iio: inv-mpu: Specify the expected format/precision for write channels

commit 6a3c45bb upstream.

The gyroscope needs IIO_VAL_INT_PLUS_NANO for the scale channel and
unless specified write returns MICRO by default.
This needs to be properly specified so that write operations into scale
have the expected behaviour.
Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/iio/imu/inv_mpu6050/inv_mpu_core.c

@@ -340,6 +340,23 @@ static int inv_mpu6050_write_fsr(struct inv_mpu6050_state *st, int fsr)
 	return 0;
 }
 
+static int inv_write_raw_get_fmt(struct iio_dev *indio_dev,
+				 struct iio_chan_spec const *chan, long mask)
+{
+	switch (mask) {
+	case IIO_CHAN_INFO_SCALE:
+		switch (chan->type) {
+		case IIO_ANGL_VEL:
+			return IIO_VAL_INT_PLUS_NANO;
+		default:
+			return IIO_VAL_INT_PLUS_MICRO;
+		}
+	default:
+		return IIO_VAL_INT_PLUS_MICRO;
+	}
+
+	return -EINVAL;
+}
 static int inv_mpu6050_write_accel_fs(struct inv_mpu6050_state *st, int fs)
 {
 	int result;
@@ -603,6 +620,7 @@ static const struct iio_info mpu_info = {
 	.driver_module = THIS_MODULE,
 	.read_raw = &inv_mpu6050_read_raw,
 	.write_raw = &inv_mpu6050_write_raw,
+	.write_raw_get_fmt = &inv_write_raw_get_fmt,
 	.attrs = &inv_attribute_group,
 	.validate_trigger = inv_mpu6050_validate_trigger,
 };