Input: update the force feedback documentation

Signed-off-by: Anssi Hannula <anssi.hannula@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

Documentation/input/ff.txt

 Force feedback for Linux.
 By Johann Deneux <deneux@ifrance.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.
 ----------------------------------------------------------------------------
 
-0. Introduction
+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.
-At the moment, only I-Force devices are supported, and not officially. That
-means I had to find out how the protocol works on my own. Of course, the
-information I managed to grasp is far from being complete, and I can not
-guarranty that this driver will work for you.
-This document only describes the force feedback part of the driver for I-Force
-devices. Please read joystick.txt before reading further this document.
+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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Here are instructions on how to compile and use the driver. In fact, this
-driver is the normal iforce, input and evdev drivers written by Vojtech
-Pavlik, plus additions to support force feedback.
+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 brake down if
+should keep a hand on your device, in order to avoid it to break down if
 something goes wrong.
 
-At the kernel's compilation:
-	- Enable IForce/Serial
-	- Enable Event interface
-
-Compile the modules, install them.
-
-You also need inputattach.
-
-You then need to insert the modules into the following order:
-% modprobe joydev
-% modprobe serport		# Only for serial
-% modprobe iforce
-% modprobe evdev
-% ./inputattach -ifor $2 &	# Only for serial
-If you are using USB, you don't need the inputattach step.
-
-Please check that you have all the /dev/input entries needed:
-cd /dev
-rm js*
-mkdir input
-mknod input/js0 c 13 0
-mknod input/js1 c 13 1
-mknod input/js2 c 13 2
-mknod input/js3 c 13 3
-ln -s input/js0 js0
-ln -s input/js1 js1
-ln -s input/js2 js2
-ln -s input/js3 js3
-
-mknod input/event0 c 13 64
-mknod input/event1 c 13 65
-mknod input/event2 c 13 66
-mknod input/event3 c 13 67
+If you have a serial iforce device, you need to start inputattach. See
+joystick.txt for details.
 
