Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 EST.
1 Force feedback for Linux. 2 By Johann Deneux <johann.deneux@gmail.com> on 2001/04/22. 3 Updated by Anssi Hannula <anssi.hannula@gmail.com> on 2006/04/09. 4 You may redistribute this file. Please remember to include shape.fig and 5 interactive.fig as well. 6 ---------------------------------------------------------------------------- 7 8 1. Introduction 9 ~~~~~~~~~~~~~~~ 10 This document describes how to use force feedback devices under Linux. The 11 goal 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 13 effects. 14 This document only describes the force feedback part of the Linux input 15 interface. Please read joystick.txt and input.txt before reading further this 16 document. 17 18 2. Instructions to the user 19 ~~~~~~~~~~~~~~~~~~~~~~~~~~~ 20 To enable force feedback, you have to: 21 22 1. have your kernel configured with evdev and a driver that supports your 23 device. 24 2. make sure evdev module is loaded and /dev/input/event* device files are 25 created. 26 27 Before you start, let me WARN you that some devices shake violently during the 28 initialisation phase. This happens for example with my "AVB Top Shot Pegasus". 29 To stop this annoying behaviour, move you joystick to its limits. Anyway, you 30 should keep a hand on your device, in order to avoid it to break down if 31 something goes wrong. 32 33 If you have a serial iforce device, you need to start inputattach. See 34 joystick.txt for details. 35 36 2.1 Does it work ? 37 ~~~~~~~~~~~~~~~~~~ 38 There is an utility called fftest that will allow you to test the driver. 39 % fftest /dev/input/eventXX 40 41 3. Instructions to the developer 42 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 43 All interactions are done using the event API. That is, you can use ioctl() 44 and write() on /dev/input/eventXX. 45 This information is subject to change. 46 47 3.1 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))) 54 unsigned long features[BITS_TO_LONGS(FF_CNT)]; 55 int ioctl(int file_descriptor, int request, unsigned long *features); 56 57 "request" must be EVIOCGBIT(EV_FF, size of features array in bytes ) 58 59 Returns the features supported by the device. features is a bitfield with the 60 following 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 78 Note: 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 82 Note: The exact syntax FF_CUSTOM is undefined for the time being as no driver 83 supports it yet. 84 85 86 int ioctl(int fd, EVIOCGEFFECTS, int *n); 87 88 Returns the number of effects the device can keep in its memory. 89 90 3.2 Uploading effects to the device 91 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 92 #include <linux/input.h> 93 #include <sys/ioctl.h> 94 95 int 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 100 uploaded, but not played. 101 The content of effect may be modified. In particular, its field "id" is set 102 to the unique id assigned by the driver. This data is required for performing 103 some operations (removing an effect, controlling the playback). 104 This if field must be set to -1 by the user in order to tell the driver to 105 allocate a new effect. 106 107 Effects are file descriptor specific. 108 109 See <linux/input.h> for a description of the ff_effect struct. You should also 110 find help in a few sketches, contained in files shape.fig and interactive.fig. 111 You need xfig to visualize these files. 112 113 3.3 Removing an effect from the device 114 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 115 int ioctl(int fd, EVIOCRMFF, effect.id); 116 117 This makes room for new effects in the device's memory. Note that this also 118 stops the effect if it was playing. 119 120 3.4 Controlling the playback of effects 121 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 122 Control 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)); 147 148 3.5 Setting the gain 149 ~~~~~~~~~~~~~~~~~~~~ 150 Not all devices have the same strength. Therefore, users should set a gain 151 factor depending on how strong they want effects to be. This setting is 152 persistent across access to the driver. 153 154 /* Set the gain of the device 155 int gain; /* between 0 and 100 */ 156 struct input_event ie; /* structure used to communicate with the driver */ 157 158 ie.type = EV_FF; 159 ie.code = FF_GAIN; 160 ie.value = 0xFFFFUL * gain / 100; 161 162 if (write(fd, &ie, sizeof(ie)) == -1) 163 perror("set gain"); 164 165 3.6 Enabling/Disabling autocenter 166 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 167 The autocenter feature quite disturbs the rendering of effects in my opinion, 168 and I think it should be an effect, which computation depends on the game 169 type. But you can enable it if you want. 170 171 int autocenter; /* between 0 and 100 */ 172 struct input_event ie; 173 174 ie.type = EV_FF; 175 ie.code = FF_AUTOCENTER; 176 ie.value = 0xFFFFUL * autocenter / 100; 177 178 if (write(fd, &ie, sizeof(ie)) == -1) 179 perror("set auto-center"); 180 181 A value of 0 means "no auto-center". 182 183 3.7 Dynamic update of an effect 184 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 185 Proceed as if you wanted to upload a new effect, except that instead of 186 setting the id field to -1, you set it to the wanted effect id. 187 Normally, the effect is not stopped and restarted. However, depending on the 188 type of device, not all parameters can be dynamically updated. For example, 189 the direction of an effect cannot be updated with iforce devices. In this 190 case, the driver stops the effect, up-load it, and restart it. 191 192 Therefore it is recommended to dynamically change direction while the effect 193 is playing only when it is ok to restart the effect with a replay count of 1. 194 195 3.8 Information about the status of effects 196 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 197 Every time the status of an effect is changed, an event is sent. The values 198 and meanings of the fields of the event are as follows: 199 200 struct 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 }; 213 214 FF_STATUS_STOPPED The effect stopped playing 215 FF_STATUS_PLAYING The effect started to play 216 217 NOTE: 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.