linux/Documentation/input/input-programming.txt
<<
>>
Prefs
   1Programming input drivers
   2~~~~~~~~~~~~~~~~~~~~~~~~~
   3
   41. Creating an input device driver
   5~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   6
   71.0 The simplest example
   8~~~~~~~~~~~~~~~~~~~~~~~~
   9
  10Here comes a very simple example of an input device driver. The device has
  11just one button and the button is accessible at i/o port BUTTON_PORT. When
  12pressed or released a BUTTON_IRQ happens. The driver could look like:
  13
  14#include <linux/input.h>
  15#include <linux/module.h>
  16#include <linux/init.h>
  17
  18#include <asm/irq.h>
  19#include <asm/io.h>
  20
  21static struct input_dev *button_dev;
  22
  23static irqreturn_t button_interrupt(int irq, void *dummy)
  24{
  25        input_report_key(button_dev, BTN_0, inb(BUTTON_PORT) & 1);
  26        input_sync(button_dev);
  27        return IRQ_HANDLED;
  28}
  29
  30static int __init button_init(void)
  31{
  32        int error;
  33
  34        if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
  35                printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
  36                return -EBUSY;
  37        }
  38
  39        button_dev = input_allocate_device();
  40        if (!button_dev) {
  41                printk(KERN_ERR "button.c: Not enough memory\n");
  42                error = -ENOMEM;
  43                goto err_free_irq;
  44        }
  45
  46        button_dev->evbit[0] = BIT_MASK(EV_KEY);
  47        button_dev->keybit[BIT_WORD(BTN_0)] = BIT_MASK(BTN_0);
  48
  49        error = input_register_device(button_dev);
  50        if (error) {
  51                printk(KERN_ERR "button.c: Failed to register device\n");
  52                goto err_free_dev;
  53        }
  54
  55        return 0;
  56
  57 err_free_dev:
  58        input_free_device(button_dev);
  59 err_free_irq:
  60        free_irq(BUTTON_IRQ, button_interrupt);
  61        return error;
  62}
  63
  64static void __exit button_exit(void)
  65{
  66        input_unregister_device(button_dev);
  67        free_irq(BUTTON_IRQ, button_interrupt);
  68}
  69
  70module_init(button_init);
  71module_exit(button_exit);
  72
  731.1 What the example does
  74~~~~~~~~~~~~~~~~~~~~~~~~~
  75
  76First it has to include the <linux/input.h> file, which interfaces to the
  77input subsystem. This provides all the definitions needed.
  78
  79In the _init function, which is called either upon module load or when
  80booting the kernel, it grabs the required resources (it should also check
  81for the presence of the device).
  82
  83Then it allocates a new input device structure with input_allocate_device()
  84and sets up input bitfields. This way the device driver tells the other
  85parts of the input systems what it is - what events can be generated or
  86accepted by this input device. Our example device can only generate EV_KEY
  87type events, and from those only BTN_0 event code. Thus we only set these
  88two bits. We could have used
  89
  90        set_bit(EV_KEY, button_dev.evbit);
  91        set_bit(BTN_0, button_dev.keybit);
  92
  93as well, but with more than single bits the first approach tends to be
  94shorter.
  95
  96Then the example driver registers the input device structure by calling
  97
  98        input_register_device(&button_dev);
  99
 100This adds the button_dev structure to linked lists of the input driver and
 101calls device handler modules _connect functions to tell them a new input
 102device has appeared. input_register_device() may sleep and therefore must
 103not be called from an interrupt or with a spinlock held.
 104
 105While in use, the only used function of the driver is
 106
 107        button_interrupt()
 108
 109which upon every interrupt from the button checks its state and reports it
 110via the
 111
 112        input_report_key()
 113
 114call to the input system. There is no need to check whether the interrupt
 115routine isn't reporting two same value events (press, press for example) to
 116the input system, because the input_report_* functions check that
 117themselves.
 118
 119Then there is the
 120
 121        input_sync()
 122
 123call to tell those who receive the events that we've sent a complete report.
 124This doesn't seem important in the one button case, but is quite important
 125for for example mouse movement, where you don't want the X and Y values
 126to be interpreted separately, because that'd result in a different movement.
 127
 1281.2 dev->open() and dev->close()
 129~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 130
 131In case the driver has to repeatedly poll the device, because it doesn't
 132have an interrupt coming from it and the polling is too expensive to be done
 133all the time, or if the device uses a valuable resource (eg. interrupt), it
 134can use the open and close callback to know when it can stop polling or
 135release the interrupt and when it must resume polling or grab the interrupt
 136again. To do that, we would add this to our example driver:
 137
 138static int button_open(struct input_dev *dev)
 139{
 140        if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
 141                printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
 142                return -EBUSY;
 143        }
 144
 145        return 0;
 146}
 147
 148static void button_close(struct input_dev *dev)
 149{
 150        free_irq(IRQ_AMIGA_VERTB, button_interrupt);
 151}
 152
 153static int __init button_init(void)
 154{
 155        ...
 156        button_dev->open = button_open;
 157        button_dev->close = button_close;
 158        ...
 159}
 160
 161Note that input core keeps track of number of users for the device and
 162makes sure that dev->open() is called only when the first user connects
 163to the device and that dev->close() is called when the very last user
 164disconnects. Calls to both callbacks are serialized.
 165
 166The open() callback should return a 0 in case of success or any nonzero value
 167in case of failure. The close() callback (which is void) must always succeed.
