Force feedback for Linux.
  By Johann Deneux <johann.deneux@gmail.com> on 2001/04/22.
  Updated by 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.
  ----------------------------------------------------------------------------
  
  1. 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.
  
  2. 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.
  
  2.1 Does it work ?
  ~~~~~~~~~~~~~~~~~~
  There is an utility called fftest that will allow you to test the driver.
  % fftest /dev/input/eventXX
  
  3. 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.
  
  3.1 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.
  
  Note: 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.
  
  3.2 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.
  
  3.3 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.
  
  3.4 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));
  
  3.5 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");
  
  3.6 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".
  
  3.7 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.
  
  3.8 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.