mirror of https://gitee.com/openkylin/linux.git
315 lines
9.3 KiB
Plaintext
315 lines
9.3 KiB
Plaintext
Joystick API Documentation -*-Text-*-
|
|
|
|
Ragnar Hojland Espinosa
|
|
<ragnar@macula.net>
|
|
|
|
7 Aug 1998
|
|
|
|
1. 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 check the initial state of the
|
|
joystick.
|
|
|
|
By default, the device is opened in blocking mode.
|
|
|
|
int fd = open ("/dev/input/js0", O_RDONLY);
|
|
|
|
|
|
2. 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.
|
|
|
|
|
|
2.1 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 a 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 */
|
|
|
|
|
|
2.2 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,
|
|
|
|
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 axis, even if the hardware doesn't allow independent movement.
|
|
|
|
|
|
2.3 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.
|
|
|
|
|
|
2.4 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.
|
|
|
|
|
|
3. 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)
|
|
|
|
|
|
3.1 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 what happened, and not
|
|
delay the processing till later.
|
|
|
|
Why can get the queue 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.
|
|
|
|
[As for 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.
|
|
|
|
|
|
4. 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);
|
|
|
|
|
|
4.1 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
|
|
|
|
|
|
4.2 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)
|
|
strncpy(name, "Unknown", sizeof(name));
|
|
printf("Name: %s\n", name);
|
|
|
|
|
|
4.3 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 axis. 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 */
|
|
|
|
|
|
5. 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 for 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.
|
|
|
|
|
|
6. 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 ;)
|
|
|
|
- Ragnar
|
|
EOF
|