/Documentation/input/ff.txt

  1Force feedback for Linux.
  2By Johann Deneux <deneux@ifrance.com> on 2001/04/22.
  3You may redistribute this file. Please remember to include shape.fig and
  4interactive.fig as well.
  5----------------------------------------------------------------------------
  6
  70. Introduction
  8~~~~~~~~~~~~~~~
  9This document describes how to use force feedback devices under Linux. The
 10goal is not to support these devices as if they were simple input-only devices
 11(as it is already the case), but to really enable the rendering of force
 12effects.
 13At the moment, only I-Force devices are supported, and not officially. That
 14means I had to find out how the protocol works on my own. Of course, the
 15information I managed to grasp is far from being complete, and I can not
 16guarranty that this driver will work for you.
 17This document only describes the force feedback part of the driver for I-Force
 18devices. Please read joystick.txt before reading further this document.
 19
 202. Instructions to the user
 21~~~~~~~~~~~~~~~~~~~~~~~~~~~
 22Here are instructions on how to compile and use the driver. In fact, this
 23driver is the normal iforce, input and evdev drivers written by Vojtech
 24Pavlik, plus additions to support force feedback.
 25
 26Before you start, let me WARN you that some devices shake violently during the
 27initialisation phase. This happens for example with my "AVB Top Shot Pegasus".
 28To stop this annoying behaviour, move you joystick to its limits. Anyway, you
 29should keep a hand on your device, in order to avoid it to brake down if
 30something goes wrong.
 31
 32At the kernel's compilation:
 33	- Enable IForce/Serial
 34	- Enable Event interface
 35
 36Compile the modules, install them.
 37
 38You also need inputattach.
 39
 40You then need to insert the modules into the following order:
 41% modprobe joydev
 42% modprobe serport		# Only for serial
 43% modprobe iforce
 44% modprobe evdev
 45% ./inputattach -ifor $2 &	# Only for serial
 46If you are using USB, you don't need the inputattach step.
 47
 48Please check that you have all the /dev/input entries needed:
 49cd /dev
 50rm js*
 51mkdir input
 52mknod input/js0 c 13 0
 53mknod input/js1 c 13 1
 54mknod input/js2 c 13 2
 55mknod input/js3 c 13 3
 56ln -s input/js0 js0
 57ln -s input/js1 js1
 58ln -s input/js2 js2
 59ln -s input/js3 js3
 60
 61mknod input/event0 c 13 64
 62mknod input/event1 c 13 65
 63mknod input/event2 c 13 66
 64mknod input/event3 c 13 67
 65
 662.1 Does it work ?
 67~~~~~~~~~~~~~~~~~~
 68There is an utility called fftest that will allow you to test the driver.
 69% fftest /dev/input/eventXX
 70
 713. Instructions to the developper
 72~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 73  All interactions are done using the event API. That is, you can use ioctl()
 74and write() on /dev/input/eventXX.
 75  This information is subject to change.
 76
 773.1 Querying device capabilities
 78~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 79#include <linux/input.h>
 80#include <sys/ioctl.h>
 81
 82unsigned long features[1 + FF_MAX/sizeof(unsigned long)];
 83int ioctl(int file_descriptor, int request, unsigned long *features);
 84
 85"request" must be EVIOCGBIT(EV_FF, size of features array in bytes )
 86
 87Returns the features supported by the device. features is a bitfield with the
 88following bits:
 89- FF_X		has an X axis (usually joysticks)
 90- FF_Y		has an Y axis (usually joysticks)
 91- FF_WHEEL	has a wheel (usually sterring wheels)
 92- FF_CONSTANT	can render constant force effects
 93- FF_PERIODIC	can render periodic effects (sine, triangle, square...)
 94- FF_RAMP       can render ramp effects
 95- FF_SPRING	can simulate the presence of a spring
 96- FF_FRICTION	can simulate friction 
 97- FF_DAMPER	can simulate damper effects
 98- FF_RUMBLE	rumble effects (normally the only effect supported by rumble
 99		pads)
100- FF_INERTIA    can simulate inertia
101
102
103int ioctl(int fd, EVIOCGEFFECTS, int *n);
104
105Returns the number of effects the device can keep in its memory.
