dht11.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 * DHT11/DHT22 bit banging GPIO driver
  4 *
  5 * Copyright (c) Harald Geyer <harald@ccbib.org>
  6 */
  7
  8#include <linux/err.h>
  9#include <linux/interrupt.h>
 10#include <linux/device.h>
 11#include <linux/kernel.h>
 12#include <linux/printk.h>
 13#include <linux/slab.h>
 14#include <linux/string_choices.h>
 15#include <linux/sysfs.h>
 16#include <linux/io.h>
 17#include <linux/mod_devicetable.h>
 18#include <linux/module.h>
 19#include <linux/platform_device.h>
 20#include <linux/wait.h>
 21#include <linux/bitops.h>
 22#include <linux/completion.h>
 23#include <linux/mutex.h>
 24#include <linux/delay.h>
 25#include <linux/gpio/consumer.h>
 26#include <linux/timekeeping.h>
 27
 28#include <linux/iio/iio.h>
 29
 30#define DHT11_DATA_VALID_TIME	2000000000  /* 2s in ns */
 31
 32#define DHT11_EDGES_PREAMBLE 2
 33#define DHT11_BITS_PER_READ 40
 34/*
 35 * Note that when reading the sensor actually 84 edges are detected, but
 36 * since the last edge is not significant, we only store 83:
 37 */
 38#define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \
 39			      DHT11_EDGES_PREAMBLE + 1)
 40
 41/*
 42 * Data transmission timing:
 43 * Data bits are encoded as pulse length (high time) on the data line.
 44 * 0-bit: 22-30uS -- typically 26uS (AM2302)
 45 * 1-bit: 68-75uS -- typically 70uS (AM2302)
 46 * The acutal timings also depend on the properties of the cable, with
 47 * longer cables typically making pulses shorter.
 48 *
 49 * Our decoding depends on the time resolution of the system:
 50 * timeres > 34uS ... don't know what a 1-tick pulse is
 51 * 34uS > timeres > 30uS ... no problem (30kHz and 32kHz clocks)
 52 * 30uS > timeres > 23uS ... don't know what a 2-tick pulse is
 53 * timeres < 23uS ... no problem
 54 *
 55 * Luckily clocks in the 33-44kHz range are quite uncommon, so we can
 56 * support most systems if the threshold for decoding a pulse as 1-bit
 57 * is chosen carefully. If somebody really wants to support clocks around
 58 * 40kHz, where this driver is most unreliable, there are two options.
 59 * a) select an implementation using busy loop polling on those systems
 60 * b) use the checksum to do some probabilistic decoding
 61 */
 62#define DHT11_START_TRANSMISSION_MIN	18000  /* us */
 63#define DHT11_START_TRANSMISSION_MAX	20000  /* us */
 64#define DHT11_MIN_TIMERES	34000  /* ns */
 65#define DHT11_THRESHOLD		49000  /* ns */
 66#define DHT11_AMBIG_LOW		23000  /* ns */
 67#define DHT11_AMBIG_HIGH	30000  /* ns */
 68
 69struct dht11 {
 70	struct device			*dev;
 71
 72	struct gpio_desc		*gpiod;
 73	int				irq;
 74
 75	struct completion		completion;
 76	/* The iio sysfs interface doesn't prevent concurrent reads: */
 77	struct mutex			lock;
 78
 79	s64				timestamp;
 80	int				temperature;
 81	int				humidity;
 82
 83	/* num_edges: -1 means "no transmission in progress" */
 84	int				num_edges;
 85	struct {s64 ts; int value; }	edges[DHT11_EDGES_PER_READ];
 86};
 87
 88#ifdef CONFIG_DYNAMIC_DEBUG
 89/*
 90 * dht11_edges_print: show the data as actually received by the
 91 *                    driver.
