On Fri, 11 Jun 2021 21:41:41 +0300 Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote: > On Fri, Jun 11, 2021 at 9:27 PM Stephan Gerhold <stephan@xxxxxxxxxxx> wrote: > > > > 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. > > Oh, oh, one more HW vendor to blame for not being mathematically precise! > We already had the very same issue in Intel and AMD documentation :-) > > In full support of such fixes! > Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx> > > > [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@xxxxxxxxxx> > > Signed-off-by: Stephan Gerhold <stephan@xxxxxxxxxxx> Given these sensors are very inaccurate at the best of times (bma222 is +- 100mg) this is unlikely to have any meaningful effect. But consistency is good so I've applied it anyway :) Applied to the togreg branch of iio.git and pushed out as testing to see if 0-day is in a spiteful mood. Jonathan > > --- > > 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 > > > >
