Force feedback for Linux. By Johann Deneux on 2001/04/22. ---------------------------------------------------------------------------- 0. 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. 2. Instructions to the user ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here are instructions on how to compile and use the driver. In fact, this driver is the normal iforce.o, input.o and evdev.o drivers written by Vojtech Pavlik, plus additions to support force feedback. 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 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 % modprobe iforce % modprobe evdev % ./inputattach -ifor $2 & # Only for serial For convenience, you may use the shell script named "ff" available from the cvs tree of the Linux Console Project at sourceforge. You can also retrieve it from http://www.esil.univ-mrs.fr/~jdeneux/projects/ff/. 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 2.1 Does it work ? ~~~~~~~~~~~~~~~~~~ There is an utility called fftest that will allow you to test the driver. % fftest /dev/eventXX 3. Instructions to the developper ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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 #include int ioctl(int file_descriptor, int request, unsigned long *features); "request" must be EVIOCGBIT(EV_FF, sizeof(unsigned long)) Returns the features supported by the device. features is a bitfield with the following bits: - FF_X has an X axis (should allways be the case) - FF_Y has an Y axis (usually not the case for wheels) - FF_CONSTANT can render constant force effects - FF_PERIODIC can render periodic effects (sine, ramp, square...) - FF_SPRING can simulate the presence of a spring - FF_FRICTION can simulate friction (aka drag, damper effect...) - FF_RUMBLE rumble effects (normally the only effect supported by rumble pads) - 8 bits from FF_N_EFFECTS_0 containing the number of effects that can be simultaneously played. 3.2 Uploading effects to the device ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #include #include 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). See for a description of the ff_effect stuct. 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. Please note this won't stop 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 #include 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 accross 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. /* 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This consists in changing some parameters of an effect while it's playing. The driver currently does not support that. You still have the brute-force method, which consists in erasing the effect and uploading the updated version. It actually works pretty well. You don't need to stop-and-start the effect.