About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / input / input-programming.txt




Custom Search

Based on kernel version 3.16. Page generated on 2014-08-06 21:40 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.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.