106
1073.2 Uploading effects to the device
108~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
109#include <linux/input.h>
110#include <sys/ioctl.h>
111 
112int ioctl(int file_descriptor, int request, struct ff_effect *effect);
113
114"request" must be EVIOCSFF.
115
116"effect" points to a structure describing the effect to upload. The effect is
117uploaded, but not played.
118The content of effect may be modified. In particular, its field "id" is set
119to the unique id assigned by the driver. This data is required for performing
120some operations (removing an effect, controlling the playback).
121This if field must be set to -1 by the user in order to tell the driver to
122allocate a new effect.
123See <linux/input.h> for a description of the ff_effect stuct. You should also
124find help in a few sketches, contained in files shape.fig and interactive.fig.
125You need xfig to visualize these files.
126
1273.3 Removing an effect from the device
128~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
129int ioctl(int fd, EVIOCRMFF, effect.id);
130
131This makes room for new effects in the device's memory. Please note this won't
132stop the effect if it was playing.
133
1343.4 Controlling the playback of effects
135~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
136Control of playing is done with write(). Below is an example:
137
138#include <linux/input.h>
139#include <unistd.h>
140
141	struct input_event play;
142	struct input_event stop;
143	struct ff_effect effect;
144	int fd;
145...
146	fd = open("/dev/input/eventXX", O_RDWR);
147...
148	/* Play three times */
149	play.type = EV_FF;
150	play.code = effect.id;
151	play.value = 3;
152	
153	write(fd, (const void*) &play, sizeof(play));
154...
155	/* Stop an effect */
156	stop.type = EV_FF;
157	stop.code = effect.id;
158	stop.value = 0;
159	
160	write(fd, (const void*) &play, sizeof(stop));
161
1623.5 Setting the gain
163~~~~~~~~~~~~~~~~~~~~
164Not all devices have the same strength. Therefore, users should set a gain
165factor depending on how strong they want effects to be. This setting is
166persistent across access to the driver, so you should not care about it if
167you are writing games, as another utility probably already set this for you.
168
169/* Set the gain of the device
170int gain;		/* between 0 and 100 */
171struct input_event ie;	/* structure used to communicate with the driver */
172
173ie.type = EV_FF;
174ie.code = FF_GAIN;
175ie.value = 0xFFFFUL * gain / 100;
176
177if (write(fd, &ie, sizeof(ie)) == -1)
178	perror("set gain");
179
1803.6 Enabling/Disabling autocenter
181~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
182The autocenter feature quite disturbs the rendering of effects in my opinion,
183and I think it should be an effect, which computation depends on the game
184type. But you can enable it if you want.
185
186int autocenter;		/* between 0 and 100 */
187struct input_event ie;
188
189ie.type = EV_FF;
190ie.code = FF_AUTOCENTER;
191ie.value = 0xFFFFUL * autocenter / 100;
192
193if (write(fd, &ie, sizeof(ie)) == -1)
194	perror("set auto-center");
195
196A value of 0 means "no auto-center".
197
1983.7 Dynamic update of an effect
199~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
200Proceed as if you wanted to upload a new effect, except that instead of
201setting the id field to -1, you set it to the wanted effect id.
202Normally, the effect is not stopped and restarted. However, depending on the
203type of device, not all parameters can be dynamically updated. For example,
204the direction of an effect cannot be updated with iforce devices. In this
205case, the driver stops the effect, up-load it, and restart it.
206
207
2083.8 Information about the status of effects
209~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
210Every time the status of an effect is changed, an event is sent. The values
211and meanings of the fields of the event are as follows:
212struct input_event {
213/* When the status of the effect changed */
214	struct timeval time;
215
216/* Set to EV_FF_STATUS */
217	unsigned short type;
218
219/* Contains the id of the effect */
220	unsigned short code;
221
222/* Indicates the status */
223	unsigned int value;
224};
225
226FF_STATUS_STOPPED	The effect stopped playing
227FF_STATUS_PLAYING	The effect started to play