Documentation/input/ff.rst

   1 ========================
   2 Force feedback for Linux
   3 ========================
   4
   5 :Author: Johann Deneux <johann.deneux@gmail.com> on 2001/04/22.
   6 :Updated: Anssi Hannula <anssi.hannula@gmail.com> on 2006/04/09.
   7
   8 You may redistribute this file. Please remember to include shape.fig and
   9 interactive.fig as well.
  10
  11 Introduction
  12 ~~~~~~~~~~~~
  13
  14 This document describes how to use force feedback devices under Linux. The
  15 goal is not to support these devices as if they were simple input-only devices
  16 (as it is already the case), but to really enable the rendering of force
  17 effects.
  18 This document only describes the force feedback part of the Linux input
  19 interface. Please read joystick.txt and input.txt before reading further this
  20 document.
  21
  22 Instructions to the user
  23 ~~~~~~~~~~~~~~~~~~~~~~~~
  24
  25 To enable force feedback, you have to:
  26
  27 1. have your kernel configured with evdev and a driver that supports your
  28    device.
  29 2. make sure evdev module is loaded and /dev/input/event* device files are
  30    created.
  31
  32 Before you start, let me WARN you that some devices shake violently during the
  33 initialisation phase. This happens for example with my "AVB Top Shot Pegasus".
  34 To stop this annoying behaviour, move you joystick to its limits. Anyway, you
  35 should keep a hand on your device, in order to avoid it to break down if
  36 something goes wrong.
  37
  38 If you have a serial iforce device, you need to start inputattach. See
  39 joystick.txt for details.
  40
  41 Does it work ?
  42 --------------
  43
  44 There is an utility called fftest that will allow you to test the driver::
  45
  46     % fftest /dev/input/eventXX
  47
  48 Instructions to the developer
  49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  50
  51 All interactions are done using the event API. That is, you can use ioctl()
  52 and write() on /dev/input/eventXX.
  53 This information is subject to change.
  54
  55 Querying device capabilities
  56 ----------------------------
  57
  58 ::
  59
  60     #include <linux/input.h>
  61     #include <sys/ioctl.h>
  62
  63     #define BITS_TO_LONGS(x) \
  64             (((x) + 8 * sizeof (unsigned long) - 1) / (8 * sizeof (unsigned long)))
  65     unsigned long features[BITS_TO_LONGS(FF_CNT)];
  66     int ioctl(int file_descriptor, int request, unsigned long *features);
  67
  68 "request" must be EVIOCGBIT(EV_FF, size of features array in bytes )
  69
  70 Returns the features supported by the device. features is a bitfield with the
  71 following bits:
  72
  73 - FF_CONSTANT   can render constant force effects
  74 - FF_PERIODIC   can render periodic effects with the following waveforms:
  75
  76   - FF_SQUARE     square waveform
  77   - FF_TRIANGLE   triangle waveform
  78   - FF_SINE       sine waveform
  79   - FF_SAW_UP     sawtooth up waveform
  80   - FF_SAW_DOWN   sawtooth down waveform
  81   - FF_CUSTOM     custom waveform
  82
  83 - FF_RAMP       can render ramp effects
  84 - FF_SPRING     can simulate the presence of a spring
  85 - FF_FRICTION   can simulate friction
  86 - FF_DAMPER     can simulate damper effects
  87 - FF_RUMBLE     rumble effects
  88 - FF_INERTIA    can simulate inertia
  89 - FF_GAIN       gain is adjustable
  90 - FF_AUTOCENTER autocenter is adjustable
  91
  92 .. note::
  93
  94     - In most cases you should use FF_PERIODIC instead of FF_RUMBLE. All
  95       devices that support FF_RUMBLE support FF_PERIODIC (square, triangle,
  96       sine) and the other way around.
  97
  98     - The exact syntax FF_CUSTOM is undefined for the time being as no driver
  99       supports it yet.
 100
 101 ::
 102
 103     int ioctl(int fd, EVIOCGEFFECTS, int *n);
 104
 105 Returns the number of effects the device can keep in its memory.
 106
 107 Uploading effects to the device
 108 -------------------------------
 109
 110 ::
 111
 112     #include <linux/input.h>
 113     #include <sys/ioctl.h>
 114
 115     int ioctl(int file_descriptor, int request, struct ff_effect *effect);
 116
 117 "request" must be EVIOCSFF.
 118
 119 "effect" points to a structure describing the effect to upload. The effect is
 120 uploaded, but not played.
