[PATCH 1/2] iio: dht11: forked a driver version that polls sensor's signal from GPIO

[Andreas Feldner]

On a BananaPi M2 Zero, the existing, IRQ based dht11 driver is not working,
but missing most IRQs. Following the hints in Harald Geyer's comments I
tried to implement a version of the driver that is polling the GPIO
sensor in a busy loop, not using IRQ altogether.

This version is actually working fair enough on this board, yielding a
valid result at every 2 or 3 read attempts.

I used the "compatible" string to allow selection of the required driver.
To select this forked driver, give compatible="dht11-poll" instead of
compatible="dht11" in the device tree (overlay).

Signed-off-by: Andreas Feldner <pelzi@xxxxxxxxxxxxxxx>
---
 drivers/iio/humidity/Kconfig         |  10 +
 drivers/iio/humidity/Makefile        |   1 +
 drivers/iio/humidity/dht11_polling.c | 348 +++++++++++++++++++++++++++
 3 files changed, 359 insertions(+)
 create mode 100644 drivers/iio/humidity/dht11_polling.c

diff --git a/drivers/iio/humidity/Kconfig b/drivers/iio/humidity/Kconfig
index 2de5494e7c22..7b06b06181d4 100644
--- a/drivers/iio/humidity/Kconfig
+++ b/drivers/iio/humidity/Kconfig
@@ -25,6 +25,16 @@ config DHT11
 	  Other sensors should work as well as long as they speak the
 	  same protocol.
 
+config DHT11_POLLING
+	tristate "DHT11 (and compatible) sensors driver using polling"
+	depends on GPIOLIB || COMPILE_TEST
+	help
+	  This driver supports reading data via a single GPIO line.
+          This version does not require the line to generate
+          interrupts. It is required to read DHT11/22 signals on
+          some boards that fail to generate IRQ quickly enough.
+          This driver is tested with DHT22 on a BananaPI M2 Zero.
+
 config HDC100X
 	tristate "TI HDC100x relative humidity and temperature sensor"
 	depends on I2C
diff --git a/drivers/iio/humidity/Makefile b/drivers/iio/humidity/Makefile
index f19ff3de97c5..908e5ecebb27 100644
--- a/drivers/iio/humidity/Makefile
+++ b/drivers/iio/humidity/Makefile
@@ -5,6 +5,7 @@
 
