1======================== 2Force 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 8You may redistribute this file. Please remember to include shape.svg and 9interactive.svg as well. 10 11Introduction 12~~~~~~~~~~~~ 13 14This document describes how to use force feedback devices under Linux. The 15goal 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 17effects. 18This document only describes the force feedback part of the Linux input 19interface. Please read joystick.txt and input.txt before reading further this 20document. 21 22Instructions to the user 23~~~~~~~~~~~~~~~~~~~~~~~~ 24 25To enable force feedback, you have to: 26 271. have your kernel configured with evdev and a driver that supports your 28 device. 292. make sure evdev module is loaded and /dev/input/event* device files are 30 created. 31 32Before you start, let me WARN you that some devices shake violently during the 33initialisation phase. This happens for example with my "AVB Top Shot Pegasus". 34To stop this annoying behaviour, move you joystick to its limits. Anyway, you 35should keep a hand on your device, in order to avoid it to break down if 36something goes wrong. 37 38If you have a serial iforce device, you need to start inputattach. See 39joystick.txt for details. 40 41Does it work ? 42-------------- 43 44There is an utility called fftest that will allow you to test the driver:: 45 46 % fftest /dev/input/eventXX 47 48Instructions to the developer 49~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 50 51All interactions are done using the event API. That is, you can use ioctl() 52and write() on /dev/input/eventXX. 53This information is subject to change. 54 55Querying 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 70Returns the features supported by the device. features is a bitfield with the 71following 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 105Returns the number of effects the device can keep in its memory. 106 107Uploading 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 120uploaded, but not played. 121The content of effect may be modified. In particular, its field "id" is set 122to the unique id assigned by the driver. This data is required for performing 123some operations (removing an effect, controlling the playback). 124This if field must be set to -1 by the user in order to tell the driver to 125allocate a new effect. 126 127Effects are file descriptor specific. 128 129See <uapi/linux/input.h> for a description of the ff_effect struct. You 130should also find help in a few sketches, contained in files shape.svg 131and interactive.svg: 132 133.. kernel-figure:: shape.svg 134 135 Shape 136 137.. kernel-figure:: interactive.svg 138 139 Interactive 140 141 142Removing an effect from the device 143---------------------------------- 144 145:: 146 147 int ioctl(int fd, EVIOCRMFF, effect.id); 148 149This makes room for new effects in the device's memory. Note that this also 150stops the effect if it was playing. 151 152Controlling the playback of effects 153----------------------------------- 154 155Control of playing is done with write(). Below is an example: 156 157:: 158 159 #include <linux/input.h> 160 #include <unistd.h> 161 162 struct input_event play; 163 struct input_event stop; 164 struct ff_effect effect; 165 int fd; 166 ... 167 fd = open("/dev/input/eventXX", O_RDWR); 168 ... 169 /* Play three times */ 170 play.type = EV_FF; 171 play.code = effect.id; 172 play.value = 3; 173 174 write(fd, (const void*) &play, sizeof(play)); 175 ... 176 /* Stop an effect */ 177 stop.type = EV_FF; 178 stop.code = effect.id; 179 stop.value = 0; 180 181 write(fd, (const void*) &play, sizeof(stop)); 182 183Setting the gain 184---------------- 185 186Not all devices have the same strength. Therefore, users should set a gain 187factor depending on how strong they want effects to be. This setting is 188persistent across access to the driver. 189 190:: 191 192 /* Set the gain of the device 193 int gain; /* between 0 and 100 */ 194 struct input_event ie; /* structure used to communicate with the driver */ 195 196 ie.type = EV_FF; 197 ie.code = FF_GAIN; 198 ie.value = 0xFFFFUL * gain / 100; 199 200 if (write(fd, &ie, sizeof(ie)) == -1) 201 perror("set gain"); 202 203Enabling/Disabling autocenter 204----------------------------- 205 206The autocenter feature quite disturbs the rendering of effects in my opinion, 207and I think it should be an effect, which computation depends on the game 208type. But you can enable it if you want. 209 210:: 211 212 int autocenter; /* between 0 and 100 */ 213 struct input_event ie; 214 215 ie.type = EV_FF; 216 ie.code = FF_AUTOCENTER; 217 ie.value = 0xFFFFUL * autocenter / 100; 218 219 if (write(fd, &ie, sizeof(ie)) == -1) 220 perror("set auto-center"); 221 222A value of 0 means "no auto-center". 223 224Dynamic update of an effect 225--------------------------- 226 227Proceed as if you wanted to upload a new effect, except that instead of 228setting the id field to -1, you set it to the wanted effect id. 229Normally, the effect is not stopped and restarted. However, depending on the 230type of device, not all parameters can be dynamically updated. For example, 231the direction of an effect cannot be updated with iforce devices. In this 232case, the driver stops the effect, up-load it, and restart it. 233 234Therefore it is recommended to dynamically change direction while the effect 235is playing only when it is ok to restart the effect with a replay count of 1. 236 237Information about the status of effects 238--------------------------------------- 239 240Every time the status of an effect is changed, an event is sent. The values 241and meanings of the fields of the event are as follows:: 242 243 struct input_event { 244 /* When the status of the effect changed */ 245 struct timeval time; 246 247 /* Set to EV_FF_STATUS */ 248 unsigned short type; 249 250 /* Contains the id of the effect */ 251 unsigned short code; 252 253 /* Indicates the status */ 254 unsigned int value; 255 }; 256 257 FF_STATUS_STOPPED The effect stopped playing 258 FF_STATUS_PLAYING The effect started to play 259 260.. note:: 261 262 - Status feedback is only supported by iforce driver. If you have 263 a really good reason to use this, please contact 264 linux-joystick@atrey.karlin.mff.cuni.cz or anssi.hannula@gmail.com 265 so that support for it can be added to the rest of the drivers. 266
