1Force feedback for Linux. 2By Johann Deneux <deneux@ifrance.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