 2.1 Does it work ?
 ~~~~~~~~~~~~~~~~~~
@@ -70,9 +40,9 @@ There is an utility called fftest that will allow you to test the driver.
 
 3. Instructions to the developper
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-  All interactions are done using the event API. That is, you can use ioctl()
+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.
+This information is subject to change.
 
 3.1 Querying device capabilities
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -86,18 +56,29 @@ int ioctl(int file_descriptor, int request, unsigned long *features);
 
 Returns the features supported by the device. features is a bitfield with the
 following bits:
-- FF_X		has an X axis (usually joysticks)
-- FF_Y		has an Y axis (usually joysticks)
-- FF_WHEEL	has a wheel (usually sterring wheels)
 - FF_CONSTANT	can render constant force effects
-- FF_PERIODIC	can render periodic effects (sine, triangle, square...)
+- 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_FRICTION	can simulate friction
 - FF_DAMPER	can simulate damper effects
-- FF_RUMBLE	rumble effects (normally the only effect supported by rumble
-		pads)
+- 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);
@@ -108,7 +89,7 @@ Returns the number of effects the device can keep in its memory.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 #include <linux/input.h>
 #include <sys/ioctl.h>
- 
+
 int ioctl(int file_descriptor, int request, struct ff_effect *effect);
 
 "request" must be EVIOCSFF.
@@ -120,6 +101,9 @@ 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.
@@ -128,8 +112,8 @@ You need xfig to visualize these files.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 int ioctl(int fd, EVIOCRMFF, effect.id);
 
-This makes room for new effects in the device's memory. Please note this won't
-stop the effect if it was playing.
+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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -149,22 +133,21 @@ Control of playing is done with write(). Below is an example:
 	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, so you should not care about it if
-you are writing games, as another utility probably already set this for you.
+persistent across access to the driver.
 
 /* Set the gain of the device
 int gain;		/* between 0 and 100 */
@@ -204,11 +187,14 @@ 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;
@@ -225,3 +211,9 @@ struct input_event {
 
 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.
+

include/linux/input.h

@@ -667,98 +667,167 @@ struct input_absinfo {
 
 /*
  * Structures used in ioctls to upload effects to a device
- * The first structures are not passed directly by using ioctls.
- * They are sub-structures of the actually sent structure (called ff_effect)
+ * They are pieces of a bigger structure (called ff_effect)
  */
 
+/*
+ * All duration values are expressed in ms. Values above 32767 ms (0x7fff)
+ * should not be used and have unspecified results.
+ */
+
+/**
+ * struct ff_replay - defines scheduling of the effect
+ * @length: duration of the effect
+ * @delay: delay before effect should start playing
+ */
 struct ff_replay {
-	__u16 length; /* Duration of an effect in ms. All other times are also expressed in ms */
-	__u16 delay;  /* Time to wait before to start playing an effect */
+	__u16 length;
+	__u16 delay;
 };
 
+/**
+ * struct ff_trigger - defines what triggers the effect
+ * @button: number of the button triggering the effect
+ * @interval: controls how soon the effect can be re-triggered
+ */
 struct ff_trigger {
-	__u16 button;   /* Number of button triggering an effect */
-	__u16 interval; /* Time to wait before an effect can be re-triggered (ms) */
+	__u16 button;
+	__u16 interval;
 };
 
+/**
+ * struct ff_envelope - generic effect envelope
+ * @attack_length: duration of the attack (ms)
+ * @attack_level: level at the beginning of the attack
+ * @fade_length: duration of fade (ms)
+ * @fade_level: level at the end of fade
+ *
+ * The @attack_level and @fade_level are absolute values; when applying
+ * envelope force-feedback core will convert to positive/negative
+ * value based on polarity of the default level of the effect.
+ * Valid range for the attack and fade levels is 0x0000 - 0x7fff
+ */
 struct ff_envelope {
-	__u16 attack_length;	/* Duration of attack (ms) */
-	__u16 attack_level;	/* Level at beginning of attack */
-	__u16 fade_length;	/* Duration of fade (ms) */
-	__u16 fade_level;	/* Level at end of fade */
+	__u16 attack_length;
+	__u16 attack_level;
+	__u16 fade_length;
+	__u16 fade_level;
 };
 
-/* FF_CONSTANT */
+/**
+ * struct ff_constant_effect - defines parameters of a constant effect
+ * @level: strength of the effect; may be negative
+ * @envelope: envelope data
+ */
 struct ff_constant_effect {
-	__s16 level;	    /* Strength of effect. Negative values are OK */
+	__s16 level;
 	struct ff_envelope envelope;
 };
 
-/* FF_RAMP */
+/**
+ * struct ff_ramp_effect - defines parameters of a ramp effect
