/*
 * DHT11/DHT22 bit banging GPIO driver
 *
 * Copyright (c) Harald Geyer <harald@ccbib.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 */

#include <linux/err.h>
#include <linux/interrupt.h>
#include <linux/device.h>
#include <linux/kernel.h>
#include <linux/printk.h>
#include <linux/slab.h>
#include <linux/of.h>
#include <linux/of_device.h>
#include <linux/sysfs.h>
#include <linux/io.h>
#include <linux/module.h>
#include <linux/platform_device.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/completion.h>
#include <linux/mutex.h>
#include <linux/delay.h>
#include <linux/gpio.h>
#include <linux/of_gpio.h>
#include <linux/timekeeping.h>

#include <linux/iio/iio.h>

#define DRIVER_NAME     "dht11"

#define DHT11_DATA_VALID_TIME   2000000000  /* 2s in ns */

#define DHT11_EDGES_PREAMBLE 2
#define DHT11_BITS_PER_READ 40
/*
 * Note that when reading the sensor actually 84 edges are detected, but
 * since the last edge is not significant, we only store 83:
 */
#define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \
                              DHT11_EDGES_PREAMBLE + 1)

/*
 * Data transmission timing:
 * Data bits are encoded as pulse length (high time) on the data line.
 * 0-bit: 22-30uS -- typically 26uS (AM2302)
 * 1-bit: 68-75uS -- typically 70uS (AM2302)
 * The acutal timings also depend on the properties of the cable, with
 * longer cables typically making pulses shorter.
 *
 * Our decoding depends on the time resolution of the system:
 * timeres > 34uS ... don't know what a 1-tick pulse is
 * 34uS > timeres > 30uS ... no problem (30kHz and 32kHz clocks)
 * 30uS > timeres > 23uS ... don't know what a 2-tick pulse is
 * timeres < 23uS ... no problem
 *
 * Luckily clocks in the 33-44kHz range are quite uncommon, so we can
 * support most systems if the threshold for decoding a pulse as 1-bit
 * is chosen carefully. If somebody really wants to support clocks around
 * 40kHz, where this driver is most unreliable, there are two options.
 * a) select an implementation using busy loop polling on those systems
 * b) use the checksum to do some probabilistic decoding
 */
#define DHT11_START_TRANSMISSION        18  /* ms */
#define DHT11_MIN_TIMERES       34000  /* ns */
#define DHT11_THRESHOLD         49000  /* ns */
#define DHT11_AMBIG_LOW         23000  /* ns */
#define DHT11_AMBIG_HIGH        30000  /* ns */

struct dht11 {
        struct device                   *dev;

        int                             gpio;
        int                             irq;

        struct completion               completion;
        /* The iio sysfs interface doesn't prevent concurrent reads: */
        struct mutex                    lock;

        s64                             timestamp;
        int                             temperature;
        int                             humidity;

        /* num_edges: -1 means "no transmission in progress" */
        int                             num_edges;
        struct {s64 ts; int value; }    edges[DHT11_EDGES_PER_READ];
};

static unsigned char dht11_decode_byte(char *bits)
{
        unsigned char ret = 0;
        int i;

        for (i = 0; i < 8; ++i) {
                ret <<= 1;
                if (bits[i])
                        ++ret;
        }

        return ret;
}

static int dht11_decode(struct dht11 *dht11, int offset)
{
        int i, t;
        char bits[DHT11_BITS_PER_READ];
        unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum;

        for (i = 0; i < DHT11_BITS_PER_READ; ++i) {
                t = dht11->edges[offset + 2 * i + 2].ts -
                        dht11->edges[offset + 2 * i + 1].ts;
                if (!dht11->edges[offset + 2 * i + 1].value)
                        return -EIO;  /* lost synchronisation */
                bits[i] = t > DHT11_THRESHOLD;
        }

        hum_int = dht11_decode_byte(bits);
        hum_dec = dht11_decode_byte(&bits[8]);
        temp_int = dht11_decode_byte(&bits[16]);
        temp_dec = dht11_decode_byte(&bits[24]);
        checksum = dht11_decode_byte(&bits[32]);

        if (((hum_int + hum_dec + temp_int + temp_dec) & 0xff) != checksum)
                return -EIO;

        dht11->timestamp = ktime_get_boot_ns();
        if (hum_int < 20) {  /* DHT22 */
                dht11->temperature = (((temp_int & 0x7f) << 8) + temp_dec) *
                                        ((temp_int & 0x80) ? -100 : 100);
                dht11->humidity = ((hum_int << 8) + hum_dec) * 100;
        } else if (temp_dec == 0 && hum_dec == 0) {  /* DHT11 */
                dht11->temperature = temp_int * 1000;
                dht11->humidity = hum_int * 1000;
        } else {
                dev_err(dht11->dev,
                        "Don't know how to decode data: %d %d %d %d\n",
                        hum_int, hum_dec, temp_int, temp_dec);
                return -EIO;
        }

        return 0;
}

/*
 * IRQ handler called on GPIO edges
 */
static irqreturn_t dht11_handle_irq(int irq, void *data)
{
        struct iio_dev *iio = data;
        struct dht11 *dht11 = iio_priv(iio);