 92 */
 93static void dht11_edges_print(struct dht11 *dht11)
 94{
 95	int i;
 96
 97	dev_dbg(dht11->dev, "%d edges detected:\n", dht11->num_edges);
 98	for (i = 1; i < dht11->num_edges; ++i) {
 99		dev_dbg(dht11->dev, "%d: %lld ns %s\n", i,
100			dht11->edges[i].ts - dht11->edges[i - 1].ts,
101			str_high_low(dht11->edges[i - 1].value));
102	}
103}
104#endif /* CONFIG_DYNAMIC_DEBUG */
105
106static unsigned char dht11_decode_byte(char *bits)
107{
108	unsigned char ret = 0;
109	int i;
110
111	for (i = 0; i < 8; ++i) {
112		ret <<= 1;
113		if (bits[i])
114			++ret;
115	}
116
117	return ret;
118}
119
120static int dht11_decode(struct dht11 *dht11, int offset)
121{
122	int i, t;
123	char bits[DHT11_BITS_PER_READ];
124	unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum;
125
126	for (i = 0; i < DHT11_BITS_PER_READ; ++i) {
127		t = dht11->edges[offset + 2 * i + 2].ts -
128			dht11->edges[offset + 2 * i + 1].ts;
129		if (!dht11->edges[offset + 2 * i + 1].value) {
130			dev_dbg(dht11->dev,
131				"lost synchronisation at edge %d\n",
132				offset + 2 * i + 1);
133			return -EIO;
134		}
135		bits[i] = t > DHT11_THRESHOLD;
136	}
137
138	hum_int = dht11_decode_byte(bits);
139	hum_dec = dht11_decode_byte(&bits[8]);
140	temp_int = dht11_decode_byte(&bits[16]);
141	temp_dec = dht11_decode_byte(&bits[24]);
142	checksum = dht11_decode_byte(&bits[32]);
143
144	if (((hum_int + hum_dec + temp_int + temp_dec) & 0xff) != checksum) {
145		dev_dbg(dht11->dev, "invalid checksum\n");
146		return -EIO;
147	}
148
149	dht11->timestamp = ktime_get_boottime_ns();
150	if (hum_int < 4) {  /* DHT22: 100000 = (3*256+232)*100 */
151		dht11->temperature = (((temp_int & 0x7f) << 8) + temp_dec) *
152					((temp_int & 0x80) ? -100 : 100);
153		dht11->humidity = ((hum_int << 8) + hum_dec) * 100;
154	} else if (temp_dec == 0 && hum_dec == 0) {  /* DHT11 */
155		dht11->temperature = temp_int * 1000;
156		dht11->humidity = hum_int * 1000;
157	} else {
158		dev_err(dht11->dev,
159			"Don't know how to decode data: %d %d %d %d\n",
160			hum_int, hum_dec, temp_int, temp_dec);
161		return -EIO;
162	}
163
164	return 0;
165}
166
167/*
168 * IRQ handler called on GPIO edges
169 */
170static irqreturn_t dht11_handle_irq(int irq, void *data)
171{
172	struct iio_dev *iio = data;
173	struct dht11 *dht11 = iio_priv(iio);
174
175	if (dht11->num_edges < DHT11_EDGES_PER_READ && dht11->num_edges >= 0) {
176		dht11->edges[dht11->num_edges].ts = ktime_get_boottime_ns();
177		dht11->edges[dht11->num_edges++].value =
178						gpiod_get_value(dht11->gpiod);
179
180		if (dht11->num_edges >= DHT11_EDGES_PER_READ)
181			complete(&dht11->completion);
182	}
183
184	return IRQ_HANDLED;
185}
186
187static int dht11_read_raw(struct iio_dev *iio_dev,
188			  const struct iio_chan_spec *chan,
189			int *val, int *val2, long m)
190{
191	struct dht11 *dht11 = iio_priv(iio_dev);
192	int ret, timeres, offset;
193
194	mutex_lock(&dht11->lock);
195	if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_boottime_ns()) {
196		timeres = ktime_get_resolution_ns();
197		dev_dbg(dht11->dev, "current timeresolution: %dns\n", timeres);
198		if (timeres > DHT11_MIN_TIMERES) {
199			dev_err(dht11->dev, "timeresolution %dns too low\n",
200				timeres);
201			/* In theory a better clock could become available
202			 * at some point ... and there is no error code
203			 * that really fits better.
