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authorMauro Carvalho Chehab <mchehab@s-opensource.com>2017-04-04 17:51:04 -0700
committerDmitry Torokhov <dmitry.torokhov@gmail.com>2017-04-05 15:45:07 -0700
commite2ba573120feadfb365467f0cdae2918926efabc (patch)
tree0274ef579fcafc4646d0c82eeb4ae826ff61cfbd /Documentation/input/ff.txt
parent1ad1473f65da8e61120e8f1b68bc92f2b71ba879 (diff)
downloadlinux-e2ba573120feadfb365467f0cdae2918926efabc.tar.bz2
Input: create a book with Linux Input documentation
Now that all files under Documentation/input follows the ReST markup language, rename them to *.rst and create a book for the Linux Input subsystem. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
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-========================
-Force feedback for Linux
-========================
-
-:Author: Johann Deneux <johann.deneux@gmail.com> on 2001/04/22.
-:Updated: Anssi Hannula <anssi.hannula@gmail.com> on 2006/04/09.
-
-You may redistribute this file. Please remember to include shape.fig and
-interactive.fig as well.
-
-Introduction
-~~~~~~~~~~~~
-
-This document describes how to use force feedback devices under Linux. The
-goal is not to support these devices as if they were simple input-only devices
-(as it is already the case), but to really enable the rendering of force
-effects.
-This document only describes the force feedback part of the Linux input
-interface. Please read joystick.txt and input.txt before reading further this
-document.
-
-Instructions to the user
-~~~~~~~~~~~~~~~~~~~~~~~~
-
-To enable force feedback, you have to:
-
-1. have your kernel configured with evdev and a driver that supports your
- device.
-2. make sure evdev module is loaded and /dev/input/event* device files are
- created.
-
-Before you start, let me WARN you that some devices shake violently during the
-initialisation phase. This happens for example with my "AVB Top Shot Pegasus".
-To stop this annoying behaviour, move you joystick to its limits. Anyway, you
-should keep a hand on your device, in order to avoid it to break down if
-something goes wrong.
-
-If you have a serial iforce device, you need to start inputattach. See
-joystick.txt for details.
-
-Does it work ?
---------------
-
-There is an utility called fftest that will allow you to test the driver::
-
- % fftest /dev/input/eventXX
-
-Instructions to the developer
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-All interactions are done using the event API. That is, you can use ioctl()
-and write() on /dev/input/eventXX.
-This information is subject to change.
-
-Querying device capabilities
-----------------------------
-
-::
-
- #include <linux/input.h>
- #include <sys/ioctl.h>
-
- #define BITS_TO_LONGS(x) \
- (((x) + 8 * sizeof (unsigned long) - 1) / (8 * sizeof (unsigned long)))
- unsigned long features[BITS_TO_LONGS(FF_CNT)];
- int ioctl(int file_descriptor, int request, unsigned long *features);
-
-"request" must be EVIOCGBIT(EV_FF, size of features array in bytes )
-
-Returns the features supported by the device. features is a bitfield with the
-following bits:
-
-- FF_CONSTANT can render constant force effects
-- FF_PERIODIC can render periodic effects with the following waveforms:
-
- - FF_SQUARE square waveform
- - FF_TRIANGLE triangle waveform
- - FF_SINE sine waveform
- - FF_SAW_UP sawtooth up waveform
- - FF_SAW_DOWN sawtooth down waveform
- - FF_CUSTOM custom waveform
-
-- FF_RAMP can render ramp effects
-- FF_SPRING can simulate the presence of a spring
-- FF_FRICTION can simulate friction
-- FF_DAMPER can simulate damper effects
-- FF_RUMBLE rumble effects
-- FF_INERTIA can simulate inertia
-- FF_GAIN gain is adjustable
-- FF_AUTOCENTER autocenter is adjustable
-
-.. note::
-
- - In most cases you should use FF_PERIODIC instead of FF_RUMBLE. All
- devices that support FF_RUMBLE support FF_PERIODIC (square, triangle,
- sine) and the other way around.
-
- - The exact syntax FF_CUSTOM is undefined for the time being as no driver
- supports it yet.
-
-::
-
- int ioctl(int fd, EVIOCGEFFECTS, int *n);
-
-Returns the number of effects the device can keep in its memory.
