Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 EST.
1 Programming input drivers 2 ~~~~~~~~~~~~~~~~~~~~~~~~~ 3 4 1. Creating an input device driver 5 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 6 7 1.0 The simplest example 8 ~~~~~~~~~~~~~~~~~~~~~~~~ 9 10 Here comes a very simple example of an input device driver. The device has 11 just one button and the button is accessible at i/o port BUTTON_PORT. When 12 pressed 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 21 static struct input_dev *button_dev; 22 23 static 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 30 static 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 64 static void __exit button_exit(void) 65 { 66 input_unregister_device(button_dev); 67 free_irq(BUTTON_IRQ, button_interrupt); 68 } 69 70 module_init(button_init); 71 module_exit(button_exit); 72 73 1.1 What the example does 74 ~~~~~~~~~~~~~~~~~~~~~~~~~ 75 76 First it has to include the <linux/input.h> file, which interfaces to the 77 input subsystem. This provides all the definitions needed. 78 79 In the _init function, which is called either upon module load or when 80 booting the kernel, it grabs the required resources (it should also check 81 for the presence of the device). 82 83 Then it allocates a new input device structure with input_allocate_device() 84 and sets up input bitfields. This way the device driver tells the other 85 parts of the input systems what it is - what events can be generated or 86 accepted by this input device. Our example device can only generate EV_KEY 87 type events, and from those only BTN_0 event code. Thus we only set these 88 two bits. We could have used 89 90 set_bit(EV_KEY, button_dev.evbit); 91 set_bit(BTN_0, button_dev.keybit); 92 93 as well, but with more than single bits the first approach tends to be 94 shorter. 95 96 Then the example driver registers the input device structure by calling 97 98 input_register_device(&button_dev); 99 100 This adds the button_dev structure to linked lists of the input driver and 101 calls device handler modules _connect functions to tell them a new input 102 device has appeared. input_register_device() may sleep and therefore must 103 not be called from an interrupt or with a spinlock held. 104 105 While in use, the only used function of the driver is 106 107 button_interrupt() 108 109 which upon every interrupt from the button checks its state and reports it 110 via the 111 112 input_report_key() 113 114 call to the input system. There is no need to check whether the interrupt 115 routine isn't reporting two same value events (press, press for example) to 116 the input system, because the input_report_* functions check that 117 themselves. 118 119 Then there is the 120 121 input_sync() 122 123 call to tell those who receive the events that we've sent a complete report. 124 This doesn't seem important in the one button case, but is quite important 125 for for example mouse movement, where you don't want the X and Y values 126 to be interpreted separately, because that'd result in a different movement. 127 128 1.2 dev->open() and dev->close() 129 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 130 131 In case the driver has to repeatedly poll the device, because it doesn't 132 have an interrupt coming from it and the polling is too expensive to be done 133 all the time, or if the device uses a valuable resource (eg. interrupt), it 134 can use the open and close callback to know when it can stop polling or 135 release the interrupt and when it must resume polling or grab the interrupt 136 again. To do that, we would add this to our example driver: 137 138 static 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 } 147 148 static void button_close(struct input_dev *dev) 149 { 150 free_irq(IRQ_AMIGA_VERTB, button_interrupt); 151 } 152 153 static int __init button_init(void) 154 { 155 ... 156 button_dev->open = button_open; 157 button_dev->close = button_close; 158 ... 159 } 160 161 Note that input core keeps track of number of users for the device and 162 makes sure that dev->open() is called only when the first user connects 163 to the device and that dev->close() is called when the very last user 164 disconnects. Calls to both callbacks are serialized. 165 166 The open() callback should return a 0 in case of success or any nonzero value 167 in case of failure. The close() callback (which is void) must always succeed. 168 169 1.3 Basic event types 170 ~~~~~~~~~~~~~~~~~~~~~ 171 172 The most simple event type is EV_KEY, which is used for keys and buttons. 173 It's reported to the input system via: 174 175 input_report_key(struct input_dev *dev, int code, int value) 176 177 See linux/input.h for the allowable values of code (from 0 to KEY_MAX). 