1=============================== 2Creating an input device driver 3=============================== 4 5The simplest example 6~~~~~~~~~~~~~~~~~~~~ 7 8Here comes a very simple example of an input device driver. The device has 9just one button and the button is accessible at i/o port BUTTON_PORT. When 10pressed or released a BUTTON_IRQ happens. The driver could look like:: 11 12 #include <linux/input.h> 13 #include <linux/module.h> 14 #include <linux/init.h> 15 16 #include <asm/irq.h> 17 #include <asm/io.h> 18 19 static struct input_dev *button_dev; 20 21 static irqreturn_t button_interrupt(int irq, void *dummy) 22 { 23 input_report_key(button_dev, BTN_0, inb(BUTTON_PORT) & 1); 24 input_sync(button_dev); 25 return IRQ_HANDLED; 26 } 27 28 static int __init button_init(void) 29 { 30 int error; 31 32 if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) { 33 printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq); 34 return -EBUSY; 35 } 36 37 button_dev = input_allocate_device(); 38 if (!button_dev) { 39 printk(KERN_ERR "button.c: Not enough memory\n"); 40 error = -ENOMEM; 41 goto err_free_irq; 42 } 43 44 button_dev->evbit[0] = BIT_MASK(EV_KEY); 45 button_dev->keybit[BIT_WORD(BTN_0)] = BIT_MASK(BTN_0); 46 47 error = input_register_device(button_dev); 48 if (error) { 49 printk(KERN_ERR "button.c: Failed to register device\n"); 50 goto err_free_dev; 51 } 52 53 return 0; 54 55 err_free_dev: 56 input_free_device(button_dev); 57 err_free_irq: 58 free_irq(BUTTON_IRQ, button_interrupt); 59 return error; 60 } 61 62 static void __exit button_exit(void) 63 { 64 input_unregister_device(button_dev); 65 free_irq(BUTTON_IRQ, button_interrupt); 66 } 67 68 module_init(button_init); 69 module_exit(button_exit); 70 71What the example does 72~~~~~~~~~~~~~~~~~~~~~ 73 74First it has to include the <linux/input.h> file, which interfaces to the 75input subsystem. This provides all the definitions needed. 76 77In the _init function, which is called either upon module load or when 78booting the kernel, it grabs the required resources (it should also check 79for the presence of the device). 80 81Then it allocates a new input device structure with input_allocate_device() 82and sets up input bitfields. This way the device driver tells the other 83parts of the input systems what it is - what events can be generated or 84accepted by this input device. Our example device can only generate EV_KEY 85type events, and from those only BTN_0 event code. Thus we only set these 86two bits. We could have used:: 87 88 set_bit(EV_KEY, button_dev.evbit); 89 set_bit(BTN_0, button_dev.keybit); 90 91as well, but with more than single bits the first approach tends to be 92shorter. 93 94Then the example driver registers the input device structure by calling:: 95 96 input_register_device(&button_dev); 97 98This adds the button_dev structure to linked lists of the input driver and 99calls device handler modules _connect functions to tell them a new input 100device has appeared. input_register_device() may sleep and therefore must 101not be called from an interrupt or with a spinlock held. 102 103While in use, the only used function of the driver is:: 104 105 button_interrupt() 106 107which upon every interrupt from the button checks its state and reports it 108via the:: 109 110 input_report_key() 111 112call to the input system. There is no need to check whether the interrupt 113routine isn't reporting two same value events (press, press for example) to 114the input system, because the input_report_* functions check that 115themselves. 116 117Then there is the:: 118 119 input_sync() 120 121call to tell those who receive the events that we've sent a complete report. 122This doesn't seem important in the one button case, but is quite important 123for for example mouse movement, where you don't want the X and Y values 124to be interpreted separately, because that'd result in a different movement. 125 126dev->open() and dev->close() 127~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 128 129In case the driver has to repeatedly poll the device, because it doesn't 130have an interrupt coming from it and the polling is too expensive to be done 131all the time, or if the device uses a valuable resource (eg. interrupt), it 132can use the open and close callback to know when it can stop polling or 133release the interrupt and when it must resume polling or grab the interrupt 134again. To do that, we would add this to our example driver:: 135 136 static int button_open(struct input_dev *dev) 137 { 138 if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) { 139 printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq); 140 return -EBUSY; 141 } 142 143 return 0; 144 } 145 146 static void button_close(struct input_dev *dev) 147 { 148 free_irq(IRQ_AMIGA_VERTB, button_interrupt); 149 } 150 151 static int __init button_init(void) 152 { 153 ... 