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Linux/drivers/iio/humidity/dht11.c

  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 
 80 struct 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 
 99 static 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 
113 static 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  */
157 static 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 
175 static 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;
250 err:
251         dht11->num_edges = -1;
252         mutex_unlock(&dht11->lock);
253         return ret;
254 }
255 
256 static const struct iio_info dht11_iio_info = {
257         .driver_module          = THIS_MODULE,
258         .read_raw               = dht11_read_raw,
259 };
260 
261 static 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 
268 static const struct of_device_id dht11_dt_ids[] = {
269         { .compatible = "dht11", },
270         { }
271 };
272 MODULE_DEVICE_TABLE(of, dht11_dt_ids);
273 
274 static 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 
322 static 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 
330 module_platform_driver(dht11_driver);
331 
332 MODULE_AUTHOR("Harald Geyer <harald@ccbib.org>");
333 MODULE_DESCRIPTION("DHT11 humidity/temperature sensor driver");
334 MODULE_LICENSE("GPL v2");
335 

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