iio: adc: hx711: Add IIO driver for AVIA HX711

This is the IIO driver for AVIA HX711 ADC which is mostly used in weighting
cells.

The protocol is quite simple and using GPIOs:
One GPIO is used as clock (SCK) while another GPIO is read (DOUT)

The raw value read from the chip is delivered.
To get a weight one needs to subtract the zero offset and scale it.
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/Kconfig

@@ -247,6 +247,25 @@ config HI8435
 	  This driver can also be built as a module. If so, the module will be
 	  called hi8435.
 
+config HX711
+	tristate "AVIA HX711 ADC for weight cells"
+	depends on GPIOLIB
+	help
+	  If you say yes here you get support for AVIA HX711 ADC which is used
+	  for weigh cells
+
+	  This driver uses two GPIOs, one acts as the clock and controls the
+	  channel selection and gain, the other one is used for the measurement
+          data
+
+	  Currently the raw value is read from the chip and delivered.
+	  To get an actual weight one needs to subtract the
+	  zero offset and multiply by a scale factor.
+	  This should be done in userspace.
+
+	  This driver can also be built as a module. If so, the module will be
+	  called hx711.
+
 config INA2XX_ADC
 	tristate "Texas Instruments INA2xx Power Monitors IIO driver"
 	depends on I2C && !SENSORS_INA2XX

drivers/iio/adc/Makefile

@@ -25,6 +25,7 @@ obj-$(CONFIG_ENVELOPE_DETECTOR) += envelope-detector.o
 obj-$(CONFIG_EXYNOS_ADC) += exynos_adc.o
 obj-$(CONFIG_FSL_MX25_ADC) += fsl-imx25-gcq.o
 obj-$(CONFIG_HI8435) += hi8435.o
+obj-$(CONFIG_HX711) += hx711.o
 obj-$(CONFIG_IMX7D_ADC) += imx7d_adc.o
 obj-$(CONFIG_INA2XX_ADC) += ina2xx-adc.o
 obj-$(CONFIG_LP8788_ADC) += lp8788_adc.o

