iio: humidity: Add support for AM2315

Add basic support for the Aosong AM2315 relative humidity
and ambient temperature sensor.
Includes support for raw readings and ACPI detection.

Datasheet:
http://www.aosong.com/asp_bin/Products/en/AM2315.pdfSigned-off-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/humidity/Kconfig

@@ -3,6 +3,16 @@
 #
 menu "Humidity sensors"
 
+config AM2315
+    tristate "Aosong AM2315 relative humidity and temperature sensor"
+    depends on I2C
+    help
+      If you say yes here you get support for the Aosong AM2315
+      relative humidity and ambient temperature sensor.
+
+      This driver can also be built as a module. If so, the module will
+      be called am2315.
+
 config DHT11
 	tristate "DHT11 (and compatible sensors) driver"
 	depends on GPIOLIB || COMPILE_TEST

drivers/iio/humidity/Makefile

@@ -2,6 +2,7 @@
 # Makefile for IIO humidity sensor drivers
 #
 
+obj-$(CONFIG_AM2315) += am2315.o
 obj-$(CONFIG_DHT11) += dht11.o
 obj-$(CONFIG_HDC100X) += hdc100x.o
 obj-$(CONFIG_HTU21) += htu21.o

drivers/iio/humidity/am2315.c

+/**
+ * Aosong AM2315 relative humidity and temperature
+ *
+ * Copyright (c) 2016, Intel Corporation.
+ *
+ * This file is subject to the terms and conditions of version 2 of
+ * the GNU General Public License. See the file COPYING in the main
+ * directory of this archive for more details.
+ *
+ * 7-bit I2C address: 0x5C.
+ */
+
+#include <linux/acpi.h>
+#include <linux/delay.h>
+#include <linux/i2c.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/iio/iio.h>
+#include <linux/iio/sysfs.h>
+
+#define AM2315_REG_HUM_MSB			0x00
+#define AM2315_REG_HUM_LSB			0x01
+#define AM2315_REG_TEMP_MSB			0x02
+#define AM2315_REG_TEMP_LSB			0x03
+
+#define AM2315_FUNCTION_READ			0x03
+#define AM2315_HUM_OFFSET			2
+#define AM2315_TEMP_OFFSET			4
+
+#define AM2315_DRIVER_NAME			"am2315"
+
+struct am2315_data {
+	struct i2c_client *client;
+	struct mutex lock;
+};
+
+struct am2315_sensor_data {
+	s16 hum_data;
+	s16 temp_data;
+};
+
+static const struct iio_chan_spec am2315_channels[] = {
+	{
+		.type = IIO_HUMIDITYRELATIVE,
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
+				      BIT(IIO_CHAN_INFO_SCALE)
+	},
+	{
+		.type = IIO_TEMP,
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
+				      BIT(IIO_CHAN_INFO_SCALE)
+	},
+};
+
+/* CRC calculation algorithm, as specified in the datasheet (page 13). */
+static u16 am2315_crc(u8 *data, u8 nr_bytes)
+{
+	int i;
+	u16 crc = 0xffff;
+
+	while (nr_bytes--) {
+		crc ^= *data++;
+		for (i = 0; i < 8; i++) {
+			if (crc & 0x01) {
+				crc >>= 1;
+				crc ^= 0xA001;
+			} else {
+				crc >>= 1;
+			}
+		}
+	}
+
+	return crc;
+}
+
+/* Simple function that sends a few bytes to the device to wake it up. */
+static void am2315_ping(struct i2c_client *client)
+{
+	i2c_smbus_read_byte_data(client, AM2315_REG_HUM_MSB);
+}
+
+static int am2315_read_data(struct am2315_data *data,
+			    struct am2315_sensor_data *sensor_data)
+{
+	int ret;
+	/* tx_buf format: <function code> <start addr> <nr of regs to read> */
+	u8 tx_buf[3] = { AM2315_FUNCTION_READ, AM2315_REG_HUM_MSB, 4 };
+	/*
+	 * rx_buf format:
+	 * <function code> <number of registers read>
+	 * <humidity MSB> <humidity LSB> <temp MSB> <temp LSB>
+	 * <CRC LSB> <CRC MSB>
+	 */
+	u8 rx_buf[8];
+	u16 crc;
+
+	/* First wake up the device. */
+	am2315_ping(data->client);
+
