About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / input / ff.txt




Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:03 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.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.