+ * @start_level: beginning strength of the effect; may be negative
+ * @end_level: final strength of the effect; may be negative
+ * @envelope: envelope data
+ */
 struct ff_ramp_effect {
 	__s16 start_level;
 	__s16 end_level;
 	struct ff_envelope envelope;
 };
 
-/* FF_SPRING of FF_FRICTION */
+/**
+ * struct ff_condition_effect - defines a spring or friction effect
+ * @right_saturation: maximum level when joystick moved all way to the right
+ * @left_saturation: same for the left side
+ * @right_coeff: controls how fast the force grows when the joystick moves
+ *	to the right
+ * @left_coeff: same for the left side
+ * @deadband: size of the dead zone, where no force is produced
+ * @center: position of the dead zone
+ */
 struct ff_condition_effect {
-	__u16 right_saturation; /* Max level when joystick is on the right */
-	__u16 left_saturation;  /* Max level when joystick in on the left */
+	__u16 right_saturation;
+	__u16 left_saturation;
 
-	__s16 right_coeff;	/* Indicates how fast the force grows when the
-				   joystick moves to the right */
-	__s16 left_coeff;	/* Same for left side */
-
-	__u16 deadband;	/* Size of area where no force is produced */
-	__s16 center;	/* Position of dead zone */
+	__s16 right_coeff;
+	__s16 left_coeff;
 
+	__u16 deadband;
+	__s16 center;
 };
 
-/* FF_PERIODIC */
+/**
+ * struct ff_periodic_effect - defines parameters of a periodic effect
+ * @waveform: kind of the effect (wave)
+ * @period: period of the wave (ms)
+ * @magnitude: peak value
+ * @offset: mean value of the wave (roughly)
+ * @phase: 'horizontal' shift
+ * @envelope: envelope data
+ * @custom_len: number of samples (FF_CUSTOM only)
+ * @custom_data: buffer of samples (FF_CUSTOM only)
+ *
+ * Known waveforms - FF_SQUARE, FF_TRIANGLE, FF_SINE, FF_SAW_UP,
+ * FF_SAW_DOWN, FF_CUSTOM. The exact syntax FF_CUSTOM is undefined
+ * for the time being as no driver supports it yet.
+ *
+ * Note: the data pointed by custom_data is copied by the driver.
+ * You can therefore dispose of the memory after the upload/update.
+ */
 struct ff_periodic_effect {
-	__u16 waveform;	/* Kind of wave (sine, square...) */
-	__u16 period;	/* in ms */
-	__s16 magnitude;	/* Peak value */
-	__s16 offset;	/* Mean value of wave (roughly) */
-	__u16 phase;		/* 'Horizontal' shift */
+	__u16 waveform;
+	__u16 period;
+	__s16 magnitude;
+	__s16 offset;
+	__u16 phase;
 
 	struct ff_envelope envelope;
 
-/* Only used if waveform == FF_CUSTOM */
-	__u32 custom_len;	/* Number of samples */
-	__s16 *custom_data;	/* Buffer of samples */
-/* Note: the data pointed by custom_data is copied by the driver. You can
- * therefore dispose of the memory after the upload/update */
+	__u32 custom_len;
+	__s16 *custom_data;
 };
 
-/* FF_RUMBLE */
-/* Some rumble pads have two motors of different weight.
-   strong_magnitude represents the magnitude of the vibration generated
-   by the heavy motor.
-*/
+/**
+ * struct ff_rumble_effect - defines parameters of a periodic effect
+ * @strong_magnitude: magnitude of the heavy motor
+ * @weak_magnitude: magnitude of the light one
+ *
+ * Some rumble pads have two motors of different weight. Strong_magnitude
+ * represents the magnitude of the vibration generated by the heavy one.
+ */
 struct ff_rumble_effect {
-	__u16 strong_magnitude;  /* Magnitude of the heavy motor */
-	__u16 weak_magnitude;    /* Magnitude of the light one */
+	__u16 strong_magnitude;
+	__u16 weak_magnitude;
 };
 
-/*
- * Structure sent through ioctl from the application to the driver
+/**
+ * struct ff_effect - defines force feedback effect
+ * @type: type of the effect (FF_CONSTANT, FF_PERIODIC, FF_RAMP, FF_SPRING,
+ *	FF_FRICTION, FF_DAMPER, FF_RUMBLE, FF_INERTIA, or FF_CUSTOM)
+ * @id: an unique id assigned to an effect
+ * @direction: direction of the effect
+ * @trigger: trigger conditions (struct ff_trigger)
+ * @replay: scheduling of the effect (struct ff_replay)
+ * @u: effect-specific structure (one of ff_constant_effect, ff_ramp_effect,
+ *	ff_periodic_effect, ff_condition_effect, ff_rumble_effect) further
+ *	defining effect parameters
+ *
+ * This structure is sent through ioctl from the application to the driver.
+ * To create a new effect aplication should set its @id to -1; the kernel
+ * will return assigned @id which can later be used to update or delete
+ * this effect.
+ *
+ * Direction of the effect is encoded as follows:
+ *	0 deg -> 0x0000 (down)
+ *	90 deg -> 0x4000 (left)
+ *	180 deg -> 0x8000 (up)
+ *	270 deg -> 0xC000 (right)
  */
 struct ff_effect {
 	__u16 type;
-/* Following field denotes the unique id assigned to an effect.
- * If user sets if to -1, a new effect is created, and its id is returned in the same field
- * Else, the user sets it to the effect id it wants to update.
- */
 	__s16 id;
-
-	__u16 direction;	/* Direction. 0 deg -> 0x0000 (down)
-					     90 deg -> 0x4000 (left)
-					    180 deg -> 0x8000 (up)
-					    270 deg -> 0xC000 (right)
-				*/
-
+	__u16 direction;
 	struct ff_trigger trigger;
 	struct ff_replay replay;