. Instructions to the developper

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

  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 

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

"request" must be EVIOCGBIT(EV_FF, sizeof(unsigned long))

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

following bits:

- FF_X          has an X axis (should allways be the case)

- FF_Y          has an Y axis (usually not the case for wheels)

- FF_CONSTANT   can render constant force effects

- FF_PERIODIC   can render periodic effects (sine, ramp, square...)

- FF_SPRING     can simulate the presence of a spring

- FF_FRICTION   can simulate friction (aka drag, damper effect...)

- FF_RUMBLE     rumble effects (normally the only effect supported by rumble

                pads)

- 8 bits from FF_N_EFFECTS_0 containing the number of effects that can be

                simultaneously played.

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).

See  for a description of the ff_effect stuct.

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. Please note this won't

stop 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 accross access to the driver, so you should not care about it if

you are writing games, as another utility probably already set this for you.

/* 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

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

This consists in changing some parameters of an effect while it's playing. The

driver currently does not support that. You still have the brute-force method,

which consists in erasing the effect and uploading the updated version. It

actually works pretty well. You don't need to stop-and-start the effect.