drivers/iio/adc/hx711.c

+/*
+ * HX711: analog to digital converter for weight sensor module
+ *
+ * Copyright (c) 2016 Andreas Klinger <ak@it-klinger.de>
+ *
+ * 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/kernel.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/property.h>
+#include <linux/slab.h>
+#include <linux/sched.h>
+#include <linux/delay.h>
+#include <linux/iio/iio.h>
+#include <linux/iio/sysfs.h>
+#include <linux/gpio/consumer.h>
+#include <linux/regulator/consumer.h>
+
+/* gain to pulse and scale conversion */
+#define HX711_GAIN_MAX		3
+
+struct hx711_gain_to_scale {
+	int			gain;
+	int			gain_pulse;
+	int			scale;
+	int			channel;
+};
+
+/*
+ * .scale depends on AVDD which in turn is known as soon as the regulator
+ * is available
+ * therefore we set .scale in hx711_probe()
+ *
+ * channel A in documentation is channel 0 in source code
+ * channel B in documentation is channel 1 in source code
+ */
+static struct hx711_gain_to_scale hx711_gain_to_scale[HX711_GAIN_MAX] = {
+	{ 128, 1, 0, 0 },
+	{  32, 2, 0, 1 },
+	{  64, 3, 0, 0 }
+};
+
+static int hx711_get_gain_to_pulse(int gain)
+{
+	int i;
+
+	for (i = 0; i < HX711_GAIN_MAX; i++)
+		if (hx711_gain_to_scale[i].gain == gain)
+			return hx711_gain_to_scale[i].gain_pulse;
+	return 1;
+}
+
+static int hx711_get_gain_to_scale(int gain)
+{
+	int i;
+
+	for (i = 0; i < HX711_GAIN_MAX; i++)
+		if (hx711_gain_to_scale[i].gain == gain)
+			return hx711_gain_to_scale[i].scale;
+	return 0;
+}
+
+static int hx711_get_scale_to_gain(int scale)
+{
+	int i;
+
+	for (i = 0; i < HX711_GAIN_MAX; i++)
+		if (hx711_gain_to_scale[i].scale == scale)
+			return hx711_gain_to_scale[i].gain;
+	return -EINVAL;
+}
+
+struct hx711_data {
+	struct device		*dev;
+	struct gpio_desc	*gpiod_pd_sck;
+	struct gpio_desc	*gpiod_dout;
+	struct regulator	*reg_avdd;
+	int			gain_set;	/* gain set on device */
+	int			gain_chan_a;	/* gain for channel A */
+	struct mutex		lock;
+};
+
+static int hx711_cycle(struct hx711_data *hx711_data)
+{
+	int val;
+
+	/*
+	 * if preempted for more then 60us while PD_SCK is high:
+	 * hx711 is going in reset
+	 * ==> measuring is false
+	 */
+	preempt_disable();
+	gpiod_set_value(hx711_data->gpiod_pd_sck, 1);
+	val = gpiod_get_value(hx711_data->gpiod_dout);
+	/*
+	 * here we are not waiting for 0.2 us as suggested by the datasheet,
+	 * because the oscilloscope showed in a test scenario
+	 * at least 1.15 us for PD_SCK high (T3 in datasheet)
+	 * and 0.56 us for PD_SCK low on TI Sitara with 800 MHz
+	 */
+	gpiod_set_value(hx711_data->gpiod_pd_sck, 0);
+	preempt_enable();
+
+	return val;
+}
+
+static int hx711_read(struct hx711_data *hx711_data)
+{
+	int i, ret;
+	int value = 0;
+	int val = gpiod_get_value(hx711_data->gpiod_dout);
+
+	/* we double check if it's really down */
+	if (val)
+		return -EIO;
+
+	for (i = 0; i < 24; i++) {
+		value <<= 1;
+		ret = hx711_cycle(hx711_data);
+		if (ret)
+			value++;
+	}
+
+	value ^= 0x800000;
+
+	for (i = 0; i < hx711_get_gain_to_pulse(hx711_data->gain_set); i++)
+		hx711_cycle(hx711_data);
+
+	return value;
+}
+
+static int hx711_wait_for_ready(struct hx711_data *hx711_data)
+{
+	int i, val;
+
+	/*
+	 * a maximum reset cycle time of 56 ms was measured.
+	 * we round it up to 100 ms
+	 */
+	for (i = 0; i < 100; i++) {
+		val = gpiod_get_value(hx711_data->gpiod_dout);
+		if (!val)
+			break;
+		/* sleep at least 1 ms */
+		msleep(1);
+	}
+	if (val)
+		return -EIO;
+
+	return 0;
+}
+
+static int hx711_reset(struct hx711_data *hx711_data)
+{
+	int ret;
+	int val = gpiod_get_value(hx711_data->gpiod_dout);
+
+	if (val) {
+		/*
+		 * an examination with the oszilloscope indicated
+		 * that the first value read after the reset is not stable
+		 * if we reset too short;
+		 * the shorter the reset cycle
+		 * the less reliable the first value after reset is;
+		 * there were no problems encountered with a value
+		 * of 10 ms or higher
+		 */
+		gpiod_set_value(hx711_data->gpiod_pd_sck, 1);
+		msleep(10);
+		gpiod_set_value(hx711_data->gpiod_pd_sck, 0);
+