178 Value is interpreted as a truth value, ie any nonzero value means key 179 pressed, zero value means key released. The input code generates events only 180 in case the value is different from before. 181 182 In addition to EV_KEY, there are two more basic event types: EV_REL and 183 EV_ABS. They are used for relative and absolute values supplied by the 184 device. A relative value may be for example a mouse movement in the X axis. 185 The mouse reports it as a relative difference from the last position, 186 because it doesn't have any absolute coordinate system to work in. Absolute 187 events are namely for joysticks and digitizers - devices that do work in an 188 absolute coordinate systems. 189 190 Having the device report EV_REL buttons is as simple as with EV_KEY, simply 191 set the corresponding bits and call the 192 193 input_report_rel(struct input_dev *dev, int code, int value) 194 195 function. Events are generated only for nonzero value. 196 197 However EV_ABS requires a little special care. Before calling 198 input_register_device, you have to fill additional fields in the input_dev 199 struct for each absolute axis your device has. If our button device had also 200 the 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; 206 207 Or, you can just say: 208 209 input_set_abs_params(button_dev, ABS_X, 0, 255, 4, 8); 210 211 This setting would be appropriate for a joystick X axis, with the minimum of 212 0, maximum of 255 (which the joystick *must* be able to reach, no problem if 213 it sometimes reports more, but it must be able to always reach the min and 214 max values), with noise in the data up to +- 4, and with a center flat 215 position of size 8. 216 217 If you don't need absfuzz and absflat, you can set them to zero, which mean 218 that the thing is precise and always returns to exactly the center position 219 (if it has any). 220 221 1.4 BITS_TO_LONGS(), BIT_WORD(), BIT_MASK() 222 ~~~~~~~~~~~~~~~~~~~~~~~~~~ 223 224 These 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 230 231 1.5 The id* and name fields 232 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 233 234 The dev->name should be set before registering the input device by the input 235 device driver. It's a string like 'Generic button device' containing a 236 user friendly name of the device. 237 238 The id* fields contain the bus ID (PCI, USB, ...), vendor ID and device ID 239 of the device. The bus IDs are defined in input.h. The vendor and device ids 240 are defined in pci_ids.h, usb_ids.h and similar include files. These fields 241 should be set by the input device driver before registering it. 242 243 The idtype field can be used for specific information for the input device 244 driver. 245 246 The id and name fields can be passed to userland via the evdev interface. 247 248 1.6 The keycode, keycodemax, keycodesize fields 249 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 250 251 These three fields should be used by input devices that have dense keymaps. 252 The keycode is an array used to map from scancodes to input system keycodes. 253 The keycode max should contain the size of the array and keycodesize the 254 size of each entry in it (in bytes). 255 256 Userspace can query and alter current scancode to keycode mappings using 257 EVIOCGKEYCODE and EVIOCSKEYCODE ioctls on corresponding evdev interface. 258 When a device has all 3 aforementioned fields filled in, the driver may 259 rely on kernel's default implementation of setting and querying keycode 260 mappings. 261 262 1.7 dev->getkeycode() and dev->setkeycode() 263 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 264 getkeycode() and setkeycode() callbacks allow drivers to override default 265 keycode/keycodesize/keycodemax mapping mechanism provided by input core 266 and implement sparse keycode maps. 267 268 1.8 Key autorepeat 269 ~~~~~~~~~~~~~~~~~~ 270 271 ... is simple. It is handled by the input.c module. Hardware autorepeat is 272 not used, because it's not present in many devices and even where it is 273 present, it is broken sometimes (at keyboards: Toshiba notebooks). To enable 274 autorepeat for your device, just set EV_REP in dev->evbit. All will be 275 handled by the input system. 276 277 1.9 Other event types, handling output events 278 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 279 280 The other event types up to now are: 281 282 EV_LED - used for the keyboard LEDs. 283 EV_SND - used for keyboard beeps. 284 285 They are very similar to for example key events, but they go in the other 286 direction - from the system to the input device driver. If your input device 287 driver 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; 291 292 int 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 } 300 301 This callback routine can be called from an interrupt or a BH (although that 302 isn't a rule), and thus must not sleep, and must not take too long to finish.