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diff --git a/Documentation/input/joydev/joystick-api.rst b/Documentation/input/joydev/joystick-api.rst new file mode 100644 index 000000000..5db6dc6fe --- /dev/null +++ b/Documentation/input/joydev/joystick-api.rst @@ -0,0 +1,348 @@ +.. _joystick-api: + +===================== +Programming Interface +===================== + +:Author: Ragnar Hojland Espinosa <ragnar@macula.net> - 7 Aug 1998 + +Introduction +============ + +.. important:: + This document describes legacy ``js`` interface. Newer clients are + encouraged to switch to the generic event (``evdev``) interface. + +The 1.0 driver uses a new, event based approach to the joystick driver. +Instead of the user program polling for the joystick values, the joystick +driver now reports only any changes of its state. See joystick-api.txt, +joystick.h and jstest.c included in the joystick package for more +information. The joystick device can be used in either blocking or +nonblocking mode, and supports select() calls. + +For backward compatibility the old (v0.x) interface is still included. +Any call to the joystick driver using the old interface will return values +that are compatible to the old interface. This interface is still limited +to 2 axes, and applications using it usually decode only 2 buttons, although +the driver provides up to 32. + +Initialization +============== + +Open the joystick device following the usual semantics (that is, with open). +Since the driver now reports events instead of polling for changes, +immediately after the open it will issue a series of synthetic events +(JS_EVENT_INIT) that you can read to obtain the initial state of the +joystick. + +By default, the device is opened in blocking mode:: + + int fd = open ("/dev/input/js0", O_RDONLY); + + +Event Reading +============= + +:: + + struct js_event e; + read (fd, &e, sizeof(e)); + +where js_event is defined as:: + + struct js_event { + __u32 time; /* event timestamp in milliseconds */ + __s16 value; /* value */ + __u8 type; /* event type */ + __u8 number; /* axis/button number */ + }; + +If the read is successful, it will return sizeof(e), unless you wanted to read +more than one event per read as described in section 3.1. + + +js_event.type +------------- + +The possible values of ``type`` are:: + + #define JS_EVENT_BUTTON 0x01 /* button pressed/released */ + #define JS_EVENT_AXIS 0x02 /* joystick moved */ + #define JS_EVENT_INIT 0x80 /* initial state of device */ + +As mentioned above, the driver will issue synthetic JS_EVENT_INIT ORed +events on open. That is, if it's issuing an INIT BUTTON event, the +current type value will be:: + + int type = JS_EVENT_BUTTON | JS_EVENT_INIT; /* 0x81 */ + +If you choose not to differentiate between synthetic or real events +you can turn off the JS_EVENT_INIT bits:: + + type &= ~JS_EVENT_INIT; /* 0x01 */ + + +js_event.number +--------------- + +The values of ``number`` correspond to the axis or button that +generated the event. Note that they carry separate numeration (that +is, you have both an axis 0 and a button 0). Generally, + + =============== ======= + Axis number + =============== ======= + 1st Axis X 0 + 1st Axis Y 1 + 2nd Axis X 2 + 2nd Axis Y 3 + ...and so on + =============== ======= + +Hats vary from one joystick type to another. Some can be moved in 8 +directions, some only in 4. The driver, however, always reports a hat as two +independent axes, even if the hardware doesn't allow independent movement. + + +js_event.value +-------------- + +For an axis, ``value`` is a signed integer between -32767 and +32767 +representing the position of the joystick along that axis. If you +don't read a 0 when the joystick is ``dead``, or if it doesn't span the +full range, you should recalibrate it (with, for example, jscal). + +For a button, ``value`` for a press button event is 1 and for a release +button event is 0. + +Though this:: + + if (js_event.type == JS_EVENT_BUTTON) { + buttons_state ^= (1 << js_event.number); + } + +may work well if you handle JS_EVENT_INIT events separately, + +:: + + if ((js_event.type & ~JS_EVENT_INIT) == JS_EVENT_BUTTON) { + if (js_event.value) + buttons_state |= (1 << js_event.number); + else + buttons_state &= ~(1 << js_event.number); + } + +is much safer since it can't lose sync with the driver. As you would +have to write a separate handler for JS_EVENT_INIT events in the first +snippet, this ends up being shorter. + + +js_event.time +------------- + +The time an event was generated is stored in ``js_event.time``. It's a time +in milliseconds since ... well, since sometime in the past. This eases the +task of detecting double clicks, figuring out if movement of axis and button +presses happened at the same time, and similar. + + +Reading +======= + +If you open the device in blocking mode, a read will block (that is, +wait) forever until an event is generated and effectively read. There +are two alternatives if you can't afford to wait forever (which is, +admittedly, a long time;) + + a) use select to wait until there's data to be read on fd, or + until it timeouts. There's a good example on the select(2) + man page. + + b) open the device in non-blocking mode (O_NONBLOCK) + + +O_NONBLOCK +---------- + +If read returns -1 when reading in O_NONBLOCK mode, this isn't +necessarily a "real" error (check errno(3)); it can just mean there +are no events pending to be read on the driver queue. You should