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1/*
2 * DHT11/DHT22 bit banging GPIO driver
3 *
4 * Copyright (c) Harald Geyer <harald@ccbib.org>
5 *
6 * This program is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 2 of the License, or
9 * (at your option) any later version.
10 *
11 * This program is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 * GNU General Public License for more details.
15 */
16
17#include <linux/err.h>
18#include <linux/interrupt.h>
19#include <linux/device.h>
20#include <linux/kernel.h>
21#include <linux/printk.h>
22#include <linux/slab.h>
23#include <linux/of.h>
24#include <linux/of_device.h>
25#include <linux/sysfs.h>
26#include <linux/io.h>
27#include <linux/module.h>
28#include <linux/platform_device.h>
29#include <linux/wait.h>
30#include <linux/bitops.h>
31#include <linux/completion.h>
32#include <linux/mutex.h>
33#include <linux/delay.h>
34#include <linux/gpio.h>
35#include <linux/of_gpio.h>
36#include <linux/timekeeping.h>
37
38#include <linux/iio/iio.h>
39
40#define DRIVER_NAME "dht11"
41
42#define DHT11_DATA_VALID_TIME 2000000000 /* 2s in ns */
43
44#define DHT11_EDGES_PREAMBLE 2
45#define DHT11_BITS_PER_READ 40
46/*
47 * Note that when reading the sensor actually 84 edges are detected, but
48 * since the last edge is not significant, we only store 83:
49 */
50#define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \
51 DHT11_EDGES_PREAMBLE + 1)
52
53/*
54 * Data transmission timing:
55 * Data bits are encoded as pulse length (high time) on the data line.
56 * 0-bit: 22-30uS -- typically 26uS (AM2302)
57 * 1-bit: 68-75uS -- typically 70uS (AM2302)
58 * The acutal timings also depend on the properties of the cable, with
59 * longer cables typically making pulses shorter.
60 *
61 * Our decoding depends on the time resolution of the system:
62 * timeres > 34uS ... don't know what a 1-tick pulse is
63 * 34uS > timeres > 30uS ... no problem (30kHz and 32kHz clocks)
64 * 30uS > timeres > 23uS ... don't know what a 2-tick pulse is
65 * timeres < 23uS ... no problem
66 *
67 * Luckily clocks in the 33-44kHz range are quite uncommon, so we can
68 * support most systems if the threshold for decoding a pulse as 1-bit
69 * is chosen carefully. If somebody really wants to support clocks around
70 * 40kHz, where this driver is most unreliable, there are two options.
71 * a) select an implementation using busy loop polling on those systems
72 * b) use the checksum to do some probabilistic decoding
73 */
74#define DHT11_START_TRANSMISSION 18 /* ms */
75#define DHT11_MIN_TIMERES 34000 /* ns */
76#define DHT11_THRESHOLD 49000 /* ns */
77#define DHT11_AMBIG_LOW 23000 /* ns */
78#define DHT11_AMBIG_HIGH 30000 /* ns */
79
80struct dht11 {
81 struct device *dev;
82
83 int gpio;
84 int irq;
85
86 struct completion completion;
87 /* The iio sysfs interface doesn't prevent concurrent reads: */
88 struct mutex lock;
89
90 s64 timestamp;