+		ret = hx711_wait_for_ready(hx711_data);
+		if (ret)
+			return ret;
+		/*
+		 * after a reset the gain is 128 so we do a dummy read
+		 * to set the gain for the next read
+		 */
+		ret = hx711_read(hx711_data);
+		if (ret < 0)
+			return ret;
+
+		/*
+		 * after a dummy read we need to wait vor readiness
+		 * for not mixing gain pulses with the clock
+		 */
+		ret = hx711_wait_for_ready(hx711_data);
+		if (ret)
+			return ret;
+	}
+
+	return val;
+}
+
+static int hx711_set_gain_for_channel(struct hx711_data *hx711_data, int chan)
+{
+	int ret;
+
+	if (chan == 0) {
+		if (hx711_data->gain_set == 32) {
+			hx711_data->gain_set = hx711_data->gain_chan_a;
+
+			ret = hx711_read(hx711_data);
+			if (ret < 0)
+				return ret;
+
+			ret = hx711_wait_for_ready(hx711_data);
+			if (ret)
+				return ret;
+		}
+	} else {
+		if (hx711_data->gain_set != 32) {
+			hx711_data->gain_set = 32;
+
+			ret = hx711_read(hx711_data);
+			if (ret < 0)
+				return ret;
+
+			ret = hx711_wait_for_ready(hx711_data);
+			if (ret)
+				return ret;
+		}
+	}
+
+	return 0;
+}
+
+static int hx711_read_raw(struct iio_dev *indio_dev,
+				const struct iio_chan_spec *chan,
+				int *val, int *val2, long mask)
+{
+	struct hx711_data *hx711_data = iio_priv(indio_dev);
+	int ret;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		mutex_lock(&hx711_data->lock);
+
+		/*
+		 * hx711_reset() must be called from here
+		 * because it could be calling hx711_read() by itself
+		 */
+		if (hx711_reset(hx711_data)) {
+			mutex_unlock(&hx711_data->lock);
+			dev_err(hx711_data->dev, "reset failed!");
+			return -EIO;
+		}
+
+		ret = hx711_set_gain_for_channel(hx711_data, chan->channel);
+		if (ret < 0) {
+			mutex_unlock(&hx711_data->lock);
+			return ret;
+		}
+
+		*val = hx711_read(hx711_data);
+
+		mutex_unlock(&hx711_data->lock);
+
+		if (*val < 0)
+			return *val;
+		return IIO_VAL_INT;
+	case IIO_CHAN_INFO_SCALE:
+		*val = 0;
+		mutex_lock(&hx711_data->lock);
+
+		*val2 = hx711_get_gain_to_scale(hx711_data->gain_set);
+
+		mutex_unlock(&hx711_data->lock);
+
+		return IIO_VAL_INT_PLUS_NANO;
+	default:
+		return -EINVAL;
+	}
+}
+
+static int hx711_write_raw(struct iio_dev *indio_dev,
+				struct iio_chan_spec const *chan,
+				int val,
+				int val2,
+				long mask)
+{
+	struct hx711_data *hx711_data = iio_priv(indio_dev);
+	int ret;
+	int gain;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_SCALE:
+		/*
+		 * a scale greater than 1 mV per LSB is not possible
+		 * with the HX711, therefore val must be 0
+		 */
+		if (val != 0)
+			return -EINVAL;
+
+		mutex_lock(&hx711_data->lock);
+
+		gain = hx711_get_scale_to_gain(val2);
+		if (gain < 0) {
+			mutex_unlock(&hx711_data->lock);
+			return gain;
+		}
+
+		if (gain != hx711_data->gain_set) {
+			hx711_data->gain_set = gain;
+			if (gain != 32)
+				hx711_data->gain_chan_a = gain;
+
+			ret = hx711_read(hx711_data);
+			if (ret < 0) {
+				mutex_unlock(&hx711_data->lock);
+				return ret;
+			}
+		}
+
+		mutex_unlock(&hx711_data->lock);
+		return 0;
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int hx711_write_raw_get_fmt(struct iio_dev *indio_dev,
+		struct iio_chan_spec const *chan,
+		long mask)
+{
+	return IIO_VAL_INT_PLUS_NANO;
+}
+
+static ssize_t hx711_scale_available_show(struct device *dev,
+				struct device_attribute *attr,
+				char *buf)
+{
+	struct iio_dev_attr *iio_attr = to_iio_dev_attr(attr);
+	int channel = iio_attr->address;
+	int i, len = 0;
+
+	for (i = 0; i < HX711_GAIN_MAX; i++)
+		if (hx711_gain_to_scale[i].channel == channel)
+			len += sprintf(buf + len, "0.%09d ",
+					hx711_gain_to_scale[i].scale);
+
+	len += sprintf(buf + len, "\n");
+
+	return len;
+}
+
+static IIO_DEVICE_ATTR(in_voltage0_scale_available, S_IRUGO,
+	hx711_scale_available_show, NULL, 0);
+
+static IIO_DEVICE_ATTR(in_voltage1_scale_available, S_IRUGO,
+	hx711_scale_available_show, NULL, 1);
+
+static struct attribute *hx711_attributes[] = {
+	&iio_dev_attr_in_voltage0_scale_available.dev_attr.attr,
+	&iio_dev_attr_in_voltage1_scale_available.dev_attr.attr,
