iio: magnetometer: ak8974: Provide scaling

The manual for the HSCDTD008A gives us a scaling for the
three axis as +/- 2.4mT (24 Gauss) per axis.

The manual for the AMI305 and AMI306 gives us a scaling
for the three axis as +/- 12 Gauss per axis.

Tests with the HSCDTD008A sensor, cat the raw values:
$ cat in_magn_*_raw
raw
45
189
-19

The scaling factor in in_magn_*_scale is 0.001464843,
which gives:
0.065 Gauss
0.277 Gauss
-0.027 Gauss

The earths magnetic field is in the range of 0.25 to 0.65
Gauss on the surface according to Wikipedia, so these
seem like reasonable values.

Again we are guessing that the AK8974 has a 12 bit ADC,
based on the similarity with AMI305 and AMI306.

Cc: Nick Reitemeyer <nick.reitemeyer@web.de>
Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/magnetometer/ak8974.c

@@ -606,6 +606,48 @@ static int ak8974_read_raw(struct iio_dev *indio_dev,
 		if (ret)
 			return ret;
 		return IIO_VAL_INT;
+	case IIO_CHAN_INFO_SCALE:
+		switch (ak8974->variant) {
+		case AK8974_WHOAMI_VALUE_AMI306:
+		case AK8974_WHOAMI_VALUE_AMI305:
+			/*
+			 * The datasheet for AMI305 and AMI306, page 6
+			 * specifies the range of the sensor to be
+			 * +/- 12 Gauss.
+			 */
+			*val = 12;
+			/*
+			 * 12 bits are used, +/- 2^11
+			 * [ -2048 .. 2047 ] (manual page 20)
+			 * [ 0xf800 .. 0x07ff ]
+			 */
+			*val2 = 11;
+			return IIO_VAL_FRACTIONAL_LOG2;
+		case AK8974_WHOAMI_VALUE_HSCDTD008A:
+			/*
+			 * The datasheet for HSCDTF008A, page 3 specifies the
+			 * range of the sensor as +/- 2.4 mT per axis, which
+			 * corresponds to +/- 2400 uT = +/- 24 Gauss.
+			 */
+			*val = 24;
+			/*
+			 * 15 bits are used (set up in CTRL4), +/- 2^14
+			 * [ -16384 .. 16383 ] (manual page 24)
+			 * [ 0xc000 .. 0x3fff ]
+			 */
+			*val2 = 14;
+			return IIO_VAL_FRACTIONAL_LOG2;
+		default:
+			/* GUESSING +/- 12 Gauss */
+			*val = 12;
+			/* GUESSING 12 bits ADC +/- 2^11 */
+			*val2 = 11;
+			return IIO_VAL_FRACTIONAL_LOG2;
+		}
+		break;
+	default:
+		/* Unknown request */
+		break;
 	}
 
 	return -EINVAL;
@@ -670,7 +712,8 @@ static const struct iio_chan_spec_ext_info ak8974_ext_info[] = {
 		.type = IIO_MAGN,					\
 		.modified = 1,						\
 		.channel2 = IIO_MOD_##axis,				\
-		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |		\
+			BIT(IIO_CHAN_INFO_SCALE),			\
 		.ext_info = ak8974_ext_info,				\
 		.address = index,					\
 		.scan_index = index,					\