[PATCH] iio: accel: bmc150: Use more consistent and accurate scale values

[Stephan Gerhold <stephan@gerhold.net>]

It is quite strange that BMA222 and BMA222E have very close, yet
subtly different values in their scale tables. Comparing the datasheets
this is simply because the "Resolution" for the different measurement
ranges are documented with different precision.

For example, for +-2g the BMA222 datasheet [1] suggests a resolution
of 15.6 mg/LSB, while the BMA222E datasheet [2] suggests 15.63 mg/LSB.

Actually, there is no need to rely on the resolution given by the Bosch
datasheets. The resolution and scale can be calculated more consistently
and accurately using the range (e.g. +-2g) and the channel size (e.g. 8 bits).

Distributing 4g (-2g to 2g) over 8 bits results in an exact resolution
of (4g / 2^8) = 15.625 mg/LSB which is the same value as in both datasheets,
just slightly more accurate. Multiplying g = 9.80665 m/s^2 we get a more
accurate value for the IIO scale table.

Generalizing this we can calculate the scale tables more accurately using
(range / 2^bits) * g * 10^6 (because of IIO_VAL_INT_PLUS_MICRO).

Document this and make the scale tables more consistent and accurate
for all the variants using that formula. Now the scale tables for
BMA222 and BMA222E are consistent and probably slightly more accurate.

[1]: https://media.digikey.com/pdf/Data%20Sheets/Bosch/BMA222.pdf
[2]: https://www.mouser.com/datasheet/2/783/BST-BMA222E-DS004-06-1021076.pdf

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
---
 drivers/iio/accel/bmc150-accel-core.c | 46 ++++++++++++++-------------
 1 file changed, 24 insertions(+), 22 deletions(-)

diff --git a/drivers/iio/accel/bmc150-accel-core.c b/drivers/iio/accel/bmc150-accel-core.c
index 43aecd4bf3a4..5ce384ebe6c7 100644
--- a/drivers/iio/accel/bmc150-accel-core.c
+++ b/drivers/iio/accel/bmc150-accel-core.c
@@ -1088,19 +1088,21 @@ static const struct iio_chan_spec bmc150_accel_channels[] =
 static const struct iio_chan_spec bma280_accel_channels[] =
 	BMC150_ACCEL_CHANNELS(14);
 
+/*
+ * The range for the Bosch sensors is typically +-2g/4g/8g/16g, distributed
+ * over the amount of bits (see above). The scale table can be calculated using
+ *     (range / 2^bits) * g = (range / 2^bits) * 9.80665 m/s^2
+ * e.g. for +-2g and 12 bits: (4 / 2^12) * 9.80665 m/s^2 = 0.0095768... m/s^2
+ * Multiply 10^6 and round to get the values listed below.
+ */
 static const struct bmc150_accel_chip_info bmc150_accel_chip_info_tbl[] = {
 	{
 		.name = "BMA222",
 		.chip_id = 0x03,
 		.channels = bma222e_accel_channels,
 		.num_channels = ARRAY_SIZE(bma222e_accel_channels),
-		/*
-		 * The datasheet page 17 says:
-		 * 15.6, 31.3, 62.5 and 125 mg per LSB.
-		 * IIO unit is m/s^2 so multiply by g = 9.80665 m/s^2.
-		 */
-		.scale_table = { {152984, BMC150_ACCEL_DEF_RANGE_2G},
-				 {306948, BMC150_ACCEL_DEF_RANGE_4G},
+		.scale_table = { {153229, BMC150_ACCEL_DEF_RANGE_2G},
+				 {306458, BMC150_ACCEL_DEF_RANGE_4G},
 				 {612916, BMC150_ACCEL_DEF_RANGE_8G},
 				 {1225831, BMC150_ACCEL_DEF_RANGE_16G} },
 	},
@@ -1109,9 +1111,9 @@ static const struct bmc150_accel_chip_info bmc150_accel_chip_info_tbl[] = {
 		.chip_id = 0xF8,
 		.channels = bma222e_accel_channels,
 		.num_channels = ARRAY_SIZE(bma222e_accel_channels),
-		.scale_table = { {153277, BMC150_ACCEL_DEF_RANGE_2G},
-				 {306457, BMC150_ACCEL_DEF_RANGE_4G},
-				 {612915, BMC150_ACCEL_DEF_RANGE_8G},
+		.scale_table = { {153229, BMC150_ACCEL_DEF_RANGE_2G},
+				 {306458, BMC150_ACCEL_DEF_RANGE_4G},
+				 {612916, BMC150_ACCEL_DEF_RANGE_8G},
 				 {1225831, BMC150_ACCEL_DEF_RANGE_16G} },
 	},
 	{
@@ -1119,30 +1121,30 @@ static const struct bmc150_accel_chip_info bmc150_accel_chip_info_tbl[] = {
 		.chip_id = 0xF9,
 		.channels = bma250e_accel_channels,
 		.num_channels = ARRAY_SIZE(bma250e_accel_channels),
-		.scale_table = { {38344, BMC150_ACCEL_DEF_RANGE_2G},
-				 {76590, BMC150_ACCEL_DEF_RANGE_4G},
-				 {153277, BMC150_ACCEL_DEF_RANGE_8G},
-				 {306457, BMC150_ACCEL_DEF_RANGE_16G} },
+		.scale_table = { {38307, BMC150_ACCEL_DEF_RANGE_2G},
+				 {76614, BMC150_ACCEL_DEF_RANGE_4G},
+				 {153229, BMC150_ACCEL_DEF_RANGE_8G},
+				 {306458, BMC150_ACCEL_DEF_RANGE_16G} },
 	},
 	{
 		.name = "BMA253/BMA254/BMA255/BMC150/BMI055",
 		.chip_id = 0xFA,
 		.channels = bmc150_accel_channels,
 		.num_channels = ARRAY_SIZE(bmc150_accel_channels),
-		.scale_table = { {9610, BMC150_ACCEL_DEF_RANGE_2G},
-				 {19122, BMC150_ACCEL_DEF_RANGE_4G},
-				 {38344, BMC150_ACCEL_DEF_RANGE_8G},
-				 {76590, BMC150_ACCEL_DEF_RANGE_16G} },
+		.scale_table = { {9577, BMC150_ACCEL_DEF_RANGE_2G},
+				 {19154, BMC150_ACCEL_DEF_RANGE_4G},
+				 {38307, BMC150_ACCEL_DEF_RANGE_8G},
+				 {76614, BMC150_ACCEL_DEF_RANGE_16G} },
 	},
 	{
 		.name = "BMA280",
 		.chip_id = 0xFB,
 		.channels = bma280_accel_channels,
 		.num_channels = ARRAY_SIZE(bma280_accel_channels),
-		.scale_table = { {2392, BMC150_ACCEL_DEF_RANGE_2G},
-				 {4785, BMC150_ACCEL_DEF_RANGE_4G},
-				 {9581, BMC150_ACCEL_DEF_RANGE_8G},
-				 {19152, BMC150_ACCEL_DEF_RANGE_16G} },
+		.scale_table = { {2394, BMC150_ACCEL_DEF_RANGE_2G},
+				 {4788, BMC150_ACCEL_DEF_RANGE_4G},
+				 {9577, BMC150_ACCEL_DEF_RANGE_8G},
+				 {19154, BMC150_ACCEL_DEF_RANGE_16G} },
 	},
 };
 
-- 
2.32.0