1============================================================
2rotary-encoder - a generic driver for GPIO connected devices
3============================================================
4
5:Author: Daniel Mack <daniel@caiaq.de>, Feb 2009
6
7Function
8--------
9
10Rotary encoders are devices which are connected to the CPU or other
11peripherals with two wires. The outputs are phase-shifted by 90 degrees
12and by triggering on falling and rising edges, the turn direction can
13be determined.
14
15Some encoders have both outputs low in stable states, others also have
16a stable state with both outputs high (half-period mode) and some have
17a stable state in all steps (quarter-period mode).
18
19The phase diagram of these two outputs look like this::
20
21                  _____       _____       _____
22                 |     |     |     |     |     |
23  Channel A  ____|     |_____|     |_____|     |____
24
25                 :  :  :  :  :  :  :  :  :  :  :  :
26            __       _____       _____       _____
27              |     |     |     |     |     |     |
28  Channel B   |_____|     |_____|     |_____|     |__
29
30                 :  :  :  :  :  :  :  :  :  :  :  :
31  Event          a  b  c  d  a  b  c  d  a  b  c  d
32
33                |<-------->|
34	          one step
35
36                |<-->|
37	          one step (half-period mode)
38
39                |<>|
40	          one step (quarter-period mode)
41
42For more information, please see
43	https://en.wikipedia.org/wiki/Rotary_encoder
44
45
46Events / state machine
47----------------------
48
49In half-period mode, state a) and c) above are used to determine the
50rotational direction based on the last stable state. Events are reported in
51states b) and d) given that the new stable state is different from the last
52(i.e. the rotation was not reversed half-way).
53
54Otherwise, the following apply:
55
56a) Rising edge on channel A, channel B in low state
57	This state is used to recognize a clockwise turn
58
59b) Rising edge on channel B, channel A in high state
60	When entering this state, the encoder is put into 'armed' state,
61	meaning that there it has seen half the way of a one-step transition.
62
63c) Falling edge on channel A, channel B in high state
64	This state is used to recognize a counter-clockwise turn
65
66d) Falling edge on channel B, channel A in low state
67	Parking position. If the encoder enters this state, a full transition
68	should have happened, unless it flipped back on half the way. The
69	'armed' state tells us about that.
70
71Platform requirements
72---------------------
73
74As there is no hardware dependent call in this driver, the platform it is
75used with must support gpiolib. Another requirement is that IRQs must be
76able to fire on both edges.
77
78
79Board integration
80-----------------
81
82To use this driver in your system, register a platform_device with the
83name 'rotary-encoder' and associate the IRQs and some specific platform
84data with it. Because the driver uses generic device properties, this can
85be done either via device tree, ACPI, or using static board files, like in
86example below:
87
88::
89
90	/* board support file example */
91
92	#include <linux/input.h>
93	#include <linux/gpio/machine.h>
94	#include <linux/property.h>
95
96	#define GPIO_ROTARY_A 1
97	#define GPIO_ROTARY_B 2
98
99	static struct gpiod_lookup_table rotary_encoder_gpios = {
100		.dev_id = "rotary-encoder.0",
101		.table = {
102			GPIO_LOOKUP_IDX("gpio-0",
103					GPIO_ROTARY_A, NULL, 0, GPIO_ACTIVE_LOW),
104			GPIO_LOOKUP_IDX("gpio-0",
105					GPIO_ROTARY_B, NULL, 1, GPIO_ACTIVE_HIGH),
106			{ },
107		},
108	};
109
110	static const struct property_entry rotary_encoder_properties[] __initconst = {
111		PROPERTY_ENTRY_U32("rotary-encoder,steps-per-period", 24),
112		PROPERTY_ENTRY_U32("linux,axis",		      ABS_X),
113		PROPERTY_ENTRY_U32("rotary-encoder,relative_axis",    0),
114		{ },
115	};
116
117	static struct platform_device rotary_encoder_device = {
118		.name		= "rotary-encoder",
119		.id		= 0,
120	};
121
122	...
123
124	gpiod_add_lookup_table(&rotary_encoder_gpios);
125	device_add_properties(&rotary_encoder_device, rotary_encoder_properties);
126	platform_device_register(&rotary_encoder_device);
127
128	...
129
130Please consult device tree binding documentation to see all properties
131supported by the driver.
132