 121 The content of effect may be modified. In particular, its field "id" is set
 122 to the unique id assigned by the driver. This data is required for performing
 123 some operations (removing an effect, controlling the playback).
 124 This if field must be set to -1 by the user in order to tell the driver to
 125 allocate a new effect.
 126
 127 Effects are file descriptor specific.
 128
 129 See <linux/input.h> for a description of the ff_effect struct. You should also
 130 find help in a few sketches, contained in files shape.fig and interactive.fig.
 131 You need xfig to visualize these files.
 132
 133
 134 Removing an effect from the device
 135 ----------------------------------
 136
 137 ::
 138
 139     int ioctl(int fd, EVIOCRMFF, effect.id);
 140
 141 This makes room for new effects in the device's memory. Note that this also
 142 stops the effect if it was playing.
 143
 144 Controlling the playback of effects
 145 -----------------------------------
 146
 147 Control of playing is done with write(). Below is an example:
 148
 149 ::
 150
 151     #include <linux/input.h>
 152     #include <unistd.h>
 153
 154         struct input_event play;
 155         struct input_event stop;
 156         struct ff_effect effect;
 157         int fd;
 158    ...
 159         fd = open("/dev/input/eventXX", O_RDWR);
 160    ...
 161         /* Play three times */
 162         play.type = EV_FF;
 163         play.code = effect.id;
 164         play.value = 3;
 165
 166         write(fd, (const void*) &play, sizeof(play));
 167    ...
 168         /* Stop an effect */
 169         stop.type = EV_FF;
 170         stop.code = effect.id;
 171         stop.value = 0;
 172
 173         write(fd, (const void*) &play, sizeof(stop));
 174
 175 Setting the gain
 176 ----------------
 177
 178 Not all devices have the same strength. Therefore, users should set a gain
 179 factor depending on how strong they want effects to be. This setting is
 180 persistent across access to the driver.
 181
 182 ::
 183
 184     /* Set the gain of the device
 185     int gain;           /* between 0 and 100 */
 186     struct input_event ie;      /* structure used to communicate with the driver */
 187
 188     ie.type = EV_FF;
 189     ie.code = FF_GAIN;
 190     ie.value = 0xFFFFUL * gain / 100;
 191
 192     if (write(fd, &ie, sizeof(ie)) == -1)
 193         perror("set gain");
 194
 195 Enabling/Disabling autocenter
 196 -----------------------------
 197
 198 The autocenter feature quite disturbs the rendering of effects in my opinion,
 199 and I think it should be an effect, which computation depends on the game
 200 type. But you can enable it if you want.
 201
 202 ::
 203
 204     int autocenter;             /* between 0 and 100 */
 205     struct input_event ie;
 206
 207     ie.type = EV_FF;
 208     ie.code = FF_AUTOCENTER;
 209     ie.value = 0xFFFFUL * autocenter / 100;
 210
 211     if (write(fd, &ie, sizeof(ie)) == -1)
 212         perror("set auto-center");
 213
 214 A value of 0 means "no auto-center".
 215
 216 Dynamic update of an effect
 217 ---------------------------
 218
 219 Proceed as if you wanted to upload a new effect, except that instead of
 220 setting the id field to -1, you set it to the wanted effect id.
 221 Normally, the effect is not stopped and restarted. However, depending on the
 222 type of device, not all parameters can be dynamically updated. For example,
 223 the direction of an effect cannot be updated with iforce devices. In this
 224 case, the driver stops the effect, up-load it, and restart it.
 225
 226 Therefore it is recommended to dynamically change direction while the effect
 227 is playing only when it is ok to restart the effect with a replay count of 1.
 228
 229 Information about the status of effects
 230 ---------------------------------------
 231
 232 Every time the status of an effect is changed, an event is sent. The values
 233 and meanings of the fields of the event are as follows::
 234
 235     struct input_event {
 236     /* When the status of the effect changed */
 237             struct timeval time;
 238
 239     /* Set to EV_FF_STATUS */
 240             unsigned short type;
 241
 242     /* Contains the id of the effect */
 243             unsigned short code;
 244
 245     /* Indicates the status */
 246             unsigned int value;
 247     };
 248
 249     FF_STATUS_STOPPED   The effect stopped playing
 250     FF_STATUS_PLAYING   The effect started to play
 251
 252 .. note::
 253
 254     - Status feedback is only supported by iforce driver. If you have
 255       a really good reason to use this, please contact
 256       linux-joystick@atrey.karlin.mff.cuni.cz or anssi.hannula@gmail.com
 257       so that support for it can be added to the rest of the drivers.