+	NULL,
+};
+
+static struct attribute_group hx711_attribute_group = {
+	.attrs = hx711_attributes,
+};
+
+static const struct iio_info hx711_iio_info = {
+	.driver_module		= THIS_MODULE,
+	.read_raw		= hx711_read_raw,
+	.write_raw		= hx711_write_raw,
+	.write_raw_get_fmt	= hx711_write_raw_get_fmt,
+	.attrs			= &hx711_attribute_group,
+};
+
+static const struct iio_chan_spec hx711_chan_spec[] = {
+	{
+		.type = IIO_VOLTAGE,
+		.channel = 0,
+		.indexed = 1,
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
+	},
+	{
+		.type = IIO_VOLTAGE,
+		.channel = 1,
+		.indexed = 1,
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
+	},
+};
+
+static int hx711_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct hx711_data *hx711_data;
+	struct iio_dev *indio_dev;
+	int ret;
+	int i;
+
+	indio_dev = devm_iio_device_alloc(dev, sizeof(struct hx711_data));
+	if (!indio_dev) {
+		dev_err(dev, "failed to allocate IIO device\n");
+		return -ENOMEM;
+	}
+
+	hx711_data = iio_priv(indio_dev);
+	hx711_data->dev = dev;
+
+	mutex_init(&hx711_data->lock);
+
+	/*
+	 * PD_SCK stands for power down and serial clock input of HX711
+	 * in the driver it is an output
+	 */
+	hx711_data->gpiod_pd_sck = devm_gpiod_get(dev, "sck", GPIOD_OUT_LOW);
+	if (IS_ERR(hx711_data->gpiod_pd_sck)) {
+		dev_err(dev, "failed to get sck-gpiod: err=%ld\n",
+					PTR_ERR(hx711_data->gpiod_pd_sck));
+		return PTR_ERR(hx711_data->gpiod_pd_sck);
+	}
+
+	/*
+	 * DOUT stands for serial data output of HX711
+	 * for the driver it is an input
+	 */
+	hx711_data->gpiod_dout = devm_gpiod_get(dev, "dout", GPIOD_IN);
+	if (IS_ERR(hx711_data->gpiod_dout)) {
+		dev_err(dev, "failed to get dout-gpiod: err=%ld\n",
+					PTR_ERR(hx711_data->gpiod_dout));
+		return PTR_ERR(hx711_data->gpiod_dout);
+	}
+
+	hx711_data->reg_avdd = devm_regulator_get(dev, "avdd");
+	if (IS_ERR(hx711_data->reg_avdd))
+		return PTR_ERR(hx711_data->reg_avdd);
+
+	ret = regulator_enable(hx711_data->reg_avdd);
+	if (ret < 0)
+		return ret;
+
+	/*
+	 * with
+	 * full scale differential input range: AVDD / GAIN
+	 * full scale output data: 2^24
+	 * we can say:
+	 *     AVDD / GAIN = 2^24
+	 * therefore:
+	 *     1 LSB = AVDD / GAIN / 2^24
+	 * AVDD is in uV, but we need 10^-9 mV
+	 * approximately to fit into a 32 bit number:
+	 * 1 LSB = (AVDD * 100) / GAIN / 1678 [10^-9 mV]
+	 */
+	ret = regulator_get_voltage(hx711_data->reg_avdd);
+	if (ret < 0) {
+		regulator_disable(hx711_data->reg_avdd);
+		return ret;
+	}
+	/* we need 10^-9 mV */
+	ret *= 100;
+
+	for (i = 0; i < HX711_GAIN_MAX; i++)
+		hx711_gain_to_scale[i].scale =
+			ret / hx711_gain_to_scale[i].gain / 1678;
+
+	hx711_data->gain_set = 128;
+	hx711_data->gain_chan_a = 128;
+
+	platform_set_drvdata(pdev, indio_dev);
+
+	indio_dev->name = "hx711";
+	indio_dev->dev.parent = &pdev->dev;
+	indio_dev->info = &hx711_iio_info;
+	indio_dev->modes = INDIO_DIRECT_MODE;
+	indio_dev->channels = hx711_chan_spec;
+	indio_dev->num_channels = ARRAY_SIZE(hx711_chan_spec);
+
+	ret = iio_device_register(indio_dev);
+	if (ret < 0) {
+		dev_err(dev, "Couldn't register the device\n");
+		regulator_disable(hx711_data->reg_avdd);
+	}
+
+	return ret;
+}
+
+static int hx711_remove(struct platform_device *pdev)
+{
+	struct hx711_data *hx711_data;
+	struct iio_dev *indio_dev;
+
+	indio_dev = platform_get_drvdata(pdev);
+	hx711_data = iio_priv(indio_dev);
+
+	iio_device_unregister(indio_dev);
+
+	regulator_disable(hx711_data->reg_avdd);
+
+	return 0;
+}
+
+static const struct of_device_id of_hx711_match[] = {
+	{ .compatible = "avia,hx711", },
+	{},
+};
+
+MODULE_DEVICE_TABLE(of, of_hx711_match);
+
+static struct platform_driver hx711_driver = {
+	.probe		= hx711_probe,
+	.remove		= hx711_remove,
+	.driver		= {
+		.name		= "hx711-gpio",
+		.of_match_table	= of_hx711_match,
+	},
+};
+
+module_platform_driver(hx711_driver);
+
+MODULE_AUTHOR("Andreas Klinger <ak@it-klinger.de>");
+MODULE_DESCRIPTION("HX711 bitbanging driver - ADC for weight cells");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:hx711-gpio");
+