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.svg and
9 interactive.svg 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 joydev/joystick.rst and input.rst before reading further
20 this 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 your 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 joydev/joystick.rst 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 The "id" 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 <uapi/linux/input.h> for a description of the ff_effect struct. You
130 should also find help in a few sketches, contained in files shape.svg
131 and interactive.svg:
132
133 .. kernel-figure:: shape.svg
134
135 Shape
136
137 .. kernel-figure:: interactive.svg
138
139 Interactive
140
141
142 Removing an effect from the device
143 ----------------------------------
144
145 ::
146
147 int ioctl(int fd, EVIOCRMFF, effect.id);
148
149 This makes room for new effects in the device's memory. Note that this also
150 stops the effect if it was playing.
151
152 Controlling the playback of effects
153 -----------------------------------
154
155 Control 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*) &stop, sizeof(stop));
182
183 Setting the gain
184 ----------------
185
186 Not all devices have the same strength. Therefore, users should set a gain
187 factor depending on how strong they want effects to be. This setting is
188 persistent 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
203 Enabling/Disabling autocenter
204 -----------------------------
205
206 The autocenter feature quite disturbs the rendering of effects in my opinion,
207 and I think it should be an effect, which computation depends on the game
208 type. 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
222 A value of 0 means "no auto-center".
223
224 Dynamic update of an effect
225 ---------------------------
226
227 Proceed as if you wanted to upload a new effect, except that instead of
228 setting the id field to -1, you set it to the wanted effect id.
229 Normally, the effect is not stopped and restarted. However, depending on the
230 type of device, not all parameters can be dynamically updated. For example,
231 the direction of an effect cannot be updated with iforce devices. In this
232 case, the driver stops the effect, up-load it, and restart it.
233
234 Therefore it is recommended to dynamically change direction while the effect
235 is playing only when it is ok to restart the effect with a replay count of 1.
236
237 Information about the status of effects
238 ---------------------------------------
239
240 Every time the status of an effect is changed, an event is sent. The values
241 and 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.
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