-
-Uploading effects to the device
--------------------------------
-
-::
-
- #include <linux/input.h>
- #include <sys/ioctl.h>
-
- int ioctl(int file_descriptor, int request, struct ff_effect *effect);
-
-"request" must be EVIOCSFF.
-
-"effect" points to a structure describing the effect to upload. The effect is
-uploaded, but not played.
-The content of effect may be modified. In particular, its field "id" is set
-to the unique id assigned by the driver. This data is required for performing
-some operations (removing an effect, controlling the playback).
-This if field must be set to -1 by the user in order to tell the driver to
-allocate a new effect.
-
-Effects are file descriptor specific.
-
-See <linux/input.h> for a description of the ff_effect struct. You should also
-find help in a few sketches, contained in files shape.fig and interactive.fig.
-You need xfig to visualize these files.
-
-
-Removing an effect from the device
-----------------------------------
-
-::
-
- int ioctl(int fd, EVIOCRMFF, effect.id);
-
-This makes room for new effects in the device's memory. Note that this also
-stops the effect if it was playing.
-
-Controlling the playback of effects
------------------------------------
-
-Control of playing is done with write(). Below is an example:
-
-::
-
- #include <linux/input.h>
- #include <unistd.h>
-
- struct input_event play;
- struct input_event stop;
- struct ff_effect effect;
- int fd;
- ...
- fd = open("/dev/input/eventXX", O_RDWR);
- ...
- /* Play three times */
- play.type = EV_FF;
- play.code = effect.id;
- play.value = 3;
-
- write(fd, (const void*) &play, sizeof(play));
- ...
- /* Stop an effect */
- stop.type = EV_FF;
- stop.code = effect.id;
- stop.value = 0;
-
- write(fd, (const void*) &play, sizeof(stop));
-
-Setting the gain
-----------------
-
-Not all devices have the same strength. Therefore, users should set a gain
-factor depending on how strong they want effects to be. This setting is
-persistent across access to the driver.
-
-::
-
- /* Set the gain of the device
- int gain; /* between 0 and 100 */
- struct input_event ie; /* structure used to communicate with the driver */
-
- ie.type = EV_FF;
- ie.code = FF_GAIN;
- ie.value = 0xFFFFUL * gain / 100;
-
- if (write(fd, &ie, sizeof(ie)) == -1)
- perror("set gain");
-
-Enabling/Disabling autocenter
------------------------------
-
-The autocenter feature quite disturbs the rendering of effects in my opinion,
-and I think it should be an effect, which computation depends on the game
-type. But you can enable it if you want.
-
-::
-
- int autocenter; /* between 0 and 100 */
- struct input_event ie;
-
- ie.type = EV_FF;
- ie.code = FF_AUTOCENTER;
- ie.value = 0xFFFFUL * autocenter / 100;
-
- if (write(fd, &ie, sizeof(ie)) == -1)
- perror("set auto-center");
-
-A value of 0 means "no auto-center".
-
-Dynamic update of an effect
----------------------------
-
-Proceed as if you wanted to upload a new effect, except that instead of
-setting the id field to -1, you set it to the wanted effect id.
-Normally, the effect is not stopped and restarted. However, depending on the
-type of device, not all parameters can be dynamically updated. For example,
-the direction of an effect cannot be updated with iforce devices. In this
-case, the driver stops the effect, up-load it, and restart it.
-
-Therefore it is recommended to dynamically change direction while the effect
-is playing only when it is ok to restart the effect with a replay count of 1.
-
-Information about the status of effects
----------------------------------------
-
-Every time the status of an effect is changed, an event is sent. The values
-and meanings of the fields of the event are as follows::
-
- struct input_event {
- /* When the status of the effect changed */
- struct timeval time;
-
- /* Set to EV_FF_STATUS */
- unsigned short type;
-
- /* Contains the id of the effect */
- unsigned short code;
-
- /* Indicates the status */
- unsigned int value;
- };
-
- FF_STATUS_STOPPED The effect stopped playing
- FF_STATUS_PLAYING The effect started to play
-
-.. note::
-
- - Status feedback is only supported by iforce driver. If you have
- a really good reason to use this, please contact
- linux-joystick@atrey.karlin.mff.cuni.cz or anssi.hannula@gmail.com
- so that support for it can be added to the rest of the drivers.