read +all events on the queue (that is, until you get a -1). + +For example, + +:: + + while (1) { + while (read (fd, &e, sizeof(e)) > 0) { + process_event (e); + } + /* EAGAIN is returned when the queue is empty */ + if (errno != EAGAIN) { + /* error */ + } + /* do something interesting with processed events */ + } + +One reason for emptying the queue is that if it gets full you'll start +missing events since the queue is finite, and older events will get +overwritten. + +The other reason is that you want to know all that happened, and not +delay the processing till later. + +Why can the queue get full? Because you don't empty the queue as +mentioned, or because too much time elapses from one read to another +and too many events to store in the queue get generated. Note that +high system load may contribute to space those reads even more. + +If time between reads is enough to fill the queue and lose an event, +the driver will switch to startup mode and next time you read it, +synthetic events (JS_EVENT_INIT) will be generated to inform you of +the actual state of the joystick. + + +.. note:: + + As of version 1.2.8, the queue is circular and able to hold 64 + events. You can increment this size bumping up JS_BUFF_SIZE in + joystick.h and recompiling the driver. + + +In the above code, you might as well want to read more than one event +at a time using the typical read(2) functionality. For that, you would +replace the read above with something like:: + + struct js_event mybuffer[0xff]; + int i = read (fd, mybuffer, sizeof(mybuffer)); + +In this case, read would return -1 if the queue was empty, or some +other value in which the number of events read would be i / +sizeof(js_event) Again, if the buffer was full, it's a good idea to +process the events and keep reading it until you empty the driver queue. + + +IOCTLs +====== + +The joystick driver defines the following ioctl(2) operations:: + + /* function 3rd arg */ + #define JSIOCGAXES /* get number of axes char */ + #define JSIOCGBUTTONS /* get number of buttons char */ + #define JSIOCGVERSION /* get driver version int */ + #define JSIOCGNAME(len) /* get identifier string char */ + #define JSIOCSCORR /* set correction values &js_corr */ + #define JSIOCGCORR /* get correction values &js_corr */ + +For example, to read the number of axes:: + + char number_of_axes; + ioctl (fd, JSIOCGAXES, &number_of_axes); + + +JSIOGCVERSION +------------- + +JSIOGCVERSION is a good way to check in run-time whether the running +driver is 1.0+ and supports the event interface. If it is not, the +IOCTL will fail. For a compile-time decision, you can test the +JS_VERSION symbol:: + + #ifdef JS_VERSION + #if JS_VERSION > 0xsomething + + +JSIOCGNAME +---------- + +JSIOCGNAME(len) allows you to get the name string of the joystick - the same +as is being printed at boot time. The 'len' argument is the length of the +buffer provided by the application asking for the name. It is used to avoid +possible overrun should the name be too long:: + + char name[128]; + if (ioctl(fd, JSIOCGNAME(sizeof(name)), name) < 0) + strscpy(name, "Unknown", sizeof(name)); + printf("Name: %s\n", name); + + +JSIOC[SG]CORR +------------- + +For usage on JSIOC[SG]CORR I suggest you to look into jscal.c They are +not needed in a normal program, only in joystick calibration software +such as jscal or kcmjoy. These IOCTLs and data types aren't considered +to be in the stable part of the API, and therefore may change without +warning in following releases of the driver. + +Both JSIOCSCORR and JSIOCGCORR expect &js_corr to be able to hold +information for all axes. That is, struct js_corr corr[MAX_AXIS]; + +struct js_corr is defined as:: + + struct js_corr { + __s32 coef[8]; + __u16 prec; + __u16 type; + }; + +and ``type``:: + + #define JS_CORR_NONE 0x00 /* returns raw values */ + #define JS_CORR_BROKEN 0x01 /* broken line */ + + +Backward compatibility +====================== + +The 0.x joystick driver API is quite limited and its usage is deprecated. +The driver offers backward compatibility, though. Here's a quick summary:: + + struct JS_DATA_TYPE js; + while (1) { + if (read (fd, &js, JS_RETURN) != JS_RETURN) { + /* error */ + } + usleep (1000); + } + +As you can figure out from the example, the read returns immediately, +with the actual state of the joystick:: + + struct JS_DATA_TYPE { + int buttons; /* immediate button state */ + int x; /* immediate x axis value */ + int y; /* immediate y axis value */ + }; + +and JS_RETURN is defined as:: + + #define JS_RETURN sizeof(struct JS_DATA_TYPE) + +To test the state of the buttons, + +:: + + first_button_state = js.buttons & 1; + second_button_state = js.buttons & 2; + +The axis values do not have a defined range in the original 0.x driver, +except that the values are non-negative. The 1.2.8+ drivers use a +fixed range for reporting the values, 1 being the minimum, 128 the +center, and 255 maximum value. + +The v0.8.0.2 driver also had an interface for 'digital joysticks', (now +called Multisystem joysticks in this driver), under /dev/djsX. This driver +doesn't try to be compatible with that interface. + + +Final Notes +=========== + +:: + + ____/| Comments, additions, and specially corrections are welcome. + \ o.O| Documentation valid for at least version 1.2.8 of the joystick + =(_)= driver and as usual, the ultimate source for documentation is + U to "Use The Source Luke" or, at your convenience, Vojtech ;) |