 168
 1691.3 Basic event types
 170~~~~~~~~~~~~~~~~~~~~~
 171
 172The most simple event type is EV_KEY, which is used for keys and buttons.
 173It's reported to the input system via:
 174
 175        input_report_key(struct input_dev *dev, int code, int value)
 176
 177See linux/input.h for the allowable values of code (from 0 to KEY_MAX).
 178Value is interpreted as a truth value, ie any nonzero value means key
 179pressed, zero value means key released. The input code generates events only
 180in case the value is different from before.
 181
 182In addition to EV_KEY, there are two more basic event types: EV_REL and
 183EV_ABS. They are used for relative and absolute values supplied by the
 184device. A relative value may be for example a mouse movement in the X axis.
 185The mouse reports it as a relative difference from the last position,
 186because it doesn't have any absolute coordinate system to work in. Absolute
 187events are namely for joysticks and digitizers - devices that do work in an
 188absolute coordinate systems.
 189
 190Having the device report EV_REL buttons is as simple as with EV_KEY, simply
 191set the corresponding bits and call the
 192
 193        input_report_rel(struct input_dev *dev, int code, int value)
 194
 195function. Events are generated only for nonzero value.
 196
 197However EV_ABS requires a little special care. Before calling
 198input_register_device, you have to fill additional fields in the input_dev
 199struct for each absolute axis your device has. If our button device had also
 200the ABS_X axis:
 201
 202        button_dev.absmin[ABS_X] = 0;
 203        button_dev.absmax[ABS_X] = 255;
 204        button_dev.absfuzz[ABS_X] = 4;
 205        button_dev.absflat[ABS_X] = 8;
 206
 207Or, you can just say:
 208
 209        input_set_abs_params(button_dev, ABS_X, 0, 255, 4, 8);
 210
 211This setting would be appropriate for a joystick X axis, with the minimum of
 2120, maximum of 255 (which the joystick *must* be able to reach, no problem if
 213it sometimes reports more, but it must be able to always reach the min and
 214max values), with noise in the data up to +- 4, and with a center flat
 215position of size 8.
 216
 217If you don't need absfuzz and absflat, you can set them to zero, which mean
 218that the thing is precise and always returns to exactly the center position
 219(if it has any).
 220
 2211.4 BITS_TO_LONGS(), BIT_WORD(), BIT_MASK()
 222~~~~~~~~~~~~~~~~~~~~~~~~~~
 223
 224These three macros from bitops.h help some bitfield computations:
 225
 226        BITS_TO_LONGS(x) - returns the length of a bitfield array in longs for
 227                           x bits
 228        BIT_WORD(x)      - returns the index in the array in longs for bit x
 229        BIT_MASK(x)      - returns the index in a long for bit x
 230
 2311.5 The id* and name fields
 232~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 233
 234The dev->name should be set before registering the input device by the input
 235device driver. It's a string like 'Generic button device' containing a
 236user friendly name of the device.
 237
 238The id* fields contain the bus ID (PCI, USB, ...), vendor ID and device ID
 239of the device. The bus IDs are defined in input.h. The vendor and device ids
 240are defined in pci_ids.h, usb_ids.h and similar include files. These fields
 241should be set by the input device driver before registering it.
 242
 243The idtype field can be used for specific information for the input device
 244driver.
 245
 246The id and name fields can be passed to userland via the evdev interface.
 247
 2481.6 The keycode, keycodemax, keycodesize fields
 249~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 250
 251These three fields should be used by input devices that have dense keymaps.
 252The keycode is an array used to map from scancodes to input system keycodes.
 253The keycode max should contain the size of the array and keycodesize the
 254size of each entry in it (in bytes).
 255
 256Userspace can query and alter current scancode to keycode mappings using
 257EVIOCGKEYCODE and EVIOCSKEYCODE ioctls on corresponding evdev interface.
 258When a device has all 3 aforementioned fields filled in, the driver may
 259rely on kernel's default implementation of setting and querying keycode
 260mappings.
 261
 2621.7 dev->getkeycode() and dev->setkeycode()
 263~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 264getkeycode() and setkeycode() callbacks allow drivers to override default
 265keycode/keycodesize/keycodemax mapping mechanism provided by input core
 266and implement sparse keycode maps.
 267
 2681.8 Key autorepeat
 269~~~~~~~~~~~~~~~~~~
 270
 271... is simple. It is handled by the input.c module. Hardware autorepeat is
 272not used, because it's not present in many devices and even where it is
 273present, it is broken sometimes (at keyboards: Toshiba notebooks). To enable
 274autorepeat for your device, just set EV_REP in dev->evbit. All will be
 275handled by the input system.
 276
 2771.9 Other event types, handling output events
 278~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 279
 280The other event types up to now are:
 281
 282EV_LED - used for the keyboard LEDs.
 283EV_SND - used for keyboard beeps.
 284
 285They are very similar to for example key events, but they go in the other
 286direction - from the system to the input device driver. If your input device
 287driver can handle these events, it has to set the respective bits in evbit,
 288*and* also the callback routine:
 289
 290        button_dev->event = button_event;
 291
 292int button_event(struct input_dev *dev, unsigned int type, unsigned int code, int value);
 293{
 294        if (type == EV_SND && code == SND_BELL) {
 295                outb(value, BUTTON_BELL);
 296                return 0;
 297        }
 298        return -1;
 299}
 300
 301This callback routine can be called from an interrupt or a BH (although that
 302isn't a rule), and thus must not sleep, and must not take too long to finish.
 303
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.