91 int temperature;
92 int humidity;
93
94 /* num_edges: -1 means "no transmission in progress" */
95 int num_edges;
96 struct {s64 ts; int value; } edges[DHT11_EDGES_PER_READ];
97};
98
99static unsigned char dht11_decode_byte(char *bits)
100{
101 unsigned char ret = 0;
102 int i;
103
104 for (i = 0; i < 8; ++i) {
105 ret <<= 1;
106 if (bits[i])
107 ++ret;
108 }
109
110 return ret;
111}
112
113static int dht11_decode(struct dht11 *dht11, int offset)
114{
115 int i, t;
116 char bits[DHT11_BITS_PER_READ];
117 unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum;
118
119 for (i = 0; i < DHT11_BITS_PER_READ; ++i) {
120 t = dht11->edges[offset + 2 * i + 2].ts -
121 dht11->edges[offset + 2 * i + 1].ts;
122 if (!dht11->edges[offset + 2 * i + 1].value)
123 return -EIO; /* lost synchronisation */
124 bits[i] = t > DHT11_THRESHOLD;
125 }
126
127 hum_int = dht11_decode_byte(bits);
128 hum_dec = dht11_decode_byte(&bits[8]);
129 temp_int = dht11_decode_byte(&bits[16]);
130 temp_dec = dht11_decode_byte(&bits[24]);
131 checksum = dht11_decode_byte(&bits[32]);
132
133 if (((hum_int + hum_dec + temp_int + temp_dec) & 0xff) != checksum)
134 return -EIO;
135
136 dht11->timestamp = ktime_get_boot_ns();
137 if (hum_int < 20) { /* DHT22 */
138 dht11->temperature = (((temp_int & 0x7f) << 8) + temp_dec) *
139 ((temp_int & 0x80) ? -100 : 100);
140 dht11->humidity = ((hum_int << 8) + hum_dec) * 100;
141 } else if (temp_dec == 0 && hum_dec == 0) { /* DHT11 */
142 dht11->temperature = temp_int * 1000;
143 dht11->humidity = hum_int * 1000;
144 } else {
145 dev_err(dht11->dev,
146 "Don't know how to decode data: %d %d %d %d\n",
147 hum_int, hum_dec, temp_int, temp_dec);
148 return -EIO;
149 }
150
151 return 0;
152}
153
154/*
155 * IRQ handler called on GPIO edges
156 */
157static irqreturn_t dht11_handle_irq(int irq, void *data)
158{
159 struct iio_dev *iio = data;
160 struct dht11 *dht11 = iio_priv(iio);
161
162 /* TODO: Consider making the handler safe for IRQ sharing */
163 if (dht11->num_edges < DHT11_EDGES_PER_READ && dht11->num_edges >= 0) {
164 dht11->edges[dht11->num_edges].ts = ktime_get_boot_ns();
165 dht11->edges[dht11->num_edges++].value =
166 gpio_get_value(dht11->gpio);
167
168 if (dht11->num_edges >= DHT11_EDGES_PER_READ)
169 complete(&dht11->completion);
170 }
171
172 return IRQ_HANDLED;
173}
174
175static int dht11_read_raw(struct iio_dev *iio_dev,
176 const struct iio_chan_spec *chan,
177 int *val, int *val2, long m)
178{
179 struct dht11 *dht11 = iio_priv(iio_dev);
180 int ret, timeres, offset;
181
182 mutex_lock(&dht11->lock);
183 if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_boot_ns()) {
184 timeres = ktime_get_resolution_ns();
185 if (timeres > DHT11_MIN_TIMERES) {
186 dev_err(dht11->dev, "timeresolution %dns too low\n",
187 timeres);
188 /* In theory a better clock could become available
189 * at some point ... and there is no error code
190 * that really fits better.