 obj-$(CONFIG_AM2315) += am2315.o
 obj-$(CONFIG_DHT11) += dht11.o
+obj-$(CONFIG_DHT11_POLLING) += dht11_polling.o
 obj-$(CONFIG_HDC100X) += hdc100x.o
 obj-$(CONFIG_HDC2010) += hdc2010.o
 obj-$(CONFIG_HID_SENSOR_HUMIDITY) += hid-sensor-humidity.o
diff --git a/drivers/iio/humidity/dht11_polling.c b/drivers/iio/humidity/dht11_polling.c
new file mode 100644
index 000000000000..ea41548144b0
--- /dev/null
+++ b/drivers/iio/humidity/dht11_polling.c
@@ -0,0 +1,348 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * DHT11/DHT22 polling version of the bit banging GPIO driver.
+ *
+ * Copyright (c) Andreas Feldner <pelzi@xxxxxxxxxxxxxxx>
+ * based on work Copyright (c) Harald Geyer <harald@xxxxxxxxx>
+ */
+
+#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/sysfs.h>
+#include <linux/io.h>
+#include <linux/mod_devicetable.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/consumer.h>
+#include <linux/timekeeping.h>
+
+#include <linux/iio/iio.h>
+
+#define DRIVER_NAME	"dht11_poll"
+
+#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 actual 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_MIN	18000  /* us */
+#define DHT11_START_TRANSMISSION_MAX	20000  /* us */
+#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;
+
+	struct gpio_desc		*gpiod;
+
+	bool				complete;
+	/* 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];
+};
+
+/*
+ * dht11_edges_print: show the data as actually received by the
+ *                    driver.
+ */
+static void dht11_edges_print(struct dht11 *dht11)
+{
+	int i;
+
+	dev_dbg(dht11->dev, "%d edges detected:\n", dht11->num_edges);
+	for (i = 1; i < dht11->num_edges; ++i) {
+		dev_dbg(dht11->dev, "%d: %lld ns %s\n", i,
+			dht11->edges[i].ts - dht11->edges[i - 1].ts,
+			dht11->edges[i - 1].value ? "high" : "low");
+	}
+}
+
+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 + 1].ts -
+			dht11->edges[offset + 2 * i].ts;
+		if (!dht11->edges[offset + 2 * i].value) {
+			dev_info(dht11->dev,
+				 "lost synchronisation at edge %d using offset %d\n",
+				 offset + 2 * i, offset);
+			return -EIO;
+		}
+		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) {
+		dev_info(dht11->dev, "invalid checksum using offset %d\n", offset);
+		return -EIO;
+	}
+
+	dht11->timestamp = ktime_get_boottime_ns();
+	if (hum_int < 4) {  /* DHT22: 100000 = (3*256+232)*100 */
+		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 void dht11_handle_edge(struct dht11 *dht11, int value)
+{
+	if (dht11->num_edges < DHT11_EDGES_PER_READ && dht11->num_edges >= 0) {
+		dht11->edges[dht11->num_edges].ts = ktime_get_boottime_ns();
+		dht11->edges[dht11->num_edges++].value = value;
+
+		if (dht11->num_edges >= DHT11_EDGES_PER_READ)
+			dht11->complete = !0;
+	}
+}
+
+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, value_prev, num_samples;
+	u64 startstamp;
+
+	mutex_lock(&dht11->lock);
+
+	startstamp = ktime_get_boottime_ns();
+	if (dht11->timestamp + DHT11_DATA_VALID_TIME < startstamp) {
+		timeres = ktime_get_resolution_ns();
+		dev_dbg(dht11->dev, "current timeresolution: %dns\n", timeres);
+		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);
+
+		dht11->complete = 0;
+
+		dht11->num_edges = 0;
+		ret = gpiod_direction_output(dht11->gpiod, 0);
+		if (ret)
+			goto err;
+		usleep_range(DHT11_START_TRANSMISSION_MIN,
+			     DHT11_START_TRANSMISSION_MAX);
+		value_prev = 0;
+		ret = gpiod_direction_input(dht11->gpiod);
+		if (ret)
+			goto err;
+
+		num_samples = 0;
+		while (!dht11->complete) {
+			int value = gpiod_get_value_cansleep(dht11->gpiod);
+
+			if (value >= 0 && value != value_prev) {
+				dht11_handle_edge(dht11, value);
+				value_prev = value;
+				num_samples = 1;
+			} else if (value < 0) {
+				ret = value;
+				break;
+			} else {
+				num_samples++;
+				if ((num_samples % 1000) == 0) {
+					dev_warn(dht11->dev, "No edge detected during 1000 reads, aborting polling.\n");
+					ret = -ETIMEDOUT;
+					dht11_handle_edge(dht11, value);
+					break;
+				}
+			}
+		}
+
+		dht11_edges_print(dht11);
+
+		if (dht11->num_edges < 2 * DHT11_BITS_PER_READ) {
+			dev_err(dht11->dev, "Only %d signal edges detected\n",
+				dht11->num_edges);
+			ret = -ETIMEDOUT;
+		} else {
+			/* there is a chance we only missed out the preamble! */
+			ret = 0;
+		}
+		if (ret < 0)
+			goto err;
+
+		offset = dht11->num_edges - 2 * DHT11_BITS_PER_READ;
+
+		for (; offset >= 0; offset--) {
+			if (dht11->edges[offset].value)
+				ret = dht11_decode(dht11, offset);
+			else
+				ret = -EIO;
+			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 = {
+	.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-poll", },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, dht11_dt_ids);
+
+static int dht11_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct dht11 *dht11;
+	struct iio_dev *iio;
+
+	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;
+	dht11->gpiod = devm_gpiod_get(dev, NULL, GPIOD_IN);
+	if (IS_ERR(dht11->gpiod))
+		return PTR_ERR(dht11->gpiod);
+
+	dht11->timestamp = ktime_get_boottime_ns() - DHT11_DATA_VALID_TIME - 1;
+	dht11->num_edges = -1;
+
+	platform_set_drvdata(pdev, iio);
+
+	dht11->complete = 0;
+	mutex_init(&dht11->lock);
+	iio->name = pdev->name;
+	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("Andreas Feldner <pelzi@xxxxxxxxxxxxxxx>");
+MODULE_DESCRIPTION("DHT11 polling humidity/temperature sensor driver");
+MODULE_LICENSE("GPL v2");
-- 
2.30.2