linux/Documentation/input/ff.txt
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   1Force feedback for Linux.
   2By Johann Deneux <johann.deneux@gmail.com> on 2001/04/22.
   3Updated by Anssi Hannula <anssi.hannula@gmail.com> on 2006/04/09.
   4You may redistribute this file. Please remember to include shape.fig and
   5interactive.fig as well.
   6----------------------------------------------------------------------------
   7
   81. Introduction
   9~~~~~~~~~~~~~~~
  10This document describes how to use force feedback devices under Linux. The
  11goal is not to support these devices as if they were simple input-only devices
  12(as it is already the case), but to really enable the rendering of force
  13effects.
  14This document only describes the force feedback part of the Linux input
  15interface. Please read joystick.txt and input.txt before reading further this
  16document.
  17
  182. Instructions to the user
  19~~~~~~~~~~~~~~~~~~~~~~~~~~~
  20To enable force feedback, you have to:
  21
  221. have your kernel configured with evdev and a driver that supports your
  23   device.
  242. make sure evdev module is loaded and /dev/input/event* device files are
  25   created.
  26
  27Before you start, let me WARN you that some devices shake violently during the
  28initialisation phase. This happens for example with my "AVB Top Shot Pegasus".
  29To stop this annoying behaviour, move you joystick to its limits. Anyway, you
  30should keep a hand on your device, in order to avoid it to break down if
  31something goes wrong.
  32
  33If you have a serial iforce device, you need to start inputattach. See
  34joystick.txt for details.
  35
  362.1 Does it work ?
  37~~~~~~~~~~~~~~~~~~
  38There is an utility called fftest that will allow you to test the driver.
  39% fftest /dev/input/eventXX
  40
  413. Instructions to the developer
  42~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  43All interactions are done using the event API. That is, you can use ioctl()
  44and write() on /dev/input/eventXX.
  45This information is subject to change.
  46
  473.1 Querying device capabilities
  48~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  49#include <linux/input.h>
  50#include <sys/ioctl.h>
  51
  52unsigned long features[1 + FF_MAX/sizeof(unsigned long)];
  53int ioctl(int file_descriptor, int request, unsigned long *features);
  54
  55"request" must be EVIOCGBIT(EV_FF, size of features array in bytes )
  56
  57Returns the features supported by the device. features is a bitfield with the
  58following bits:
  59- FF_CONSTANT   can render constant force effects
  60- FF_PERIODIC   can render periodic effects with the following waveforms:
  61  - FF_SQUARE     square waveform
  62  - FF_TRIANGLE   triangle waveform
  63  - FF_SINE       sine waveform
  64  - FF_SAW_UP     sawtooth up waveform
  65  - FF_SAW_DOWN   sawtooth down waveform
  66  - FF_CUSTOM     custom waveform
  67- FF_RAMP       can render ramp effects
  68- FF_SPRING     can simulate the presence of a spring
  69- FF_FRICTION   can simulate friction
  70- FF_DAMPER     can simulate damper effects
  71- FF_RUMBLE     rumble effects
  72- FF_INERTIA    can simulate inertia
  73- FF_GAIN       gain is adjustable
  74- FF_AUTOCENTER autocenter is adjustable
  75
  76Note: In most cases you should use FF_PERIODIC instead of FF_RUMBLE. All
  77      devices that support FF_RUMBLE support FF_PERIODIC (square, triangle,
  78      sine) and the other way around.
  79
  80Note: The exact syntax FF_CUSTOM is undefined for the time being as no driver
  81      supports it yet.
  82
  83
  84int ioctl(int fd, EVIOCGEFFECTS, int *n);
  85
  86Returns the number of effects the device can keep in its memory.
  87
  883.2 Uploading effects to the device
  89~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  90#include <linux/input.h>
  91#include <sys/ioctl.h>
  92
  93int ioctl(int file_descriptor, int request, struct ff_effect *effect);
  94
  95"request" must be EVIOCSFF.
  96
  97"effect" points to a structure describing the effect to upload. The effect is
  98uploaded, but not played.
  99The content of effect may be modified. In particular, its field "id" is set
 100to the unique id assigned by the driver. This data is required for performing
 101some operations (removing an effect, controlling the playback).
