Add basic IIO support for the Analog Devices ADXL345 3-axis accelerometer.
The datasheet can be found here:
http://www.analog.com/media/en/technical-documentation/data-sheets/ADXL345.pdf

comments below
 

buffered mode would be useful in order to get consistent measurements of 
all three axis at once

+ */
+
+#include <linux/i2c.h>
+#include <linux/module.h>
+
+#include <linux/iio/iio.h>
+
+#define ADXL345_REG_DEVID		0x00
+#define ADXL345_REG_POWER_CTL		0x2D
+#define ADXL345_REG_DATA_FORMAT		0x31
+#define ADXL345_REG_DATAX0		0x32
+#define ADXL345_REG_DATAY0		0x34
+#define ADXL345_REG_DATAZ0		0x36
+
+#define ADXL345_POWER_CTL_MEASURE	BIT(3)
+#define ADXL345_POWER_CTL_STANDBY	0x00
+
+#define ADXL345_DATA_FORMAT_FULL_RES	BIT(3)

maybe state in comment that FULL_RES is 12 bit

+#define ADXL345_DATA_FORMAT_2G		0
+#define ADXL345_DATA_FORMAT_4G		1
+#define ADXL345_DATA_FORMAT_8G		2
+#define ADXL345_DATA_FORMAT_16G		3
+
+#define ADXL345_DEVID			0xE5
+
+struct adxl345_data {
+	struct i2c_client *client;
+	u8 data_range;
+};
+
+#define ADXL345_CHANNEL(reg, axis) {					\
+	.type = IIO_ACCEL,						\
+	.modified = 1,							\
+	.channel2 = IIO_MOD_##axis,					\
+	.address = reg,							\
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),			\

haven't checked but likely we need _SCALE to present the correct unit of 
measurement

+}
+
+static const struct iio_chan_spec adxl345_channels[] = {
+	ADXL345_CHANNEL(ADXL345_REG_DATAX0, X),
+	ADXL345_CHANNEL(ADXL345_REG_DATAY0, Y),
+	ADXL345_CHANNEL(ADXL345_REG_DATAZ0, Z),
+};
+
+static int adxl345_read_raw(struct iio_dev *indio_dev,
+			    struct iio_chan_spec const *chan,
+			    int *val, int *val2, long mask)
+{
+	struct adxl345_data *data = iio_priv(indio_dev);
+	int ret;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		/*
+		 * Data is stored in adjacent registers:
+		 * ADXL345_REG_DATA(X0/Y0/Z0) contain the least significant byte
+		 * and ADXL345_REG_DATA(X0/Y0/Z0) + 1 the most significant byte
+		 */

there is no data ready? we might get old data?

+		ret = i2c_smbus_read_word_data(data->client, chan->address);
+

drop newline

+		if (ret < 0)
+			return ret;
+
+		*val = sign_extend32(le16_to_cpu(ret), 12);

le16_to_cpu() not needed, I2C read_word_data does it for you

+		return IIO_VAL_INT;
+	}
+
+	return -EINVAL;
+}
+
+static const struct iio_info adxl345_info = {
+	.driver_module	= THIS_MODULE,
+	.read_raw	= adxl345_read_raw,
+};
+
+static int adxl345_probe(struct i2c_client *client,
+			 const struct i2c_device_id *id)
+{
+	struct adxl345_data *data;
+	struct iio_dev *indio_dev;
+	int ret;
+
+	ret = i2c_smbus_read_byte_data(client, ADXL345_REG_DEVID);
+	if (ret < 0) {
+		dev_err(&client->dev, "Error reading device ID: %d\n", ret);
+		return ret;
+	}
+
+	if (ret != ADXL345_DEVID) {
+		dev_err(&client->dev, "Invalid device ID: %d\n", ret);
+		return -ENODEV;
+	}
+
+	indio_dev = devm_iio_device_alloc(&client->dev, sizeof(*data));
+	if (!indio_dev)
+		return -ENOMEM;
+
+	data = iio_priv(indio_dev);
+	i2c_set_clientdata(client, indio_dev);
+	data->client = client;
+	/* Enable full-resolution mode */
+	data->data_range = ADXL345_DATA_FORMAT_FULL_RES;
+
+	ret = i2c_smbus_write_byte_data(data->client, ADXL345_REG_DATA_FORMAT,
+					data->data_range);
+	if (ret < 0) {
+		dev_err(&client->dev, "Failed to set data range: %d\n", ret);
+		return ret;
+	}
+
+	indio_dev->dev.parent = &client->dev;
+	indio_dev->name = id->name;
+	indio_dev->info = &adxl345_info;
+	indio_dev->modes = INDIO_DIRECT_MODE;
+	indio_dev->channels = adxl345_channels;
+	indio_dev->num_channels = ARRAY_SIZE(adxl345_channels);
+
+	/* Enable measurement mode */
+	ret = i2c_smbus_write_byte_data(data->client, ADXL345_REG_POWER_CTL,
+					ADXL345_POWER_CTL_MEASURE);
+	if (ret < 0) {
+		dev_err(&client->dev, "Failed to enable measurement mode: %d\n",
+			ret);
+		return ret;
+	}
+
+	return iio_device_register(indio_dev);

stop measurement mode if registration fails

+}
+
+static int adxl345_remove(struct i2c_client *client)
+{
+	struct iio_dev *indio_dev = i2c_get_clientdata(client);
+	struct adxl345_data *data = iio_priv(indio_dev);
+
+	iio_device_unregister(indio_dev);
+
+	return i2c_smbus_write_byte_data(data->client, ADXL345_REG_POWER_CTL,
+					 ADXL345_POWER_CTL_STANDBY);
+}
+
+static const struct i2c_device_id adxl345_i2c_id[] = {
+	{ "adxl345", 0 },
+	{ }
+};
+
+MODULE_DEVICE_TABLE(i2c, adxl345_i2c_id);
+
+static struct i2c_driver adxl345_driver = {
+	.driver = {
+		.name	= "adxl345",
+	},
+	.probe		= adxl345_probe,
+	.remove		= adxl345_remove,
+	.id_table	= adxl345_i2c_id,
+};
+
+module_i2c_driver(adxl345_driver);
+
+MODULE_AUTHOR("Eva Rachel Retuya [off-list ref]");
+MODULE_DESCRIPTION("ADXL345 3-Axis Digital Accelerometer driver");
+MODULE_LICENSE("GPL v2");

-- 

Peter Meerwald-Stadler
+43-664-2444418 (mobile)