linux/drivers/iio/humidity/dht11.c
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   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/of.h>
  15#include <linux/of_device.h>
  16#include <linux/sysfs.h>
  17#include <linux/io.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 DRIVER_NAME     "dht11"
  31
  32#define DHT11_DATA_VALID_TIME   2000000000  /* 2s in ns */
  33
  34#define DHT11_EDGES_PREAMBLE 2
  35#define DHT11_BITS_PER_READ 40
  36/*
  37 * Note that when reading the sensor actually 84 edges are detected, but
  38 * since the last edge is not significant, we only store 83:
  39 */
  40#define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \
  41                              DHT11_EDGES_PREAMBLE + 1)
  42
  43/*
  44 * Data transmission timing:
  45 * Data bits are encoded as pulse length (high time) on the data line.
  46 * 0-bit: 22-30uS -- typically 26uS (AM2302)
  47 * 1-bit: 68-75uS -- typically 70uS (AM2302)
  48 * The acutal timings also depend on the properties of the cable, with
  49 * longer cables typically making pulses shorter.
  50 *
  51 * Our decoding depends on the time resolution of the system:
  52 * timeres > 34uS ... don't know what a 1-tick pulse is
  53 * 34uS > timeres > 30uS ... no problem (30kHz and 32kHz clocks)
  54 * 30uS > timeres > 23uS ... don't know what a 2-tick pulse is
  55 * timeres < 23uS ... no problem
  56 *
  57 * Luckily clocks in the 33-44kHz range are quite uncommon, so we can
  58 * support most systems if the threshold for decoding a pulse as 1-bit
  59 * is chosen carefully. If somebody really wants to support clocks around
  60 * 40kHz, where this driver is most unreliable, there are two options.
  61 * a) select an implementation using busy loop polling on those systems
  62 * b) use the checksum to do some probabilistic decoding
  63 */
  64#define DHT11_START_TRANSMISSION_MIN    18000  /* us */
  65#define DHT11_START_TRANSMISSION_MAX    20000  /* us */
  66#define DHT11_MIN_TIMERES       34000  /* ns */
  67#define DHT11_THRESHOLD         49000  /* ns */
  68#define DHT11_AMBIG_LOW         23000  /* ns */
  69#define DHT11_AMBIG_HIGH        30000  /* ns */
  70
  71struct dht11 {
  72        struct device                   *dev;
  73
  74        struct gpio_desc                *gpiod;
  75        int                             irq;
  76
  77        struct completion               completion;
  78        /* The iio sysfs interface doesn't prevent concurrent reads: */
  79        struct mutex                    lock;
  80
  81        s64                             timestamp;
  82        int                             temperature;
  83        int                             humidity;
  84
  85        /* num_edges: -1 means "no transmission in progress" */
  86        int                             num_edges;
  87        struct {s64 ts; int value; }    edges[DHT11_EDGES_PER_READ];
  88};
  89
  90#ifdef CONFIG_DYNAMIC_DEBUG
  91/*
  92 * dht11_edges_print: show the data as actually received by the
  93 *                    driver.
  94 */
  95static void dht11_edges_print(struct dht11 *dht11)
  96{
  97        int i;
  98
  99        dev_dbg(dht11->dev, "%d edges detected:\n", dht11->num_edges);
 100        for (i = 1; i < dht11->num_edges; ++i) {
 101                dev_dbg(dht11->dev, "%d: %lld ns %s\n", i,
 102                        dht11->edges[i].ts - dht11->edges[i - 1].ts,
 103                        dht11->edges[i - 1].value ? "high" : "low");
 104        }
 105}
 106#endif /* CONFIG_DYNAMIC_DEBUG */
 107
 108static unsigned char dht11_decode_byte(char *bits)
 109{
 110        unsigned char ret = 0;
 111        int i;
 112
 113        for (i = 0; i < 8; ++i) {
 114                ret <<= 1;
 115                if (bits[i])
 116                        ++ret;
 117        }