154 button_dev->open = button_open; 155 button_dev->close = button_close; 156 ... 157 } 158 159Note that input core keeps track of number of users for the device and 160makes sure that dev->open() is called only when the first user connects 161to the device and that dev->close() is called when the very last user 162disconnects. Calls to both callbacks are serialized. 163 164The open() callback should return a 0 in case of success or any nonzero value 165in case of failure. The close() callback (which is void) must always succeed. 166 167Inhibiting input devices 168~~~~~~~~~~~~~~~~~~~~~~~~ 169 170Inhibiting a device means ignoring input events from it. As such it is about 171maintaining relationships with input handlers - either already existing 172relationships, or relationships to be established while the device is in 173inhibited state. 174 175If a device is inhibited, no input handler will receive events from it. 176 177The fact that nobody wants events from the device is exploited further, by 178calling device's close() (if there are users) and open() (if there are users) on 179inhibit and uninhibit operations, respectively. Indeed, the meaning of close() 180is to stop providing events to the input core and that of open() is to start 181providing events to the input core. 182 183Calling the device's close() method on inhibit (if there are users) allows the 184driver to save power. Either by directly powering down the device or by 185releasing the runtime-pm reference it got in open() when the driver is using 186runtime-pm. 187 188Inhibiting and uninhibiting are orthogonal to opening and closing the device by 189input handlers. Userspace might want to inhibit a device in anticipation before 190any handler is positively matched against it. 191 192Inhibiting and uninhibiting are orthogonal to device's being a wakeup source, 193too. Being a wakeup source plays a role when the system is sleeping, not when 194the system is operating. How drivers should program their interaction between 195inhibiting, sleeping and being a wakeup source is driver-specific. 196 197Taking the analogy with the network devices - bringing a network interface down 198doesn't mean that it should be impossible be wake the system up on LAN through 199this interface. So, there may be input drivers which should be considered wakeup 200sources even when inhibited. Actually, in many I2C input devices their interrupt 201is declared a wakeup interrupt and its handling happens in driver's core, which 202is not aware of input-specific inhibit (nor should it be). Composite devices 203containing several interfaces can be inhibited on a per-interface basis and e.g. 204inhibiting one interface shouldn't affect the device's capability of being a 205wakeup source. 206 207If a device is to be considered a wakeup source while inhibited, special care 208must be taken when programming its suspend(), as it might need to call device's 209open(). Depending on what close() means for the device in question, not 210opening() it before going to sleep might make it impossible to provide any 211wakeup events. The device is going to sleep anyway. 212 213Basic event types 214~~~~~~~~~~~~~~~~~ 215 216The most simple event type is EV_KEY, which is used for keys and buttons. 217It's reported to the input system via:: 218 219 input_report_key(struct input_dev *dev, int code, int value) 220 221See uapi/linux/input-event-codes.h for the allowable values of code (from 0 to 222KEY_MAX). Value is interpreted as a truth value, ie any nonzero value means key 223pressed, zero value means key released. The input code generates events only 224in case the value is different from before. 225 226In addition to EV_KEY, there are two more basic event types: EV_REL and 227EV_ABS. They are used for relative and absolute values supplied by the 228device. A relative value may be for example a mouse movement in the X axis. 229The mouse reports it as a relative difference from the last position, 230because it doesn't have any absolute coordinate system to work in. Absolute 231events are namely for joysticks and digitizers - devices that do work in an 232absolute coordinate systems. 233 234Having the device report EV_REL buttons is as simple as with EV_KEY, simply 235set the corresponding bits and call the:: 236 237 input_report_rel(struct input_dev *dev, int code, int value) 238 239function. Events are generated only for nonzero value. 