191 */
192 ret = -EAGAIN;
193 goto err;
194 }
195 if (timeres > DHT11_AMBIG_LOW && timeres < DHT11_AMBIG_HIGH)
196 dev_warn(dht11->dev,
197 "timeresolution: %dns - decoding ambiguous\n",
198 timeres);
199
200 reinit_completion(&dht11->completion);
201
202 dht11->num_edges = 0;
203 ret = gpio_direction_output(dht11->gpio, 0);
204 if (ret)
205 goto err;
206 msleep(DHT11_START_TRANSMISSION);
207 ret = gpio_direction_input(dht11->gpio);
208 if (ret)
209 goto err;
210
211 ret = request_irq(dht11->irq, dht11_handle_irq,
212 IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING,
213 iio_dev->name, iio_dev);
214 if (ret)
215 goto err;
216
217 ret = wait_for_completion_killable_timeout(&dht11->completion,
218 HZ);
219
220 free_irq(dht11->irq, iio_dev);
221
222 if (ret == 0 && dht11->num_edges < DHT11_EDGES_PER_READ - 1) {
223 dev_err(&iio_dev->dev,
224 "Only %d signal edges detected\n",
225 dht11->num_edges);
226 ret = -ETIMEDOUT;
227 }
228 if (ret < 0)
229 goto err;
230
231 offset = DHT11_EDGES_PREAMBLE +
232 dht11->num_edges - DHT11_EDGES_PER_READ;
233 for (; offset >= 0; --offset) {
234 ret = dht11_decode(dht11, offset);
235 if (!ret)
236 break;
237 }
238
239 if (ret)
240 goto err;
241 }
242
243 ret = IIO_VAL_INT;
244 if (chan->type == IIO_TEMP)
245 *val = dht11->temperature;
246 else if (chan->type == IIO_HUMIDITYRELATIVE)
247 *val = dht11->humidity;
248 else
249 ret = -EINVAL;
250err:
251 dht11->num_edges = -1;
252 mutex_unlock(&dht11->lock);
253 return ret;
254}
255
256static const struct iio_info dht11_iio_info = {
257 .driver_module = THIS_MODULE,
258 .read_raw = dht11_read_raw,
259};
260
261static const struct iio_chan_spec dht11_chan_spec[] = {
262 { .type = IIO_TEMP,
263 .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), },
264 { .type = IIO_HUMIDITYRELATIVE,
265 .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), }
266};
267
268static const struct of_device_id dht11_dt_ids[] = {
269 { .compatible = "dht11", },
270 { }
271};
272MODULE_DEVICE_TABLE(of, dht11_dt_ids);
273
274static int dht11_probe(struct platform_device *pdev)
275{
276 struct device *dev = &pdev->dev;
277 struct device_node *node = dev->of_node;
278 struct dht11 *dht11;
279 struct iio_dev *iio;
280 int ret;
281
282 iio = devm_iio_device_alloc(dev, sizeof(*dht11));
283 if (!iio) {
284 dev_err(dev, "Failed to allocate IIO device\n");
285 return -ENOMEM;
286 }
287
288 dht11 = iio_priv(iio);
289 dht11->dev = dev;
290
291 ret = of_get_gpio(node, 0);
292 if (ret < 0)
293 return ret;
294 dht11->gpio = ret;
295 ret = devm_gpio_request_one(dev, dht11->gpio, GPIOF_IN, pdev->name);
296 if (ret)
297 return ret;
298
299 dht11->irq = gpio_to_irq(dht11->gpio);
300 if (dht11->irq < 0) {
301 dev_err(dev, "GPIO %d has no interrupt\n", dht11->gpio);
302 return -EINVAL;
303 }
304
305 dht11->timestamp = ktime_get_boot_ns() - DHT11_DATA_VALID_TIME - 1;
306 dht11->num_edges = -1;
307
308 platform_set_drvdata(pdev, iio);
309
310 init_completion(&dht11->completion);
311 mutex_init(&dht11->lock);
312 iio->name = pdev->name;
313 iio->dev.parent = &pdev->dev;
314 iio->info = &dht11_iio_info;
315 iio->modes = INDIO_DIRECT_MODE;
316 iio->channels = dht11_chan_spec;
317 iio->num_channels = ARRAY_SIZE(dht11_chan_spec);
318
319 return devm_iio_device_register(dev, iio);
320}
321
322static struct platform_driver dht11_driver = {
323 .driver = {
324 .name = DRIVER_NAME,
325 .of_match_table = dht11_dt_ids,
326 },
327 .probe = dht11_probe,
328};
329
330module_platform_driver(dht11_driver);
331
332MODULE_AUTHOR("Harald Geyer <harald@ccbib.org>");
333MODULE_DESCRIPTION("DHT11 humidity/temperature sensor driver");
334MODULE_LICENSE("GPL v2");