204			 */
205			ret = -EAGAIN;
206			goto err;
207		}
208		if (timeres > DHT11_AMBIG_LOW && timeres < DHT11_AMBIG_HIGH)
209			dev_warn(dht11->dev,
210				 "timeresolution: %dns - decoding ambiguous\n",
211				 timeres);
212
213		reinit_completion(&dht11->completion);
214
215		dht11->num_edges = 0;
216		ret = gpiod_direction_output(dht11->gpiod, 0);
217		if (ret)
218			goto err;
219		usleep_range(DHT11_START_TRANSMISSION_MIN,
220			     DHT11_START_TRANSMISSION_MAX);
221		ret = gpiod_direction_input(dht11->gpiod);
222		if (ret)
223			goto err;
224
225		ret = request_irq(dht11->irq, dht11_handle_irq,
226				  IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING,
227				  iio_dev->name, iio_dev);
228		if (ret)
229			goto err;
230
231		ret = wait_for_completion_killable_timeout(&dht11->completion,
232							   HZ);
233
234		free_irq(dht11->irq, iio_dev);
235
236#ifdef CONFIG_DYNAMIC_DEBUG
237		dht11_edges_print(dht11);
238#endif
239
240		if (ret == 0 && dht11->num_edges < DHT11_EDGES_PER_READ - 1) {
241			dev_err(dht11->dev, "Only %d signal edges detected\n",
242				dht11->num_edges);
243			ret = -ETIMEDOUT;
244		}
245		if (ret < 0)
246			goto err;
247
248		offset = DHT11_EDGES_PREAMBLE +
249				dht11->num_edges - DHT11_EDGES_PER_READ;
250		for (; offset >= 0; --offset) {
251			ret = dht11_decode(dht11, offset);
252			if (!ret)
253				break;
254		}
255
256		if (ret)
257			goto err;
258	}
259
260	ret = IIO_VAL_INT;
261	if (chan->type == IIO_TEMP)
262		*val = dht11->temperature;
263	else if (chan->type == IIO_HUMIDITYRELATIVE)
264		*val = dht11->humidity;
265	else
266		ret = -EINVAL;
267err:
268	dht11->num_edges = -1;
269	mutex_unlock(&dht11->lock);
270	return ret;
271}
272
273static const struct iio_info dht11_iio_info = {
274	.read_raw		= dht11_read_raw,
275};
276
277static const struct iio_chan_spec dht11_chan_spec[] = {
278	{ .type = IIO_TEMP,
279		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), },
280	{ .type = IIO_HUMIDITYRELATIVE,
281		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), }
282};
283
284static const struct of_device_id dht11_dt_ids[] = {
285	{ .compatible = "dht11", },
286	{ }
287};
288MODULE_DEVICE_TABLE(of, dht11_dt_ids);
289
290static int dht11_probe(struct platform_device *pdev)
291{
292	struct device *dev = &pdev->dev;
293	struct dht11 *dht11;
294	struct iio_dev *iio;
295
296	iio = devm_iio_device_alloc(dev, sizeof(*dht11));
297	if (!iio)
298		return -ENOMEM;
299
300	dht11 = iio_priv(iio);
301	dht11->dev = dev;
302	dht11->gpiod = devm_gpiod_get(dev, NULL, GPIOD_IN);
303	if (IS_ERR(dht11->gpiod))
304		return PTR_ERR(dht11->gpiod);
305
306	dht11->irq = gpiod_to_irq(dht11->gpiod);
307	if (dht11->irq < 0) {
308		dev_err(dev, "GPIO %d has no interrupt\n", desc_to_gpio(dht11->gpiod));
309		return -EINVAL;
310	}
311
312	dht11->timestamp = ktime_get_boottime_ns() - DHT11_DATA_VALID_TIME - 1;
313	dht11->num_edges = -1;
314
315	platform_set_drvdata(pdev, iio);
316
317	init_completion(&dht11->completion);
318	mutex_init(&dht11->lock);
319	iio->name = pdev->name;
320	iio->info = &dht11_iio_info;
321	iio->modes = INDIO_DIRECT_MODE;
322	iio->channels = dht11_chan_spec;
323	iio->num_channels = ARRAY_SIZE(dht11_chan_spec);
324
325	return devm_iio_device_register(dev, iio);
326}
327
328static struct platform_driver dht11_driver = {
329	.driver = {
330		.name	= "dht11",
331		.of_match_table = dht11_dt_ids,
332	},
333	.probe  = dht11_probe,
334};
335
336module_platform_driver(dht11_driver);
337
338MODULE_AUTHOR("Harald Geyer <harald@ccbib.org>");
339MODULE_DESCRIPTION("DHT11 humidity/temperature sensor driver");
340MODULE_LICENSE("GPL v2");