 102This if field must be set to -1 by the user in order to tell the driver to
 103allocate a new effect.
 104
 105Effects are file descriptor specific.
 106
 107See <linux/input.h> for a description of the ff_effect struct. You should also
 108find help in a few sketches, contained in files shape.fig and interactive.fig.
 109You need xfig to visualize these files.
 110
 1113.3 Removing an effect from the device
 112~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 113int ioctl(int fd, EVIOCRMFF, effect.id);
 114
 115This makes room for new effects in the device's memory. Note that this also
 116stops the effect if it was playing.
 117
 1183.4 Controlling the playback of effects
 119~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 120Control of playing is done with write(). Below is an example:
 121
 122#include <linux/input.h>
 123#include <unistd.h>
 124
 125        struct input_event play;
 126        struct input_event stop;
 127        struct ff_effect effect;
 128        int fd;
 129...
 130        fd = open("/dev/input/eventXX", O_RDWR);
 131...
 132        /* Play three times */
 133        play.type = EV_FF;
 134        play.code = effect.id;
 135        play.value = 3;
 136
 137        write(fd, (const void*) &play, sizeof(play));
 138...
 139        /* Stop an effect */
 140        stop.type = EV_FF;
 141        stop.code = effect.id;
 142        stop.value = 0;
 143
 144        write(fd, (const void*) &play, sizeof(stop));
 145
 1463.5 Setting the gain
 147~~~~~~~~~~~~~~~~~~~~
 148Not all devices have the same strength. Therefore, users should set a gain
 149factor depending on how strong they want effects to be. This setting is
 150persistent across access to the driver.
 151
 152/* Set the gain of the device
 153int gain;               /* between 0 and 100 */
 154struct input_event ie;  /* structure used to communicate with the driver */
 155
 156ie.type = EV_FF;
 157ie.code = FF_GAIN;
 158ie.value = 0xFFFFUL * gain / 100;
 159
 160if (write(fd, &ie, sizeof(ie)) == -1)
 161        perror("set gain");
 162
 1633.6 Enabling/Disabling autocenter
 164~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 165The autocenter feature quite disturbs the rendering of effects in my opinion,
 166and I think it should be an effect, which computation depends on the game
 167type. But you can enable it if you want.
 168
 169int autocenter;         /* between 0 and 100 */
 170struct input_event ie;
 171
 172ie.type = EV_FF;
 173ie.code = FF_AUTOCENTER;
 174ie.value = 0xFFFFUL * autocenter / 100;
 175
 176if (write(fd, &ie, sizeof(ie)) == -1)
 177        perror("set auto-center");
 178
 179A value of 0 means "no auto-center".
 180
 1813.7 Dynamic update of an effect
 182~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 183Proceed as if you wanted to upload a new effect, except that instead of
 184setting the id field to -1, you set it to the wanted effect id.
 185Normally, the effect is not stopped and restarted. However, depending on the
 186type of device, not all parameters can be dynamically updated. For example,
 187the direction of an effect cannot be updated with iforce devices. In this
 188case, the driver stops the effect, up-load it, and restart it.
 189
 190Therefore it is recommended to dynamically change direction while the effect
 191is playing only when it is ok to restart the effect with a replay count of 1.
 192
 1933.8 Information about the status of effects
 194~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 195Every time the status of an effect is changed, an event is sent. The values
 196and meanings of the fields of the event are as follows:
 197
 198struct input_event {
 199/* When the status of the effect changed */
 200        struct timeval time;
 201
 202/* Set to EV_FF_STATUS */
 203        unsigned short type;
 204
 205/* Contains the id of the effect */
 206        unsigned short code;
 207
 208/* Indicates the status */
 209        unsigned int value;
 210};
 211
 212FF_STATUS_STOPPED       The effect stopped playing
 213FF_STATUS_PLAYING       The effect started to play
 214
 215NOTE: Status feedback is only supported by iforce driver. If you have
 216      a really good reason to use this, please contact
 217      linux-joystick@atrey.karlin.mff.cuni.cz or anssi.hannula@gmail.com
 218      so that support for it can be added to the rest of the drivers.
 219
 220