iio: magnetometer: ak8974: Correct realbits

The HSCDTD008A has 15 bits of actual ADC valie, and the
AMI305 and AMI306 has 12 bits.

Correct this by providing an extra parameter to the channel
macro and define a separate set of channels for each variant
of the chip. The resolution is the actual resolution of the
internal ADC of the chip.

The values are stored in a S16 in 2's complement so all 16
bits are used for storing (no shifting needed).

The AMI305, AMI306 and HSCDTD008A valid bits are picked from
respective datasheet.

My best educated guess is that AK8974 is also 12 bits. The
AK8973 is an 8 bit and earlier version, and the sibling
drivers AMI305 and AMI306 are 12 bits, so it makes sense
to assume that the AK8974 is also 12 bits.

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

@@ -651,7 +651,7 @@ static const struct iio_chan_spec_ext_info ak8974_ext_info[] = {
 	{ },
 };
 
-#define AK8974_AXIS_CHANNEL(axis, index)				\
+#define AK8974_AXIS_CHANNEL(axis, index, bits)				\
 	{								\
 		.type = IIO_MAGN,					\
 		.modified = 1,						\
@@ -662,16 +662,32 @@ static const struct iio_chan_spec_ext_info ak8974_ext_info[] = {
 		.scan_index = index,					\
 		.scan_type = {						\
 			.sign = 's',					\
-			.realbits = 16,					\
+			.realbits = bits,				\
 			.storagebits = 16,				\
 			.endianness = IIO_LE				\
 		},							\
 	}
 
-static const struct iio_chan_spec ak8974_channels[] = {
-	AK8974_AXIS_CHANNEL(X, 0),
-	AK8974_AXIS_CHANNEL(Y, 1),
-	AK8974_AXIS_CHANNEL(Z, 2),
+/*
+ * We have no datasheet for the AK8974 but we guess that its
+ * ADC is 12 bits. The AMI305 and AMI306 certainly has 12bit
+ * ADC.
+ */
+static const struct iio_chan_spec ak8974_12_bits_channels[] = {
+	AK8974_AXIS_CHANNEL(X, 0, 12),
+	AK8974_AXIS_CHANNEL(Y, 1, 12),
+	AK8974_AXIS_CHANNEL(Z, 2, 12),
+	IIO_CHAN_SOFT_TIMESTAMP(3),
+};
+
+/*
+ * The HSCDTD008A has 15 bits resolution the way we set it up
+ * in CTRL4.
+ */
+static const struct iio_chan_spec ak8974_15_bits_channels[] = {
+	AK8974_AXIS_CHANNEL(X, 0, 15),
+	AK8974_AXIS_CHANNEL(Y, 1, 15),
+	AK8974_AXIS_CHANNEL(Z, 2, 15),
 	IIO_CHAN_SOFT_TIMESTAMP(3),
 };
 
@@ -820,8 +836,21 @@ static int ak8974_probe(struct i2c_client *i2c,
 	pm_runtime_put(&i2c->dev);
 
 	indio_dev->dev.parent = &i2c->dev;
-	indio_dev->channels = ak8974_channels;
-	indio_dev->num_channels = ARRAY_SIZE(ak8974_channels);
+	switch (ak8974->variant) {
+	case AK8974_WHOAMI_VALUE_AMI306:
+	case AK8974_WHOAMI_VALUE_AMI305:
+		indio_dev->channels = ak8974_12_bits_channels;
+		indio_dev->num_channels = ARRAY_SIZE(ak8974_12_bits_channels);
+		break;
+	case AK8974_WHOAMI_VALUE_HSCDTD008A:
+		indio_dev->channels = ak8974_15_bits_channels;
+		indio_dev->num_channels = ARRAY_SIZE(ak8974_15_bits_channels);
+		break;
+	default:
+		indio_dev->channels = ak8974_12_bits_channels;
+		indio_dev->num_channels = ARRAY_SIZE(ak8974_12_bits_channels);
+		break;
+	}
 	indio_dev->info = &ak8974_info;
 	indio_dev->available_scan_masks = ak8974_scan_masks;
 	indio_dev->modes = INDIO_DIRECT_MODE;