        /* TODO: Consider making the handler safe for IRQ sharing */
        if (dht11->num_edges < DHT11_EDGES_PER_READ && dht11->num_edges >= 0) {
                dht11->edges[dht11->num_edges].ts = ktime_get_boot_ns();
                dht11->edges[dht11->num_edges++].value =
                                                gpio_get_value(dht11->gpio);

                if (dht11->num_edges >= DHT11_EDGES_PER_READ)
                        complete(&dht11->completion);
        }

        return IRQ_HANDLED;
}

static int dht11_read_raw(struct iio_dev *iio_dev,
                          const struct iio_chan_spec *chan,
                        int *val, int *val2, long m)
{
        struct dht11 *dht11 = iio_priv(iio_dev);
        int ret, timeres, offset;

        mutex_lock(&dht11->lock);
        if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_boot_ns()) {
                timeres = ktime_get_resolution_ns();
                if (timeres > DHT11_MIN_TIMERES) {
                        dev_err(dht11->dev, "timeresolution %dns too low\n",
                                timeres);
                        /* In theory a better clock could become available
                         * at some point ... and there is no error code
                         * that really fits better.
                         */
                        ret = -EAGAIN;
                        goto err;
                }
                if (timeres > DHT11_AMBIG_LOW && timeres < DHT11_AMBIG_HIGH)
                        dev_warn(dht11->dev,
                                 "timeresolution: %dns - decoding ambiguous\n",
                                 timeres);

                reinit_completion(&dht11->completion);

                dht11->num_edges = 0;
                ret = gpio_direction_output(dht11->gpio, 0);
                if (ret)
                        goto err;
                msleep(DHT11_START_TRANSMISSION);
                ret = gpio_direction_input(dht11->gpio);
                if (ret)
                        goto err;

                ret = request_irq(dht11->irq, dht11_handle_irq,
                                  IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING,
                                  iio_dev->name, iio_dev);
                if (ret)
                        goto err;

                ret = wait_for_completion_killable_timeout(&dht11->completion,
                                                           HZ);

                free_irq(dht11->irq, iio_dev);

                if (ret == 0 && dht11->num_edges < DHT11_EDGES_PER_READ - 1) {
                        dev_err(&iio_dev->dev,
                                "Only %d signal edges detected\n",
                                        dht11->num_edges);
                        ret = -ETIMEDOUT;
                }
                if (ret < 0)
                        goto err;

                offset = DHT11_EDGES_PREAMBLE +
                                dht11->num_edges - DHT11_EDGES_PER_READ;
                for (; offset >= 0; --offset) {
                        ret = dht11_decode(dht11, offset);
                        if (!ret)
                                break;
                }

                if (ret)
                        goto err;
        }

        ret = IIO_VAL_INT;
        if (chan->type == IIO_TEMP)
                *val = dht11->temperature;
        else if (chan->type == IIO_HUMIDITYRELATIVE)
                *val = dht11->humidity;
        else
                ret = -EINVAL;
err:
        dht11->num_edges = -1;
        mutex_unlock(&dht11->lock);
        return ret;
}

static const struct iio_info dht11_iio_info = {
        .driver_module          = THIS_MODULE,
        .read_raw               = dht11_read_raw,
};

static const struct iio_chan_spec dht11_chan_spec[] = {
        { .type = IIO_TEMP,
                .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), },
        { .type = IIO_HUMIDITYRELATIVE,
                .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), }
};

static const struct of_device_id dht11_dt_ids[] = {
        { .compatible = "dht11", },
        { }
};
MODULE_DEVICE_TABLE(of, dht11_dt_ids);

static int dht11_probe(struct platform_device *pdev)
{
        struct device *dev = &pdev->dev;
        struct device_node *node = dev->of_node;
        struct dht11 *dht11;
        struct iio_dev *iio;
        int ret;

        iio = devm_iio_device_alloc(dev, sizeof(*dht11));
        if (!iio) {
                dev_err(dev, "Failed to allocate IIO device\n");
                return -ENOMEM;
        }

        dht11 = iio_priv(iio);
        dht11->dev = dev;

        ret = of_get_gpio(node, 0);
        if (ret < 0)
                return ret;
        dht11->gpio = ret;
        ret = devm_gpio_request_one(dev, dht11->gpio, GPIOF_IN, pdev->name);
        if (ret)
                return ret;

        dht11->irq = gpio_to_irq(dht11->gpio);
        if (dht11->irq < 0) {
                dev_err(dev, "GPIO %d has no interrupt\n", dht11->gpio);
                return -EINVAL;
        }

        dht11->timestamp = ktime_get_boot_ns() - DHT11_DATA_VALID_TIME - 1;
        dht11->num_edges = -1;

        platform_set_drvdata(pdev, iio);

        init_completion(&dht11->completion);
        mutex_init(&dht11->lock);
        iio->name = pdev->name;
        iio->dev.parent = &pdev->dev;
        iio->info = &dht11_iio_info;
        iio->modes = INDIO_DIRECT_MODE;
        iio->channels = dht11_chan_spec;
        iio->num_channels = ARRAY_SIZE(dht11_chan_spec);

        return devm_iio_device_register(dev, iio);
}

static struct platform_driver dht11_driver = {
        .driver = {
                .name   = DRIVER_NAME,
                .of_match_table = dht11_dt_ids,
        },
        .probe  = dht11_probe,
};

module_platform_driver(dht11_driver);

MODULE_AUTHOR("Harald Geyer <harald@ccbib.org>");
MODULE_DESCRIPTION("DHT11 humidity/temperature sensor driver");
MODULE_LICENSE("GPL v2");