 118
 119        return ret;
 120}
 121
 122static int dht11_decode(struct dht11 *dht11, int offset)
 123{
 124        int i, t;
 125        char bits[DHT11_BITS_PER_READ];
 126        unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum;
 127
 128        for (i = 0; i < DHT11_BITS_PER_READ; ++i) {
 129                t = dht11->edges[offset + 2 * i + 2].ts -
 130                        dht11->edges[offset + 2 * i + 1].ts;
 131                if (!dht11->edges[offset + 2 * i + 1].value) {
 132                        dev_dbg(dht11->dev,
 133                                "lost synchronisation at edge %d\n",
 134                                offset + 2 * i + 1);
 135                        return -EIO;
 136                }
 137                bits[i] = t > DHT11_THRESHOLD;
 138        }
 139
 140        hum_int = dht11_decode_byte(bits);
 141        hum_dec = dht11_decode_byte(&bits[8]);
 142        temp_int = dht11_decode_byte(&bits[16]);
 143        temp_dec = dht11_decode_byte(&bits[24]);
 144        checksum = dht11_decode_byte(&bits[32]);
 145
 146        if (((hum_int + hum_dec + temp_int + temp_dec) & 0xff) != checksum) {
 147                dev_dbg(dht11->dev, "invalid checksum\n");
 148                return -EIO;
 149        }
 150
 151        dht11->timestamp = ktime_get_boottime_ns();
 152        if (hum_int < 4) {  /* DHT22: 100000 = (3*256+232)*100 */
 153                dht11->temperature = (((temp_int & 0x7f) << 8) + temp_dec) *
 154                                        ((temp_int & 0x80) ? -100 : 100);
 155                dht11->humidity = ((hum_int << 8) + hum_dec) * 100;
 156        } else if (temp_dec == 0 && hum_dec == 0) {  /* DHT11 */
 157                dht11->temperature = temp_int * 1000;
 158                dht11->humidity = hum_int * 1000;
 159        } else {
 160                dev_err(dht11->dev,
 161                        "Don't know how to decode data: %d %d %d %d\n",
 162                        hum_int, hum_dec, temp_int, temp_dec);
 163                return -EIO;
 164        }
 165
 166        return 0;
 167}
 168
 169/*
 170 * IRQ handler called on GPIO edges
 171 */
 172static irqreturn_t dht11_handle_irq(int irq, void *data)
 173{
 174        struct iio_dev *iio = data;
 175        struct dht11 *dht11 = iio_priv(iio);
 176
 177        /* TODO: Consider making the handler safe for IRQ sharing */
 178        if (dht11->num_edges < DHT11_EDGES_PER_READ && dht11->num_edges >= 0) {
 179                dht11->edges[dht11->num_edges].ts = ktime_get_boottime_ns();
 180                dht11->edges[dht11->num_edges++].value =
 181                                                gpiod_get_value(dht11->gpiod);
 182
 183                if (dht11->num_edges >= DHT11_EDGES_PER_READ)
 184                        complete(&dht11->completion);
 185        }
 186
 187        return IRQ_HANDLED;
 188}
 189
 190static int dht11_read_raw(struct iio_dev *iio_dev,
 191                          const struct iio_chan_spec *chan,
 192                        int *val, int *val2, long m)
 193{
 194        struct dht11 *dht11 = iio_priv(iio_dev);
 195        int ret, timeres, offset;
 196
 197        mutex_lock(&dht11->lock);
 198        if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_boottime_ns()) {
 199                timeres = ktime_get_resolution_ns();
 200                dev_dbg(dht11->dev, "current timeresolution: %dns\n", timeres);
 201                if (timeres > DHT11_MIN_TIMERES) {
 202                        dev_err(dht11->dev, "timeresolution %dns too low\n",
 203                                timeres);
 204                        /* In theory a better clock could become available
 205                         * at some point ... and there is no error code
 206                         * that really fits better.
