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Force feedback for Linux.
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By Johann Deneux <deneux@ifrance.com> on 2001/04/22.
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----------------------------------------------------------------------------
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This document describes how to use force feedback devices under Linux. The
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goal is not to support these devices as if they were simple input-only devices
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(as it is already the case), but to really enable the rendering of force
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At the moment, only I-Force devices are supported, and not officially. That
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means I had to find out how the protocol works on my own. Of course, the
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information I managed to grasp is far from being complete, and I can not
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guarranty that this driver will work for you.
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This document only describes the force feedback part of the driver for I-Force
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devices. Please read joystick.txt before reading further this document.
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2. Instructions to the user
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Here are instructions on how to compile and use the driver. In fact, this
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driver is the normal iforce.o, input.o and evdev.o drivers written by Vojtech
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Pavlik, plus additions to support force feedback.
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Before you start, let me WARN you that some devices shake violently during the
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initialisation phase. This happens for example with my "AVB Top Shot Pegasus".
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To stop this annoying behaviour, move you joystick to its limits. Anyway, you
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should keep a hand on your device, in order to avoid it to brake down if
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At the kernel's compilation:
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- Enable IForce/Serial
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- Enable Event interface
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Compile the modules, install them.
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You also need inputattach.
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You then need to insert the modules into the following order:
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% ./inputattach -ifor $2 & # Only for serial
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For convenience, you may use the shell script named "ff" available from
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the cvs tree of the Linux Console Project at sourceforge. You can also
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retrieve it from http://www.esil.univ-mrs.fr/~jdeneux/projects/ff/.
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If you are using USB, you don't need the inputattach step.
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Please check that you have all the /dev/input entries needed:
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mknod input/js0 c 13 0
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mknod input/js1 c 13 1
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mknod input/js2 c 13 2
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mknod input/js3 c 13 3
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mknod input/event0 c 13 64
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mknod input/event1 c 13 65
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mknod input/event2 c 13 66
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mknod input/event3 c 13 67
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There is an utility called fftest that will allow you to test the driver.
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% fftest /dev/input/eventXX
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3. Instructions to the developper
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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All interactions are done using the event API. That is, you can use ioctl()
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and write() on /dev/input/eventXX.
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This information is subject to change.
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3.1 Querying device capabilities
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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#include <linux/input.h>
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#include <sys/ioctl.h>
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int ioctl(int file_descriptor, int request, unsigned long *features);
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"request" must be EVIOCGBIT(EV_FF, sizeof(unsigned long))
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Returns the features supported by the device. features is a bitfield with the
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- FF_X has an X axis (should allways be the case)
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- FF_Y has an Y axis (usually not the case for wheels)
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- FF_CONSTANT can render constant force effects
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- FF_PERIODIC can render periodic effects (sine, ramp, square...)
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- FF_SPRING can simulate the presence of a spring
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- FF_FRICTION can simulate friction (aka drag, damper effect...)
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- FF_RUMBLE rumble effects (normally the only effect supported by rumble
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- 8 bits from FF_N_EFFECTS_0 containing the number of effects that can be
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simultaneously played.
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3.2 Uploading effects to the device
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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#include <linux/input.h>
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#include <sys/ioctl.h>
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int ioctl(int file_descriptor, int request, struct ff_effect *effect);
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"request" must be EVIOCSFF.
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"effect" points to a structure describing the effect to upload. The effect is
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uploaded, but not played.
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The content of effect may be modified. In particular, its field "id" is set
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to the unique id assigned by the driver. This data is required for performing
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some operations (removing an effect, controlling the playback).
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This if field must be set to -1 by the user in order to tell the driver to
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allocate a new effect.
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See <linux/input.h> for a description of the ff_effect stuct.
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3.3 Removing an effect from the device
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int ioctl(int fd, EVIOCRMFF, effect.id);
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This makes room for new effects in the device's memory. Please note this won't
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stop the effect if it was playing.
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3.4 Controlling the playback of effects
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Control of playing is done with write(). Below is an example:
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#include <linux/input.h>
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struct input_event play;
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struct input_event stop;
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struct ff_effect effect;
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fd = open("/dev/input/eventXX", O_RDWR);
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/* Play three times */
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play.code = effect.id;
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write(fd, (const void*) &play, sizeof(play));
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stop.code = effect.id;
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write(fd, (const void*) &play, sizeof(stop));
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Not all devices have the same strength. Therefore, users should set a gain
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factor depending on how strong they want effects to be. This setting is
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persistent accross access to the driver, so you should not care about it if
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you are writing games, as another utility probably already set this for you.
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/* Set the gain of the device
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int gain; /* between 0 and 100 */
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struct input_event ie; /* structure used to communicate with the driver */
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ie.value = 0xFFFFUL * gain / 100;
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if (write(fd, &ie, sizeof(ie)) == -1)
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3.6 Enabling/Disabling autocenter
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The autocenter feature quite disturbs the rendering of effects in my opinion,
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and I think it should be an effect, which computation depends on the game
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type. But you can enable it if you want.
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int autocenter; /* between 0 and 100 */
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struct input_event ie;
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ie.code = FF_AUTOCENTER;
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ie.value = 0xFFFFUL * autocenter / 100;
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if (write(fd, &ie, sizeof(ie)) == -1)
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perror("set auto-center");
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A value of 0 means "no auto-center".
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3.7 Dynamic update of an effect
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Proceed as if you wanted to upload a new effect, except that instead of
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setting the id field to -1, you set it to the wanted effect id.
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If you update very often, some device may stop playing effects. The driver
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checks that, and re-start the updated effect if necessary. However, this
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prevents effects from being palyed smoothly.
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3.8 Information about the status of effects
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Every time the status of an effect is changed, an event is sent. The values
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and meanings of the fields of the event are as follows:
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/* When the status of the effect changed */
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/* Set to EV_FF_STATUS */
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/* Contains the id of the effect */
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/* Indicates the status */
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FF_STATUS_STOPPED The effect stopped playing
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FF_STATUS_PLAYING The effect started to play
added
removed
linux/Documentation/input/ff.txt
