Message-ID: <ee14f0c8bfbae887d21f827baece8b6e@ccbib.org>
Date:   Tue, 31 Jan 2023 11:18:14 +0100
From:   harald@...ib.org
To:     Andreas Feldner <pelzi@...ing-snail.de>
Cc:     jic23@...nel.org, lars@...afoo.de, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iio: dht11: forked a driver version that polls
 sensor's signal from GPIO

On 2023-01-30 21:42, Andreas Feldner wrote:
> On a BananaPi M2 Zero, the existing, IRQ based dht11 driver is not 
> working,
> but missing most IRQs.

That's quite surprising as the driver works well on many similar systems
based on Allwinner SoCs. I suspect the problem is with your setup. Maybe
some other (polling?) driver is slowing everything down.

Eg. I remember one user reporting, that the driver stops working 
whenever
he enables debug output. After much back and forth it was found out, 
that
the driver for the system consoles UART was blocking things just long
enough to jumble the dht11 timings.

> 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.

IIRC one readout takes about 80 milliseconds. That's a very long time 
for
a busy loop. I doubt this is acceptable for inclusion in the kernel. Of
course also Jonathan's comments apply.

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

A success rate of 50% is pretty bad (for a polling driver). This adds to
my suspicion, that the problem is with your setup.

There is one known way to improve the reliability of the driver:
The initial version first enabled the interrupt, then sent the start
condition and after that switched the line from output to input. Usually
the silicon supports getting interrupts for your own output and this
actually made the driver more reliable.

I guess the extra reliability comes from the interrupt handler being
loaded into the cache before the first significant interrupt and thus
improving the timing.

However, when the driver was finally ready to merge, the GPIO subsystem
had a new check rejecting IRQ enabling on output lines, because it was
considered inconsistent. I couldn't convince the GPIO maintainer to add
it back, so we had to drop that little trick.

HTH,
Harald


> 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@...ing-snail.de>
> ---
>  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@...ing-snail.de>
> + * based on work Copyright (c) Harald Geyer <harald@...ib.org>
> + */
> +
> +#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@...ing-snail.de>");
> +MODULE_DESCRIPTION("DHT11 polling humidity/temperature sensor 
> driver");
> +MODULE_LICENSE("GPL v2");

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