 207                         */
 208                        ret = -EAGAIN;
 209                        goto err;
 210                }
 211                if (timeres > DHT11_AMBIG_LOW && timeres < DHT11_AMBIG_HIGH)
 212                        dev_warn(dht11->dev,
 213                                 "timeresolution: %dns - decoding ambiguous\n",
 214                                 timeres);
 215
 216                reinit_completion(&dht11->completion);
 217
 218                dht11->num_edges = 0;
 219                ret = gpiod_direction_output(dht11->gpiod, 0);
 220                if (ret)
 221                        goto err;
 222                usleep_range(DHT11_START_TRANSMISSION_MIN,
 223                             DHT11_START_TRANSMISSION_MAX);
 224                ret = gpiod_direction_input(dht11->gpiod);
 225                if (ret)
 226                        goto err;
 227
 228                ret = request_irq(dht11->irq, dht11_handle_irq,
 229                                  IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING,
 230                                  iio_dev->name, iio_dev);
 231                if (ret)
 232                        goto err;
 233
 234                ret = wait_for_completion_killable_timeout(&dht11->completion,
 235                                                           HZ);
 236
 237                free_irq(dht11->irq, iio_dev);
 238
 239#ifdef CONFIG_DYNAMIC_DEBUG
 240                dht11_edges_print(dht11);
 241#endif
 242
 243                if (ret == 0 && dht11->num_edges < DHT11_EDGES_PER_READ - 1) {
 244                        dev_err(dht11->dev, "Only %d signal edges detected\n",
 245                                dht11->num_edges);
 246                        ret = -ETIMEDOUT;
 247                }
 248                if (ret < 0)
 249                        goto err;
 250
 251                offset = DHT11_EDGES_PREAMBLE +
 252                                dht11->num_edges - DHT11_EDGES_PER_READ;
 253                for (; offset >= 0; --offset) {
 254                        ret = dht11_decode(dht11, offset);
 255                        if (!ret)
 256                                break;
 257                }
 258
 259                if (ret)
 260                        goto err;
 261        }
 262
 263        ret = IIO_VAL_INT;
 264        if (chan->type == IIO_TEMP)
 265                *val = dht11->temperature;
 266        else if (chan->type == IIO_HUMIDITYRELATIVE)
 267                *val = dht11->humidity;
 268        else
 269                ret = -EINVAL;
 270err:
 271        dht11->num_edges = -1;
 272        mutex_unlock(&dht11->lock);
 273        return ret;
 274}
 275
 276static const struct iio_info dht11_iio_info = {
 277        .read_raw               = dht11_read_raw,
 278};
 279
 280static const struct iio_chan_spec dht11_chan_spec[] = {
 281        { .type = IIO_TEMP,
 282                .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), },
 283        { .type = IIO_HUMIDITYRELATIVE,
 284                .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), }
 285};
 286
 287static const struct of_device_id dht11_dt_ids[] = {
 288        { .compatible = "dht11", },
 289        { }
 290};
 291MODULE_DEVICE_TABLE(of, dht11_dt_ids);
 292
 293static int dht11_probe(struct platform_device *pdev)
 294{
 295        struct device *dev = &pdev->dev;
 296        struct dht11 *dht11;
 297        struct iio_dev *iio;
 298
 299        iio = devm_iio_device_alloc(dev, sizeof(*dht11));
 300        if (!iio) {
 301                dev_err(dev, "Failed to allocate IIO device\n");
 302                return -ENOMEM;
 303        }
 304
 305        dht11 = iio_priv(iio);
 306        dht11->dev = dev;
 307        dht11->gpiod = devm_gpiod_get(dev, NULL, GPIOD_IN);
 308        if (IS_ERR(dht11->gpiod))
 309                return PTR_ERR(dht11->gpiod);
 310
 311        dht11->irq = gpiod_to_irq(dht11->gpiod);
 312        if (dht11->irq < 0) {
 313                dev_err(dev, "GPIO %d has no interrupt\n", desc_to_gpio(dht11->gpiod));
 314                return -EINVAL;
 315        }
 316
 317        dht11->timestamp = ktime_get_boottime_ns() - DHT11_DATA_VALID_TIME - 1;
 318        dht11->num_edges = -1;
 319
 320        platform_set_drvdata(pdev, iio);
 321
 322        init_completion(&dht11->completion);
 323        mutex_init(&dht11->lock);
 324        iio->name = pdev->name;
 325        iio->dev.parent = &pdev->dev;
 326        iio->info = &dht11_iio_info;
 327        iio->modes = INDIO_DIRECT_MODE;
 328        iio->channels = dht11_chan_spec;
 329        iio->num_channels = ARRAY_SIZE(dht11_chan_spec);
 330
 331        return devm_iio_device_register(dev, iio);
 332}
 333
 334static struct platform_driver dht11_driver = {
 335        .driver = {
 336                .name   = DRIVER_NAME,
 337                .of_match_table = dht11_dt_ids,
 338        },
 339        .probe  = dht11_probe,
 340};
 341
 342module_platform_driver(dht11_driver);
 343
 344MODULE_AUTHOR("Harald Geyer <harald@ccbib.org>");
 345MODULE_DESCRIPTION("DHT11 humidity/temperature sensor driver");
 346MODULE_LICENSE("GPL v2");
 347