+	mutex_lock(&data->lock);
+	ret = i2c_master_send(data->client, tx_buf, sizeof(tx_buf));
+	if (ret < 0) {
+		dev_err(&data->client->dev, "failed to send read request\n");
+		goto exit_unlock;
+	}
+	/* Wait 2-3 ms, then read back the data sent by the device. */
+	usleep_range(2000, 3000);
+	/* Do a bulk data read, then pick out what we need. */
+	ret = i2c_master_recv(data->client, rx_buf, sizeof(rx_buf));
+	if (ret < 0) {
+		dev_err(&data->client->dev, "failed to read sensor data\n");
+		goto exit_unlock;
+	}
+	mutex_unlock(&data->lock);
+	/*
+	 * Do a CRC check on the data and compare it to the value
+	 * calculated by the device.
+	 */
+	crc = am2315_crc(rx_buf, sizeof(rx_buf) - 2);
+	if ((crc & 0xff) != rx_buf[6] || (crc >> 8) != rx_buf[7]) {
+		dev_err(&data->client->dev, "failed to verify sensor data\n");
+		return -EIO;
+	}
+
+	sensor_data->hum_data = (rx_buf[AM2315_HUM_OFFSET] << 8) |
+				 rx_buf[AM2315_HUM_OFFSET + 1];
+	sensor_data->temp_data = (rx_buf[AM2315_TEMP_OFFSET] << 8) |
+				  rx_buf[AM2315_TEMP_OFFSET + 1];
+
+	return ret;
+
+exit_unlock:
+	mutex_unlock(&data->lock);
+	return ret;
+}
+
+static int am2315_read_raw(struct iio_dev *indio_dev,
+			   struct iio_chan_spec const *chan,
+			   int *val, int *val2, long mask)
+{
+	int ret;
+	struct am2315_sensor_data sensor_data;
+	struct am2315_data *data = iio_priv(indio_dev);
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		ret = am2315_read_data(data, &sensor_data);
+		if (ret < 0)
+			return ret;
+		*val = (chan->type == IIO_HUMIDITYRELATIVE) ?
+				sensor_data.hum_data : sensor_data.temp_data;
+		return IIO_VAL_INT;
+	case IIO_CHAN_INFO_SCALE:
+		*val = 100;
+		return IIO_VAL_INT;
+	}
+
+	return -EINVAL;
+}
+
+static const struct iio_info am2315_info = {
+	.driver_module		= THIS_MODULE,
+	.read_raw		= am2315_read_raw,
+};
+
+static int am2315_probe(struct i2c_client *client,
+			const struct i2c_device_id *id)
+{
+	struct iio_dev *indio_dev;
+	struct am2315_data *data;
+
+	indio_dev = devm_iio_device_alloc(&client->dev, sizeof(*data));
+	if (!indio_dev) {
+		dev_err(&client->dev, "iio allocation failed!\n");
+		return -ENOMEM;
+	}
+
+	data = iio_priv(indio_dev);
+	data->client = client;
+	i2c_set_clientdata(client, indio_dev);
+	mutex_init(&data->lock);
+
+	indio_dev->dev.parent = &client->dev;
+	indio_dev->info = &am2315_info;
+	indio_dev->name = AM2315_DRIVER_NAME;
+	indio_dev->modes = INDIO_DIRECT_MODE;
+	indio_dev->channels = am2315_channels;
+	indio_dev->num_channels = ARRAY_SIZE(am2315_channels);
+
+	return iio_device_register(indio_dev);
+}
+
+static int am2315_remove(struct i2c_client *client)
+{
+	struct iio_dev *indio_dev = i2c_get_clientdata(client);
+
+	iio_device_unregister(indio_dev);
+
+	return 0;
+}
+
+static const struct i2c_device_id am2315_i2c_id[] = {
+	{"am2315", 0},
+	{}
+};
+
+static const struct acpi_device_id am2315_acpi_id[] = {
+	{"AOS2315", 0},
+	{}
+};
+
+MODULE_DEVICE_TABLE(acpi, am2315_acpi_id);
+
+static struct i2c_driver am2315_driver = {
+	.driver = {
+		.name = "am2315",
+		.acpi_match_table = ACPI_PTR(am2315_acpi_id),
+	},
+	.probe =            am2315_probe,
+	.remove =	    am2315_remove,
+	.id_table =         am2315_i2c_id,
+};
+
+module_i2c_driver(am2315_driver);
+
+MODULE_AUTHOR("Tiberiu Breana <tiberiu.a.breana@intel.com>");
+MODULE_DESCRIPTION("Aosong AM2315 relative humidity and temperature");
+MODULE_LICENSE("GPL v2");