240 241However EV_ABS requires a little special care. Before calling 242input_register_device, you have to fill additional fields in the input_dev 243struct for each absolute axis your device has. If our button device had also 244the ABS_X axis:: 245 246 button_dev.absmin[ABS_X] = 0; 247 button_dev.absmax[ABS_X] = 255; 248 button_dev.absfuzz[ABS_X] = 4; 249 button_dev.absflat[ABS_X] = 8; 250 251Or, you can just say:: 252 253 input_set_abs_params(button_dev, ABS_X, 0, 255, 4, 8); 254 255This setting would be appropriate for a joystick X axis, with the minimum of 2560, maximum of 255 (which the joystick *must* be able to reach, no problem if 257it sometimes reports more, but it must be able to always reach the min and 258max values), with noise in the data up to +- 4, and with a center flat 259position of size 8. 260 261If you don't need absfuzz and absflat, you can set them to zero, which mean 262that the thing is precise and always returns to exactly the center position 263(if it has any). 264 265BITS_TO_LONGS(), BIT_WORD(), BIT_MASK() 266~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 267 268These three macros from bitops.h help some bitfield computations:: 269 270 BITS_TO_LONGS(x) - returns the length of a bitfield array in longs for 271 x bits 272 BIT_WORD(x) - returns the index in the array in longs for bit x 273 BIT_MASK(x) - returns the index in a long for bit x 274 275The id* and name fields 276~~~~~~~~~~~~~~~~~~~~~~~ 277 278The dev->name should be set before registering the input device by the input 279device driver. It's a string like 'Generic button device' containing a 280user friendly name of the device. 281 282The id* fields contain the bus ID (PCI, USB, ...), vendor ID and device ID 283of the device. The bus IDs are defined in input.h. The vendor and device ids 284are defined in pci_ids.h, usb_ids.h and similar include files. These fields 285should be set by the input device driver before registering it. 286 287The idtype field can be used for specific information for the input device 288driver. 289 290The id and name fields can be passed to userland via the evdev interface. 291 292The keycode, keycodemax, keycodesize fields 293~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 294 295These three fields should be used by input devices that have dense keymaps. 296The keycode is an array used to map from scancodes to input system keycodes. 297The keycode max should contain the size of the array and keycodesize the 298size of each entry in it (in bytes). 299 300Userspace can query and alter current scancode to keycode mappings using 301EVIOCGKEYCODE and EVIOCSKEYCODE ioctls on corresponding evdev interface. 302When a device has all 3 aforementioned fields filled in, the driver may 303rely on kernel's default implementation of setting and querying keycode 304mappings. 305 306dev->getkeycode() and dev->setkeycode() 307~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 308 309getkeycode() and setkeycode() callbacks allow drivers to override default 310keycode/keycodesize/keycodemax mapping mechanism provided by input core 311and implement sparse keycode maps. 312 313Key autorepeat 314~~~~~~~~~~~~~~ 315 316... is simple. It is handled by the input.c module. Hardware autorepeat is 317not used, because it's not present in many devices and even where it is 318present, it is broken sometimes (at keyboards: Toshiba notebooks). To enable 319autorepeat for your device, just set EV_REP in dev->evbit. All will be 320handled by the input system. 321 322Other event types, handling output events 323~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 324 325The other event types up to now are: 326 327- EV_LED - used for the keyboard LEDs. 328- EV_SND - used for keyboard beeps. 329 330They are very similar to for example key events, but they go in the other 331direction - from the system to the input device driver. If your input device 332driver can handle these events, it has to set the respective bits in evbit, 333*and* also the callback routine:: 334 335 button_dev->event = button_event; 336 337 int button_event(struct input_dev *dev, unsigned int type, 338 unsigned int code, int value) 339 { 340 if (type == EV_SND && code == SND_BELL) { 341 outb(value, BUTTON_BELL); 342 return 0; 343 } 344 return -1; 345 } 346 347This callback routine can be called from an interrupt or a BH (although that 348isn't a rule), and thus must not sleep, and must not take too long to finish. 349