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
  52#define BITS_TO_LONGS(x) \
  53        (((x) + 8 * sizeof (unsigned long) - 1) / (8 * sizeof (unsigned long)))
  54unsigned long features[BITS_TO_LONGS(FF_CNT)];
  55int ioctl(int file_descriptor, int request, unsigned long *features);
  56
  57"request" must be EVIOCGBIT(EV_FF, size of features array in bytes )
  58
  59Returns the features supported by the device. features is a bitfield with the
  60following bits:
  61- FF_CONSTANT   can render constant force effects
  62- FF_PERIODIC   can render periodic effects with the following waveforms:
  63  - FF_SQUARE     square waveform
  64  - FF_TRIANGLE   triangle waveform
  65  - FF_SINE       sine waveform
  66  - FF_SAW_UP     sawtooth up waveform
  67  - FF_SAW_DOWN   sawtooth down waveform
  68  - FF_CUSTOM     custom waveform
  69- FF_RAMP       can render ramp effects
  70- FF_SPRING     can simulate the presence of a spring
  71- FF_FRICTION   can simulate friction
  72- FF_DAMPER     can simulate damper effects
  73- FF_RUMBLE     rumble effects
  74- FF_INERTIA    can simulate inertia
  75- FF_GAIN       gain is adjustable
  76- FF_AUTOCENTER autocenter is adjustable
  77
  78Note: In most cases you should use FF_PERIODIC instead of FF_RUMBLE. All
  79      devices that support FF_RUMBLE support FF_PERIODIC (square, triangle,
  80      sine) and the other way around.
  81
  82Note: The exact syntax FF_CUSTOM is undefined for the time being as no driver
  83      supports it yet.
  84
  85
  86int ioctl(int fd, EVIOCGEFFECTS, int *n);
  87
  88Returns the number of effects the device can keep in its memory.
  89
  903.2 Uploading effects to the device
  91~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  92#include <linux/input.h>
  93#include <sys/ioctl.h>
  94
  95int ioctl(int file_descriptor, int request, struct ff_effect *effect);
  96
  97"request" must be EVIOCSFF.
  98
  99"effect" points to a structure describing the effect to upload. The effect is
 100uploaded, but not played.
 101The content of effect may be modified. In particular, its field "id" is set
 102to the unique id assigned by the driver. This data is required for performing
 103some operations (removing an effect, controlling the playback).
 104This if field must be set to -1 by the user in order to tell the driver to
 105allocate a new effect.
 106
 107Effects are file descriptor specific.
 108
 109See <linux/input.h> for a description of the ff_effect struct. You should also
 110find help in a few sketches, contained in files shape.fig and interactive.fig.
 111You need xfig to visualize these files.
 112
 1133.3 Removing an effect from the device
 114~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 115int ioctl(int fd, EVIOCRMFF, effect.id);
 116
 117This makes room for new effects in the device's memory. Note that this also
 118stops the effect if it was playing.
 119
 1203.4 Controlling the playback of effects
 121~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 122Control of playing is done with write(). Below is an example:
 123
 124#include <linux/input.h>
 125#include <unistd.h>
 126
 127        struct input_event play;
 128        struct input_event stop;
 129        struct ff_effect effect;
 130        int fd;
 131...
 132        fd = open("/dev/input/eventXX", O_RDWR);
 133...
 134        /* Play three times */
 135        play.type = EV_FF;
 136        play.code = effect.id;
 137        play.value = 3;
 138
 139        write(fd, (const void*) &play, sizeof(play));
 140...
 141        /* Stop an effect */
 142        stop.type = EV_FF;
 143        stop.code = effect.id;
 144        stop.value = 0;
 145
 146        write(fd, (const void*) &play, sizeof(stop));
 147
 1483.5 Setting the gain
 149~~~~~~~~~~~~~~~~~~~~
 150Not all devices have the same strength. Therefore, users should set a gain
 151factor depending on how strong they want effects to be. This setting is
 152persistent across access to the driver.
 153
 154/* Set the gain of the device
 155int gain;               /* between 0 and 100 */
 156struct input_event ie;  /* structure used to communicate with the driver */
 157
 158ie.type = EV_FF;
 159ie.code = FF_GAIN;
 160ie.value = 0xFFFFUL * gain / 100;
 161
 162if (write(fd, &ie, sizeof(ie)) == -1)
 163        perror("set gain");
 164
 1653.6 Enabling/Disabling autocenter
 166~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 167The autocenter feature quite disturbs the rendering of effects in my opinion,
 168and I think it should be an effect, which computation depends on the game
 169type. But you can enable it if you want.
 170
 171int autocenter;         /* between 0 and 100 */
 172struct input_event ie;
 173
 174ie.type = EV_FF;
 175ie.code = FF_AUTOCENTER;
 176ie.value = 0xFFFFUL * autocenter / 100;
 177
 178if (write(fd, &ie, sizeof(ie)) == -1)
 179        perror("set auto-center");
 180
 181A value of 0 means "no auto-center".
 182
 1833.7 Dynamic update of an effect
 184~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 185Proceed as if you wanted to upload a new effect, except that instead of
 186setting the id field to -1, you set it to the wanted effect id.
 187Normally, the effect is not stopped and restarted. However, depending on the
 188type of device, not all parameters can be dynamically updated. For example,
 189the direction of an effect cannot be updated with iforce devices. In this
 190case, the driver stops the effect, up-load it, and restart it.
 191
 192Therefore it is recommended to dynamically change direction while the effect
 193is playing only when it is ok to restart the effect with a replay count of 1.
 194
 1953.8 Information about the status of effects
 196~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 197Every time the status of an effect is changed, an event is sent. The values
 198and meanings of the fields of the event are as follows:
 199
 200struct input_event {
 201/* When the status of the effect changed */
 202        struct timeval time;
 203
 204/* Set to EV_FF_STATUS */
 205        unsigned short type;
 206
 207/* Contains the id of the effect */
 208        unsigned short code;
 209
 210/* Indicates the status */
 211        unsigned int value;
 212};
 213
 214FF_STATUS_STOPPED       The effect stopped playing
 215FF_STATUS_PLAYING       The effect started to play
 216
 217NOTE: Status feedback is only supported by iforce driver. If you have
 218      a really good reason to use this, please contact
 219      linux-joystick@atrey.karlin.mff.cuni.cz or anssi.hannula@gmail.com
 220      so that support for it can be added to the rest of the drivers.
 221
 222
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